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Xref: spies comp.sys.ibm.pc.hardware:48481 comp.sys.ibm.pc.programmer:226 Path: spies!sgiblab!spool.mu.edu!yale.edu!ira.uka.de!sbusol.rz.uni-sb.de!coli.uni-sb.de!sbustd!chbl From: chbl@sbustd.rz.uni-sb.de (Christian Blum) Newsgroups: comp.sys.ibm.pc.hardware,comp.sys.ibm.pc.programmer Subject: FAQ: serial ports of the IBM PC Date: 6 Jan 1993 23:13:20 GMT Organization: Studenten-Mail, Rechenzentrum Universitaet des Saarlandes Lines: 1127 Message-ID: <1ifp2gINNffa@coli-gate.coli.uni-sb.de> NNTP-Posting-Host: sbustd.stud.uni-sb.de This is a FAQ answer on serial communications using the TTY protocol. It contains information on the TTY protocol and hardware and software implemen- tations on IBM PCs which is derived from National Semiconductor data sheets and practical experience of the author. If you want to contribute to this FAQ in any way, please email me (probably by replying to this posting). My email address is: chbl@stud.uni-sb.de See the end for details. Introduction ============ One of the most universal parts of the PC is its serial port. You can connect a mouse, a modem, a printer, a plotter, another PC, ... But its usage (both software and hardware) is one of the best-kept secrets for most users, besides that it is not difficult to understand how to connect (not plug-in) devices to it and how to program it. Regard this FAQ as a manual of the serial port of your PC for both hardware and software. Historical summary ------------------ In early days of telecommunications, errand-boys and optical signals (flags, lights, clouds of smoke) were the only methods of transmitting information across long distances. With increasing requirements on speed and growing amount of information, more practical methods were developed. One milestone was the first wire-bound transmission on May 24th, 1844 ("What hath God wrought", using the famous Morse-alphabet). Well, technology improved a bit, and soon there were machines that could be used like typewriters, except that you typed not only on your own piece of paper but also on somebody elses. The only thing that has been changed on the step from the teletyper to your PC is speed. The TTY (teletyping) protocol ----------------------------- Definition: A protocol is a clear description of the LOGICAL method of transmitting information. This does NOT include physical realisation. The TTYp uses two different states of the line called 'mark' and 'space'. If no data is transmitted, the line is in the 'space' state. Data looks like mark +---+ +---+ +---+ | | | | | | space ----------+ +-------+ +---+ +------- (1) --------(2)-------- (3) (1) start bit (2) data bits (3) stop bit(s) Both transmitter (TX) and receiver (RX) use the same data rate (measured in baud, which is the reciprocal value of the smallest time interval between two changes of the line state. TX and RX know about the number of data bits (probably with a parity bit added), and both know about the size of the stop step (called the stop bit or the stop bits, depending on the size of the stop step; normally 1, 1.5 or 2 times the size of a data bit). Data is transmitted bit-synchroneously and word-asynchroneously, which means that the size of the bits, the length of the word etc.pp. is clearly defined but the time between two words is undefined. The start bit indicates the beginning of a new data word. It is used to synchronise transmitter and receiver. Data is transmitted LSB to MSB, which means that the least significant bit (Bit 0) is transmitted first with 4 to 7 bits of data following. This results in 5 to 8 bits of data. A logical '0' is transmitted by the 'space' state of the line, a logical '1' by 'mark'. A parity bit can be added to the data bits to allow error detection. There are two (well, actually five) kinds of parity: odd and even (plus none, mark and space). Odd parity means that the number of 'mark' steps in the data word (including parity bit) is always odd, so the parity bit is set accordingly (I don't have to explain 'even' parity, must I?). It is also possible to set the parity bit to a fixed state or to omit it. The stop bit does not indicate the end of the word (as it could be derived from its name); it rather separates two consecutive words by putting the line into the 'space' state for a minimum time. The protocol is usually described by a sequence of numbers and letters, e.g. 8n1 means 1 start bit (always), 8 bits of data, no parity bit, 1 stop bit. 7e2 would indicate 7 bits of data, even parity, 2 stop bits (but I've never