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- -----------------------------------------------------------------------------
- 5th July 1994
- -----------------------------------------------------------------------------
- Support Group Application Note
- Number: 234
- Issue: 3.00
- Author: DW
- -----------------------------------------------------------------------------
-
- Peripheral Interfacing via the Serial Port
-
- -----------------------------------------------------------------------------
- This document describes the hardware configurations of the serial port over
- the history of Acorn 32 bit computers, and details some of the software
- protocols needed to drive them.
- -----------------------------------------------------------------------------
- Applicable Hardware: All RISC OS and RISC iX computers.
-
- Related Application Notes: None.
-
- -----------------------------------------------------------------------------
- Copyright (C) 1994 Acorn Computers Limited
-
- Every effort has been made to ensure that the information in this leaflet is
- true and correct at the time of printing. However, the products described in
- this leaflet are subject to continuous development and improvements and
- Acorn Computers Limited reserves the right to change its specifications at
- any time. Acorn Computers Limited cannot accept liability for any loss or
- damage arising from the use of any information or particulars in this
- leaflet. ACORN, ECONET and ARCHIMEDES are trademarks of Acorn Computers
- Limited.
- -----------------------------------------------------------------------------
- Support Group
- Acorn Computers Limited
- Acorn House
- Vision Park
- Histon
- Cambridge
- CB4 4AE
- -----------------------------------------------------------------------------
- Table of Contents
-
- Introduction 3
-
- The Hardware Layer 3
-
- Communication via the Serial Port 3
-
- A Note on Baud Rates 5
-
- Communication Protocols and Flow Control 5
-
- Communication with BBC Model B and Master Series Computers 6
-
- Telecommunication Standards and Data Compression 7
-
- The Programmer's Interface to the Serial Port 9
-
- Connecting Printers to the Serial Port 9
-
- Troubleshooting 9
-
- Appendix A: Cable Configurations 11
-
- Appendix B: Useful Contacts 13
-
- Appendix C: RS232 Signals 14
-
- Glossary 15
-
-
- Throughout this Application Note, terms are used which may be unfamiliar to
- the reader. Signals are defined in Appendix C, and a Glossary has been
- provided which aims to supply sufficient background information.
-
- Introduction
- ------------
-
- A serial port allows data to be transferred between two
- computers, or a computer and peripheral device, as a stream of data bits
- sent one after the other down a single wire. This is fundamentally different
- from parallel communications (commonly used with printers), which transfers
- data many bits at a time over a number of wires. Serial communication can be
- synchronous or asynchronous.
-
- As the practice of serial communication between devices dates from the very
- early history of computing, many different standards have sprung up over the
- years; today, as a result of this, there is no de-facto software
- communication protocol and no de-facto hardware connector. However, there is
- a set of standards which most manufacturers tend to agree on; these are the
- RS232 physical connector specification and the RS423 signal level electrical
- specification.
-
- The Hardware Layer
- ------------------
-
- The full RS232 standard connector has 25 pins, usually
- built into a D-type connector. Most modern computers and peripherals do not
- implement the full RS232 standard, but employ the signal specification from
- the RS423 standard, again with a D connector. The Acorn serial port is
- conformant to the RS423 serial port electrical standard.
-
- There are two main classifications of RS232 cable; straight-through and
- crossover. Broadly speaking, if the cable is linking the computer to a
- communications device, such as a modem, then a straight-through cable is
- used - in other words, one where the transmit line is connected to the
- same-numbered pins at either end of the cable, as is the receive line, etc.
- A modem is a piece of Data Communication Equipment (DCE); hence the
- straight-through cable is sometimes referred to as "DTE to DCE."
-
- If the cable is connecting the computer to a peripheral such as a printer,
- then the transmit line from the computer needs to be connected to the
- peripheral's receive line and vice versa; the two lines have to be crossed
- over. A printer, like the computer itself, is a piece of Data Terminal
- Equipment (the terminology goes back to the concept of a large computer
- which had a number of user-interface terminals connected to it by serial
- lines), and so the cross-over cable is sometimes referred to as "DTE to DTE"
- or a "null modem" cable.
-
- Consult the manufacturer of your peripheral to determine which cable type is
- required.