seen this one...). The usual thing is 8n1 or 7e1. Early teletypers used the neckbreaking speed of 50 baud (which means that one step is 20ms), 5 bits of data, no parity and 1.5 stop bits (don't ask me why!). Your PC is capable of serial transmission at up to 115,200 baud (step size of 8.68 microseconds!). Typical rates are 300 baud, 1200 baud, 2400 baud and 9600 baud. The physical transmission ------------------------- Teletypers used a closed-loop line with a space current of 20ma and a mark current of 0ma (typical), which allowed to detect a 'broken line'. The RS232C port of your PC uses voltages rather than currents to indicate logical states: 'space' is signaled by -3v to -15v (typical -12v), 'mark' by +3v to +15v (typical +12V). The typical output impedance of the serial port of a PC is 2 kiloohms (resulting in about 5ma @ 10v), the typical input impedance is about 4.3 kiloohms, so there should be a maximum fan-out of 5 (5 inputs can be connected to 1 output). Please don't rely on this, it may differ from PC to PC. Three lines (RX, TX & ground) are needed. Q. Why does my PC have a 25pin/9pin connector if there are only 3 lines needed? A. There are several status lines that are only used with a modem. See the software section of this FAQ. Q. How can I easily connect two PCs by a three-wire lead? A. This connection is called a 'null-modem' connection. RX1 is connected to TX2 and vice versa, GND1 to GND2. In addition to this, connect RTS to CTS & DCD and DTR to DSR (modem software often relies on that). See the hardware section for further details. Hardware ======== The connectors -------------- PCs have 9pin/25pin male SUB-D connectors. The pin layout is as follows (looking at the back side of your PC): 1 13 1 5 _______________________________ _______________ \ . . . . . . . . . . . . . / \ . . . . . / \ . . . . . . . . . . . . / \ . . . . / --------------------------- ----------- 14 25 6 9 Name (V24) 25pin 9pin Dir Full name Remarks -------------------------------------------------------------------------- TxD 2 2 o Transmit Data RxD 3 3 i Receive Data RTS 4 5 o Request To Send CTS 5 8 i Clear To Send DTR 20 4 o Data Terminal Ready DSR 6 6 i Data Set Ready RI 22 9 i Ring Indicator DCD 8 1 i Data Carrier Detect GND 7 5 - Signal ground - 1 - - Protective ground Don't use this one! SCTE 24 - - Sync. clock trans. end Several PCs only SCT 15 - o Sync. clock TX dito. SCR 17 - i Sync. clock RX dito. The most important lines are RxD, TxD, and GND. Others are used with modems, printers and plotters to indicate internal states or to use synchroneous transmission (this is rarely used, and most PCs don't support it). '0' means -3v to -15V, '1' means +3v to +15v. '1' is the active state. The lines are: RxD, TxD: These lines carry the data. RTS, CTS: Are used by the PC and the modem/printer/whatsoever (further on referred to as the data set) to start/stop a communication. The PC sets RTS to its active state ('1'), and the data set responds with CTS '1' (always in this order). If the data set wants to stop/interrupt the communication (e.g. buffer overflow), it drops CTS to '0'; the PC uses RTS to control the data flow. DTR, DSR: Are used to establish a connection at the very beginning, i.e. the PC and the data set 'shake hands' first to assure they are both present. The PC sets DTR to '1', and the data set answers with DSR '1'. Modems often indicate hang-up by resetting DSR to '0'. (These six lines plus GND are often referred to as '7 wire'-connection or 'hand shake'-connection.) DCD: The modem uses this line to indicate that it has detected the carrier of the modem on the other side of the line. RI: The modem uses this line to signal that 'the phone rings' (even if there isn't a bell fitted to your modem). SCTE, SCT, SCR: forget about these. Protective ground: This line is connected to the power ground of the serial adapter. It should not be used as a signal ground, and it MUST NOT be connected to GND (even if your DMM shows up a connection!). Connect this line to the screen of the lead (if there is one). Technical data (typical): Signal level: -10.5v/+11v Short circuit current: 6.8ma (yes, that's enough for your mouse!) Output impedance: 2 kiloohms Input impedance: 4.3 kiloohms Connecting devices ------------------ Normally, a 7 wire connection is used. Connect: GND1 to GND2 RxD1 to TxD2 TxD1 to RxD2 DTR1 to DSR2 DSR1 to DTR2 RTS1 to CTS2 CTS1 to RTS2 If a modem is connected, add lines for the following: RI, DCD If software wants it, connect DCD1 to CTS1 and DCD2 to CTS2. BEWARE! While PCs use