-
- Communication via the Serial Port
- ---------------------------------
- Data is transferred down a serial line in
- packets, which comprise a data "word" wrapped up with a "parity" bit and
- one or more "stop" bits. The waveform looks rather like this:
-
- Insert Figure 1: A Typical Serial Data Stream
-
- where the "on" and "off" voltages fall into this range:
-
- Insert Figure 2: Serial Voltage Ranges
-
- The format of the packet (number of data bits, type of parity etc) and the
- serial baud rate can be set up using the *CONFIGURE BAUD n and *CONFIGURE
- DATA n commands from the command line; the following table details the
- settings. Consult the documentation supplied with your peripheral to
- determine which settings are appropriate.
-
- Insert Figure 3: Baud Rate and Word Format Configuration Settings
-
- eg *CONFIGURE BAUD 7 would make the system default to 9600 baud
- communications.
-
- Note that baud rates above 19200 are only available on the Risc PC. For high
- baud rates, the transmit and receive rates must be set to the same value; at
- low baud rates (1200 and below), a system timer can be used for the
- "receive" rate, hence a different receive rate can be set using *FX7, eg
- *FX7,3 would set a receive rate of 300 baud.
-
- A Note on Baud Rates
- --------------------
- A common misconception in serial communications
- concerns the definition of "baud" as the number of bits transmitted per
- second (bps), particularly when used in reference to modems.
-
- The baud rate is the number of changes in signal state per second. The
- Public Switched Telephone Network (PSTN) bandwidth will not support a "true"
- baud rate greater than 600 under any circumstances; however, each change of
- signal state may represent a 1, 2, 4, 8 or 16 bit sequence, depending on the
- coding method in use. Thus the term "baud" is incorrectly used if the
- transmission speed is classed as greater than 600 baud; however, at
- transmission rates of 300 and 600 bits per second including one start, one
- parity and two stop bits, a 1:1 mapping of bits to baud is achieved.
-
- The data rate, however, is (bits per second)*(word length)/(total packet
- size in bits), which usually amounts to 300*8/11 or 600*8/11 "useful" bps.
-
- Fax transmission rates of 9600 bps require Quadrature Amplitude Modulation,
- in which each combination of phase angle and amplitude represents one of 16
- 4-bit patterns; the baud rate is therefore 9600/16=600 baud, which is still
- just within the PSTN bandwidth. At higher transmission speeds, other coding
- methods allow each line state to represent an 8, 16 or 32 bit pattern.
-
- In short, the "old" definition of the baud (ie as the reciprocal of the
- duration of the shortest signalling element) was meaningful for Morse code,
- in which a dot was the shortest-duration element and a dash had a duration
- three times as long. It no longer has any real meaning in digital
- transmission, as all line states last for an equal time.
-
- Communication Protocols and Flow Control
- ----------------------------------------
- Owing to the nature of serial
- communications, it is necessary to have a signalling system by which the
- peripheral can inform the host that it is present, switched on, and ready to
- receive data or has data to transmit, and vice versa. The last two items in
- this list comprise flow control.
-
- Unlike IBM PC compatibles, Acorn machines default to using the DSR line to
- flag readiness to transmit, and require the presence of the DCD signal. IBM
- PC compatibles use CTS instead of DSR, and do not necessarily use DCD. On
- machines fitted with the 82710 or 82711 serial controllers (these machines
- being the A3010, A3020, A4000, A5000, A4 and Risc PC), it is possible to
- change which signals are used for flagging in software, using the
- SWI"OS_SerialOp",0 command. Thus an IBM compatible cable may be used; the
- option to reprogram the serial port in this manner is currently provided by
- some serial communications software. If you have a suitable machine and
- prefer to use an over-the-counter IBM standard cable rather than resort to
- making your own, first check with the supplier of your communications
- software that there is an option to reprogram the serial controller.
-
- If you have an older Acorn machine (Archimedes 300 series, 440, 400/1
- series, 540, R140 or R200 series), you will need to have a cable wired to a
- specification based on the figures in Appendix A of this document.
-
- The need for flow control and "handshaking," in which the receiving device
- verifies the parity bits included in words and can request the re-sending of
- a corrupted word, has led to the development of a number of serial
- communications protocol standards. These vary in their ease of
- implementation, speed and robustness of transfer, and a number of them are
- described below.
-
- XON / XOFF
- This is the earliest software flow control system which is still
- in use, and makes use of the flow control characters &11 (XON) and &13
- (XOFF). There is no error checking implemented as standard. XON / XOFF is
- incompatible with SLIP and PPP, as they require the full 8 bits of the data
- word to send Internet frame data; ie under Internet Protocol, &11 and &13
- cannot be reserved for flow control.