pin 2 for RxD and pin 3 for TxD, modems normally have those pins reversed! This allows to easily connect pin1 to pin1, pin2 to pin 2 etc. If you connect two PCs, cross RxD and TxD. If hardware handshaking is not needed, a so-called null-modem connection can be used. Connect: GND1 to GND2 RxD1 to TxD2 TxD1 to RxD2 Additionally, connect (if software needs it): RTS1 to CTS1 & DCD1 RTS2 to CTS2 & DCD2 DTR1 to DSR1 DTR2 to DSR2 You won't need long wires for these! The null-modem connection is used to establish an XON/XOFF-transmission between two PCs (see software section for details). Remember: the names DTR, DSR, CTS & RTS refer to the lines as seen from the PC. This means that for your data set DTR & RTS are incoming signals and DSR & CTS are outputs! Base addresses & interrupts --------------------------- Normally, the following list is correct: Port Base address Int # COM1 0x3F8 0xC COM2 0x2F8 0xB COM3 0x3E8 0xC COM4 0x2E8 0xB Internal realisation -------------------- This is for completion only. In PCs, serial communication is realized with a set of three chips (there are no further components needed!): a UART (Universal Asynchroneous Receiver/Transmitter) and two line drivers. Normally, the 82450/16450 does the 'brain work' while the 1488 and 1489 drive the lines. Only one supply voltage (+5v) is needed; the 1488/1489 generate their voltages internally. The chips are produced by many manufacturers; it's of no importance which letters are printed in front of the numbers (mostly NS for National Semiconductor). Don't regard the letters behind the number also; they just indicate special features and packaging (Advanced, Fifo, New, MILitary etc.). You might have heard of the possibility to replace the 16450 by a 16550 to improve reliability and software throughput. This is only useful if your software is able to use the fifo (first in-first out) buffer features. The chips are fully pin-compatible except for two pins that are not used by any board known to the author: pin 24 (CSOUT, chip select out) and pin 29 (NC, no connection to be made). With the 16550, pin 24 is -TXRDY and pin 29 is -RXRDY, signals that aren't needed and that even won't care if they are shorted to +5v or ground. Therefore it should always be possible to simply replace the 16450 by the 16550 - even if it's not always useful due to lacking software capabilities. IT IS DEFINITELY NOT NECESSARY FOR COMMUNICATION UP TO LOUSY 9600 BAUD! These rates can easily be handled by any CPU and the interrupt-driven communication won't slow down the computer substantially. But if you want to use high-speed transfer without using the interrupt features (i.e. by 'polling'), it is recommended to use the 16550 in order to make transmission more reliable. Some hardware information taken from the data sheet of National Semiconductor (shortened and commented): NOTE: All line signals (RxD, TxD, RTS, CTS, DCD, DTR, DSR, RI) are inverted on their way to the port! Pin description of the 16450(16550) [Dual-In-Line package]: +-----+ +-----+ D0 -| 1 +-+ 40|- VCC D1 -| 2 39|- -RI D2 -| 3 38|- -DCD D3 -| 4 37|- -DSR D4 -| 5 36|- -CTS D5 -| 6 35|- MR D6 -| 7 34|- -OUT1 D7 -| 8 33|- -DTR RCLK -| 9 32|- -RTS SIN -| 10 31|- -OUT2 SOUT -| 11 30|- INTR CS0 -| 12 29|- NC (-RXRDY) CS1 -| 13 28|- A0 -CS2 -| 14 27|- A1 -BAUDOUT -| 15 26|- A2 XIN -| 16 25|- -ADS XOUT -| 17 24|- CSOUT (-TXRDY) -WR -| 18 23|- DDIS WR -| 19 22|- RD VSS -| 20 21|- -RD +-------------+ A0, A1, A2, Register Select, Pins 26-28: Address signals connected to these 3 inputs select a UART register for the CPU to read from or to write to during data transfer. A table of registers and their addresses is shown below. Note that the state of the Divisor Latch Access Bit (DLAB), which is the most significant bit of the Line Control Register, affects the selection of certain UART registers. The DLAB must be set high by the system software to access the Baud Generator Divisor Latches. DLAB A2 A1 A0 Register 0 0 0 0 Receive Buffer (read) Transmitter Holding Reg. (write) 0 0 0 1 Interrupt Enable x 0 1 0 Interrupt Identification (read) x 0 1 0 FIFO Control (write) (undefined on the 16450. CB) x 0 1 1 Line Control x 1 0 0 Modem Control x 1 0 1 Line Status x 1 1 0 Modem Status x 1 1 1 Scratch (special use on some boards. CB) 1 0 0 0 Divisor Latch (LSB) 1 0 0 1 Divisor Latch (MSB) -ADS, Address Strobe, Pin 25: The positive edge of an active Address Strobe (-ADS) signal latches the Register Select (A0, A1, A2) and Chip Select (CS0, CS1, -CS2) signals. Note: An active -ADS input is required when Register Select and Chip Select signals are not stable for the duration of a read or write operation. If not required, tie the -ADS input permanently low. (As it is done in your PC. CB) -BAUDOUT, Baud Out, Pin 15: This is the 16 x clock signal from the transmitter section of the UART. The clock rate is equal to the main reference oscillator frequency divided by the specified divisor in the Baud Generator Divisor Latches. The -BAUDOUT may also be used for the receiver section by tying this output to the RCLK input of the chip. (Yep, that's true for your PC. CB). CS0, CS1, -CS2, Chip Select, Pins 12-14: When CS0 and CS1 are high and CS2 is low, the chip is selected. This enables communication between the UART and the CPU. -CTS, Clear To Send, Pin 36: When low, this indicates that the modem or data set is ready to exchange data. This signal can be tested by reading bit 4 of the MSR. Bit 4 is the complement of this signal, and Bit 0 is '1' if -CTS has changed state since the previous reading (bit0=1 generates an interrupt if the modem status interrupt has been enabled). D0-D7, Data Bus, Pins 1-8: Connected to the data bus of the CPU. -DCD, Data Carrier Detect, Pin 38: blah blah blah, can be tested by reading bit 7 / bit 3 of the MSR. Same text as -CTS. DDIS, Driver Disable, Pin 23: This goes low whenever the CPU is reading data from the UART. -DSR, Data Set Ready, Pin 37: blah, blah, blah, bit 5 / bit 1 of MSR. -DTR, Data Terminal Ready, Pin 33: can be set active low by programming bit 0 of the MCR '1'. Loop mode operation holds this signal in its inactive state. INTR, Interrupt, Pin 30: goes high when an interrupt is requested by the UART. Reset low by the MR. MR, Master Reset, Pin 35: Schmitt Trigger input, resets internal registers to their initial values (see below). -OUT1, Out 1, Pin 34: user-designated output, can be set low by programming bit 2 of the MCR '1' and vice versa. Loop mode operation holds this signal inactive high. -OUT2, Out 2, Pin 31: blah blah blah, bit 3. (Used in your PC to connect the UART to the interrupt line of the slot when '1'. CB) RCLK, Receiver Clock, Pin 9: This input is the 16 x baud rate clock for the receiver section of the chip. (Normally connected to -BAUDOUT, as in your PC. CB) RD, -RD, Read, Pins 22 and 21: When Rd is high *or* -RD is low while the chip is selected, the CPU can read data from the UART. (One of these is normally tied. CB) -RI, Ring Indicator, Pin 39: blah blah blah, Bit 6 / Bit 2 of the MSR. (Bit 2 indicates only change from active low to inactive high! Curious, isn't it? CB) -RTS, Request To Send, Pin 32: blah blah blah, see DTR (Bit 1). SIN, Serial Input, Pin 10. SOUT, Serial Output, Pin 11: ... Set to 'mark' (high) upon MR. -RXRDY, -TYRDY: refer to NS data sheet. Those pins are used for DMA channeling. Since they are not connected in your PC, I won't describe them here. VCC, Pin 40, +5v VSS, Pin 20, GND WR, -WR: same as Rd, -RD for writing data. XIN, XOUT, Pins 16 and 17: Connect a crystal here (1.5k betw. xtal & pin 17) and pin 16 with a capacitor of approx. 20p to GND and other xtal conn. 40p to GND. Resistor of approx. 1meg parallel to xtal. Or use pin 16 as an input and pin 17 as an output for an external clock signal. Absolute Maximum Ratings: Temperature under bias: 0 C to +70 C Storage Temperature: -65 C to 150 C All input or output voltages with respect to VSS: -0.5v to +7.0v Power dissipation: 1W Further electrical characteristics see the very good data sheet of NS. UART Reset Configuration Register/Signal Reset Control Reset State -------------------------------------------------------------------- IER MR 0000 0000 IIR MR 0000 0001 FCR MR 0000 0000 LCR MR 0000 0000 MCR MR 0000 0000 LSR MR 0110 0000 MSR MR xxxx 0000 (according to signals) SOUT MR high INTR (RCVR errs) Read LSR/MR low INTR (data ready) Read RBR/MR low INTR (THRE) Rd IIR/Wr THR/MR low INTR (modem status) Read MSR/MR low -OUT2 MR high -RTS MR high -DTR MR high -OUt1 MR high RCVR FIFO MR/FCR1^FCR0/DFCR0 all bits low XMIT FIFO MR/FCR1^FCR0/DFCR0 all bits low Software ======== First some information from the data sheet. Then: HOW TO PROGRAM. Register Description -------------------- See "Hardware" for addresses. Register Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 RBR (r/o) ----------------------- data bits received ------------------------ THR (w/o) ------------------ data bits to be transmitted -------------------- IER ERBFI ETBEI ELSI EDSSI 0 0 0 0 IIR (r/o) pending IID0 IID1 IID2 0 0 FIFO en FIFOen FCR (w/o) enable RFres XFres DMAsel 0 0 - RX trigger - LCR - word length - stopbits PAR en even sel stick par SBR DLAB MCR DTR RTS OUT1 OUT2 Loop 0 0 0 LSR RBF OE PE FE Break THRE TEMT FIFOerr MSR DCTS DDSR TERI DDCD CTS DSR RI DCD ERBFI: Enable Receiver Buffer Full Interrupt ETBEI: Enable Transmitter Buffer Empty Interrupt ELSI: Enable Line Status Interrupt EDSSI: Enable Delta Status Signals Interrupt IID?: Interrupt IDentification RFres: Receiver FIFO reset XFres: Transmitter FIFO reset SBR: Set BReak RBF: Receiver Buffer Full (Data Available) OE: Overrun Error PE: Parity Error FE: Framing Error THRE: Transmitter Holding Register Empty (new data can be written to THR) TEMT: Last word has been sent DCTS: Delta Clear To Send DDSR: Delta Data Set Ready TERI: Trailing Edge Ring Indicator DDCD: Delta Data Carrier Detect LCR (Line Control Register): Bit 1 Bit 0 word length Bit 2 Stop bits 0 0 5 bits 0 1 0 1 6 bits 1 1.5/2 1 0 7 bits (1.5 if word length is 5) 1 1 8 bits Bit 5 Bit 4 Bit 3 Parity type Bit 6 SOUT condition x x 0 no parity 0 normal operation 0 0 1 odd parity 1 force 'space' 0 1 1 even parity Bit 7 DLAB 1 0 1 mark parity 0 normal registers 1 1 1 space parity 1 divisor at reg 0, 1 Baud Rate Generator: DLAB must be set. Write word (16 bits) to address 0 of the UART to program baud rate as follows: xtal frequency in Hz / 16 / rate = divisor Your PC uses an xtal frequency of 1.8432 MHz. Do *NOT* use 0! An Error of up to 5 percent is irrelevant. Some values: Baud rate Divisor (hex) Percent Error 50 900 0.0% 75 600 0.0% 110 417 0.026% 134.5 359 0.058% 150 300 0.0% 300 180 0.0% 600 C0 0.0% 1200 60 0.0% 1800 40 0.0% 2000 3A 0.69% 2400 30 0.0% 3600 20 0.0% 4800 18 0.0% 7200 10 0.0% 9600 C 0.0% 19200 6 0.0% 38400 3 0.0% 56000 2 2.86% 115200 1 0.0% LSR (Line Status Register): Bit 0 Data Ready (DR). Reset by reading RBR. Bit 1 Overrun Error (OE). Reset by reading LSR. Indicates loss of data. Bit 2 Parity Error (PE). Indicates transmission error. Reset by LSR. Bit 3 Framing Error (FE). Indicates missing stop bit. Reset by LSR. Bit 4 Break Indicator (BI). Set if 'space' for more than 1 word. Reset by LSR. Bit 5 Transmitter Holding Register Empty (THRE). Indicates that a new word can be written to THR. Reset by writing THR. Bit 6 Transmitter Empty (TEMT). Indicates that no transmission is running. Reset by reading LSR. Bit 7 Set if at least 1 word in FIFO has been received with an error. Cleared by reading LSR if there is no further error in the FIFO. FCR (FIFO Control Register): Bit 0: FIFO enable. Bit 1: Clear receiver FIFO. This bit is self-clearing. Bit 2: Clear transmitter FIFO. This bit is self-clearing. Bit 3: DMA mode (pins -RXRDY and -TXRDY), see sheet Bits 6-7:Trigger level of the DR-interrupt. Bit 7 Bit 6 Receiver FIFO trigger level 0 0 01 0 1 04 1 0 08 1 1 14 IIR (Interrupt Identification Register): Bit 3 Bit 2 Bit 1 Bit 0 Priority Source Description 0 0 0 1 none 0 1 1 0 highest Status OE, PE, FE or BI of the LSR set. Serviced by reading the LSR. 0 1 0 0 second Receiver DR or trigger level rea- ched. Serviced by read- ing RBR 'til under level 1 1 0 0 second FIFO No Receiver FIFO action since 4 words' time (neither in nor out) but data in FIFO. Serviced by reading RBR. 0 0 1 0 third Transm. THRE. Serviced by read- ing IIR (if source of int only!!) or writing to THR. 0 0 0 0 fourth Modem One of the delta flags in the MSR set. Serviced by reading MSR. Bit 6 & 7: set if FCR bit 0 set. In most software applications bits 3, 6 & 7 should be masked before servicing since they are not relevant. NOTE! Even if some of these interrupts are masked, the service routine can be confronted with *all* states shown above when the IIR is loop-polled until bit 0 is set. IER (Interrupt Enable Register): Bit 0: If set, DR interrupt is enabled. Bit 1: If set, THRE interrupt is enabled. Bit 2: If set, Status interrupt is enabled. Bit 3: If set, Modem status interrupt is enabled. MCR (Modem Control Register): Bit 0: Programs -DTR. If set, -DTR is low and the DTR pin of the port is '1'. Bit 1: Programs -RTS. Bit 2: Programs -OUT1. Not used in a PC. Bit 3: Programs -OUT2. If set, interrupts generated by the UART are trans- ferred to the ICU (Interrupt Control Unit). Bit 4: '1': local loopback. All outputs disables. MSR (Modem Status Register): Bit 0: Delta CTS. Set if CTS has changed state since last reading. Bit 1: Delta DSR. Set if DSR has changed state since last reading. Bit 2: TERI. Set if -RI has changed from low to high (i.e. RI at port has changed from '1' to '0'). Bit 3: Delta DCD. Set if DCD has changed state since last reading. Bit 4: CTS. 1 if '1' at port. Bit 5: DSR. Bit 6: RI. If loopback is selected, it is equivalent to OUT1. Bit 7: DCD. Programming ----------- Now for the clickety-clickety thing. I hope you're a bit keen in assembler programming (if not, you've got a problem B-). Programming the UART in high level languages is, of course, possible, but not at very high rates or interrupt-driven. I give you several routines in assembler (and, wherever possible, in C) that do the dirty work for you. First, the non-interrupt version. Let's assume the following constants are set correctly (either by 'CONSTANT EQU value' or by '#define CONSTANT value'). UART_BASEADDR the base address of the UART UART_BAUDRATE the divisor value (e.g. 12 for 9600 baud) UART_LCRVAL the value to be written to the LCR (e.g. 0x1b for 8N1) UART_FCRVAL the value to be written to the FCR. Bit 0, 1 and 2 set, bits 6 & 7 according to wished trigger level (see above) First thing to do is initializing the UART. This works as follows: init_uart proc near push ax ; we are 'clean guys' push dx mov dx,UART_BASEADDR+3 ; LCR mov al,80h ; set DLAB out dx,al mov dx,UART_BASEADDR ; divisor mov ax,UART_BAUDRATE out dx,ax mov dx,UART_BASEADDR+3 ; LCR mov al,UART_LCRVAL ; params out dx,al mov dx,UART_BASEADDR+4 ; MCR xor ax,ax ; clear loopback out dx,al ;*** pop dx pop ax ret init_uart endp void init_uart() { outp(UART_BASEADDR+3,0x80); outpw(UART_BASEADDR,UART_BAUDRATE); outp(UART_BASEADDR+3,UART_LCRVAL); outp(UART_BASEADDR+4,0); //*** } If we wanted to use the FIFO functions of the 16550, we'd have to add some lines to the routines above (where the ***s are). In assembler: mov dx,UART_BASEADDR+2 ; FCR mov al,UART_FCRVAL out dx,al And in C: outp(UART_BASEADDR+2,UART_FCRVAL); Not very complex, isn't it? Well, I told you so at the very beginning, and we want to start easy. Now let's send a character. uart_send proc near ; character to be sent in AL push dx push ax mov dx,UART_BASEADDR+5 us_wait: in al,dx ; wait until we are allowed to write a byte to the THR test al,20h jz us_wait pop ax mov dx,UART_BASEADDR out dx,al ; then write the byte pop ax pop dx ret uart_send endp void uart_send(char character) { while ((inp(UART_BASEADDR+5)&0x20)!=0) {;} outp(UART_BASEADDR,(int)character); } This one sends a null-terminated string. uart_send_string proc near ; DS:SI contains a pointer to the string to be sent. push si push ax push dx cld ; we want to read the string in its correct order uss_loop: lodsb or al,al ; last character sent? jz uss_ende ;*1* mov dx,UART_BASEADDR+5 push ax uss_wait: in al,dx test al,20h jz uss_wait mov dx,UART_BASEADDR pop ax out dx,al ;*2* jmp uss_loop uss_ende: pop dx pop ax pop si ret uart_send_string endp void uart_send_string(char *string) { int i; for (i=0; string[i]!=0; i++) { //*1* while ((inp(UART_BASEADDR+5)&0x20)!=0) {;} outp(UART_BASEADDR,(int)string[i]); //*2* } } Of course, we could have used our already programmed function/procedure uart_send instead of the piece of code limited by *1* and *2*, but we are interested in high-speed code. It shouldn't be a hard nut for you to modify the above function/procedure so that it sends a block of data rather than a null-terminated string. I'll omit that here. Now for reception. We want to program routines that do the following: - check if a character received or an error occured - read a character if there's one available Both the C and the assembler routines return 0 (in AX) if there is neither an error condition nor a character available. If a character is available, Bit 8 is set and AL or the lower byte of the return value contains the character. Bit 9 is set if we lost data (overrun), bit 10 signals a parity error, bit 11 signals a framing error, bit 12 shows if there is a break in the data stream and bit 15 signals if there are any errors in the FIFO (if we turned it on). The procedure/function is much smaller than this paragraph: uart_get_char proc near push dx mov dx,UART_BASEADDR+5 in al,dx mov ah,al and ah,9fh test al,1 jz ugc_nochar mov dx,UART_BASEADDR in al,dx ugc_nochar: pop dx ret uart_get_char endp unsigned uart_get_char() { unsigned x; x=(inp(UART_BASEADDR+5)<<8)&0x9f; if (x&0x100) x|=((unsigned)inp(UART_BASEADDR))&0xff); return x; } This procedure/function lets us easily keep track of what's happening with the RxD pin. It does not provide any information on the modem status lines! We'll program that later on. If we wanted to show what's happening with th RxD pin, we'd just have to write a routine like the following (I use a macro in the assembler version to shorten the code): DOS_print macro pointer ; prints a string in the code segment push ax push ds push cs pop ds mov dx,pointer mov ah,9 int 21h pop ds pop ax endm uart_watch_rxd proc near uwr_loop: ; check if keyboard hit; we want a possibility to break the loop mov ah,1 int 16h jnz uwr_exit call uart_get_char or ax,ax jz uwr_loop test ah,1 ; is there a character in AL? jz uwr_nodata push ax ; yes, print it mov dl,al mov ah,2 int 21h pop ax uwr_nodata: test ah,0eh ; any error at all? jz uwr_loop ; this speeds up things test ah,2 ; overrun error? jz uwr_noover DOS_print overrun_text uwr_noover: test ah,4 ; parity error? jz uwr_nopar DOS_print parity_text uwr_nopar: test ah,8 ; framing error? jz uwr_loop DOS_print framing_text jmp uwr_loop overrun_text db "*** Overrun Error ***$" parity_text db "*** Parity Error ***$" framing_text db "*** Framing Error ***$" uart_watch_rxd endp void uart_watch_rxd() { union _useful_ { unsigned val; char character; } x; while (!kbhit()) { x.val=uart_get_char(); if (!x.val) continue; // nothing? Continue if (x.val&0x100) putc(x.character); // character? Print it if (!(x.val&0x0e00)) continue; // any error condidion? No, continue if (x.val&0x200) printf("*** Overrun Error ***"); if (x.val&0x400) printf("*** Parity Error ***"); if (x.val&0x800) printf("*** Framing Error ***"); } } If you call these routines from a function/procedure as shown below, you've got a small terminal program! terminal proc near ter_loop: call uart_watch_rxd ; watch line until a key is pressed xor ax,ax ; get that key from the buffer int 16h cmp al,27 ; is it ESC? jz ter_end ; yes, then end this function call uart_send ; send the character typed if it's not ESC jmp ter_loop ; don't forget to check if data comes in ter_end: ret terminal endp void terminal() { int key; while (1) { uart_watch_rxd(); key=getche(); if (key==27) break; uart_send((char)key); } } These, of course, should be called from an embedding routine like the following (the assembler routines concatenated will assemble as an .EXE file. Put the lines 'code segment' and 'assume ss:stack' to the front). main proc near call uart_init call terminal mov ax,4c00h int 21h main endp code ends stack segment stack 'stack' dw 128 dup (?) stack ends end main void main() { uart_init(); terminal(); } Here we are. Now you've got everything you need to program null-modem polling UART software. You know the way. Now go and add functions to check if a data set is there, then establish a connection. Don't know how? Set DTR, wait for DSR. If you want to send, set RTS and wait for CTS before you actually transmit data. You don't need to store old values of the MCR: this register is readable. Just read in the data, AND/OR the bit required and write the byte back. Now for the interrupt-driven version of the program. This is going to be a bit voluminous, so I draw the scene and leave the painting to you. If you want to implement interrupt-driven routines in a C program use either the inline-assembler feature or link the objects together. First thing to do is initialize the UART the same way as shown above. But there is some more work to be done before you enable the UART interrupt: FIRST SET THE INTERRUPT VECTOR CORRECTLY! Use Function 0x25 of the DOS interrupt 0x21. UART_INT EQU 0Ch ; for COM2 / COM4 use 0bh UART_ONMASK EQU 11101111b ; for COM2 / COM4 use 11110111b UART_OFFMASK EQU 00010000b ; for COM2 / COM4 use 00001000b UART_IERVAL EQU ? ; replace ? by any value between 0h and 0fh ; (dependent on which ints you want) ; DON'T SET bit 1 yet! initialize_UART_interrupt proc near push ds push cs ; build a pointer in DS:DX pop ds lea dx,interrupt_service_routine mov ax,2500h+UART_INT int 21h pop ds mov dx,UART_BASEADDR+4 ; MCR in al,dx or al,8 ; set OUT2 bit to enable interrupts out dx,al mov dx,UART_BASEADDR+1 ; IER mov al,UART_IERVAL out dx,al in al,21h ; last thing to do is unmask the int in the ICU and al,UART_ONMASK out 21h,al sti ; and free interrupts if they have been disabled ret initialize_UART_interrupt endp Now the interrupt service routine. It has to follow several rules: first, it MUST NOT change the contents of any register of the CPU! Then it has to tell the ICU (did I tell you that this is the interrupt control unit?) that the interrupt is being serviced. Next thing is test which part of the UART needs service. Let's have a look at the following procedure: interupt_service_routine proc far ; define as near if you want to link .COM ;*1* push ax push cx push dx push bx push sp push bp push si push di ;*2* replace the part between *1* and *2* by pusha on an 80186+ system push ds push es mov al,20h ; remember: first thing to do in interrupt routines is tell out 20h,al ; the ICU about it int_loop: mov dx,UART_BASEADDR+2 ; IIR xor ax,ax ; clear AH; this is the fastest and shortest possibility in al,dx ; check IIR info test al,1 jnz int_end and al,6 ; we're interested in bit 1 & 2 (see data sheet info) mov si,ax ; this is already an index! Well-devised, huh? call word ptr cs:int_servicetab[si] ; ensure a near call is used... jmp int_loop int_end: pop es pop ds ;*3* pop di pop si pop bp pop sp pop bx pop dx pop cx pop ax ;*4* *3* - *4* can be replaced by popa on an 80186+ based system iret interupt_service_routine endp This is the part of the service routine that does the decisions. Now we need four different service routines to cover all four interrupt source possibilities (EVEN IF WE DIDN'T ENABLE THEM!! Since 'unexpected' interrupts can have higher priority than 'expected' ones, they can appear if an expected [not masked] interrupt situation shows up). int_servicetab DW int_modem, int_tx, int_rx, int_status int_modem proc near mov dx,UART_BASE+6 ; MSR in al,dx ; do with the info what you like; probably just ignore it... ; but YOU MUST READ THE MSR or you'll lock up the system! ret int_modem endp int_tx proc near ; get next byte of data from a buffer or something ; (remember to set the segment registers correctly!) ; and write it to the THR (offset 0) ; if no more data is to be sent, disable the THRE interrupt ; end of data to be sent? ; no, jump to end_int_tx mov dx,UART_BASEADDR+1 in al,dx and al,00001101b out dx,al end_int_tx: ret int_tx endp int_rx proc near mov dx,UART_BASEADDR in al,dx ; do with the character what you like (best write it to a ; FIFO buffer) ret int_rx endp int_status proc near mov dx,UART_BASEADDR+5 in al,dx ; do what you like. It's just important to read the LSR ret int_status endp How is data sent now? Write it to a fifo buffer that is read by the interrupt routine. Then set bit 1 of the IER and check if this has already started transmission. If not, you'll have to start it by yourself... THIS IS DUE TO THOSE NUTTY GUYS AT BIG BLUE WHO DECIDED TO USE EDGE TRIGGERED INTERRUPTS INSTEAD OF PROVIDING ONE SINGLE FLIP FLOP FOR THE 8253/8254! This procedure can be a C function, too. It is not time-critical at all. ; copy data to buffer mov dx,UART_BASEADDR+1 ; IER in al,dx or al,2 ; set bit 1 out dx,al mov dx,UART_BASEADDR+5 ; LSR in al,dx test al,40h ; is there a transmission running? jz dont_crank ; yes, so don't mess it up call int_tx ; no, crank it up dont_crank: Well, that's it! Your main program has to take care about the buffers, nothing else! One more thing: always remember that at 115,200 baud there is service to be done at least every 8 microseconds! On an XT with 4.77 MHz this is about 5 assembler commands!! So forget about servicing the serial port at this rate in an interrupt-driven manner on such computers. An AT with 12 MHz probably will manage it if you use 'macro commands' such as pusha. An AT can perform about 20 instructions between two characters, a 386 with 25 MHz will do about 55, and a 486 with 33 MHz will manage about 145. Using a 16550 will calm down the situation a bit, but 16 bytes FIFO isn't that much... The interrupt service routines can be accelerated by not pushing that much registers, and pusha and popa are fast instructions. Another last thing: due to the poor construction of the PC interrupt system, one interrupt line can only be driven by one device. This means if you want to use COM3 and your mouse is connected to COM1, you can't use interrupt features without disabling the mouse (write 0x0 to the mouse's MCR). Well, that's the end of my short summary. Don't hesitate to correct me if I'm wrong (preferably via email) in the details (I hope not, but it's not easy to find typographical and other errors in a text that you've written yourself). And please help to complete this list! If you've got anything to add, email it to me or post it as an additional FAQ answer. Maybe anybody can bring himself to write a summary on modem communication? Please tell me what you think about it! Is it worth while to do the work? Is anybody interested in it? Shall I carry on, complete and re-post the list? Please give me feedback! Yours Chris -- --------------------------------------------------------------------- Christian Blum Universitaet des Saarlandes, Germany Friedrich-Ebert-Str. 50 chbl@stud.uni-sb.de W-6685 Heiligenwald chbl@sbustd.rz.uni-sb.de Germany Tel (+49) (0) 6821 67476

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