-
- XModem
- This is probably the most widely available communications protocol,
- and is supported by a large number of communications packages on many types
- of host. XModem transfers files in blocks of 128 bytes. Each block has a
- checksum, or CRC (Cyclic Redundancy Checksum) added to the end, which is
- calculated by summing the values of the bytes in the packet and taking a
- pair of bits from the sum. From this checksum, the receiving system can
- determine whether the packet was corrupted during transmission, and if so,
- it can ask the sending system to retransmit that block. Although fairly
- slow in transferring data, XModem will produce reliable transfers.
-
- YModem
- Based on the same protocol set as XModem, YModem uses 1024 byte
- packets wherever possible, and a different system for packet integrity
- checking. Although YModem can fall back on smaller packets where applicable,
- there is no backward compatibility with XModem's checksum system.
-
- Kermit
- This is another packet-oriented serial communication protocol.
- Developed at the University of Columbia, the protocol standard is Public
- Domain, and hence Kermit ranks with XModem for widespread use. Although
- fairly intensive on encoding and checksumming, and hence fairly slow, Kermit
- connections are very robust.
-
- SLIP
- SLIP provides a point-to-point connection between two devices for the
- transmission of Internet datagrams; the devices can be either two computers,
- or a computer and an Internet router. SLIP modifies a standard Internet
- datagram by appending a special SLIP END character to it, which allows
- datagrams to be distinguished as separate. Normal parameters for an
- asynchronous lines apply to SLIP; SLIP requires a port configuration of 8
- data bits, no parity, and EIA or hardware flow control. SLIP does not
- provide any protection against line errors and data corruption, being
- reliant on other high-layer protocols for this. Over a particularly
- error-prone dialup link, therefore, SLIP on its own would not be
- satisfactory. A SLIP system also needs to have its IP address configuration
- set every time the SLIP is loaded and configured, as it cannot determine
- network addresses dynamically.
-
- PPP
- The Point-to-Point Protocol has a number of advantages over SLIP; it is
- designed to operate both over asynchronous connections and bit-oriented
- synchronous systems, can configure connections to a remote network
- dynamically, and test that the link is usable. Full information on PPP can
- be obtained from Internet RFC 1171 (see Glossary), and RFC 1220 describes
- how PPP can be used with remote bridging.
-
- Communication with BBC Model B and Master Series Computers
- ----------------------------------------------------------
- It is possible to
- transfer data between a 32-bit Acorn computer and an 8 bit BBC Model B /
- Master series computer via the Serial Port; the wiring diagram for the
- appropriate cable is presented as Figure 9 in Appendix A.
-
- If no suitable communications packages are available, it is possible to
- transfer data without using one of the recognised communications protocols;
- this is not recommended except in exceptional circumstances and with a very
- short cable.
-
- On the transmitting system (assumed to be the Model B / Master), issue:
-
- *FX8,4 *FX3,1
-
- The first call sets the transmission rate to 1200 baud, and the second
- selects the serial port as the output device.
-
- On the receiving machine, (assumed to be a 32 bit Acorn machine), issue:
-
- *FX156,20 *FX7,4 *FX2,1
-
- The first command configures the parity and word size appropriately: note
- that the Model B defaults to one stop bit per word, whereas the 32 bit range
- defaults to two stop bits. The second sets the receive rate to 1200 baud,
- and the third causes standard input to be via the serial port.
-
- Performing a LIST operation on a BASIC program stored in the BBC's RAM, or a
- *TYPE on a plain text file stored on its disc, will cause the program or
- text to be loaded into whichever RISC OS application has the caret on the
- receiving machine.
-
- Communication is terminated by issuing
-
- *FX3,0 *FX2,0
-
- on both machines; it is suggested that the receiving machine terminates
- communication first.
-
- Telecommunication Standards and Data Compression
- ------------------------------------------------
- The CCITT (European
- Telecommunication Standards body) has defined a number of standards for
- modem-to-modem communications over the PSTN network, and translation from
- these standards codes to communication capabilities is often a point of
- confusion. The following list covers most of the standards currently
- available.
-
- V.21 and Bell 103
- Specifies 300 bps 2-wire full duplex communications using
- FSK (Frequency Shift Keying) modulation schemes. AT&T created the Bell 103
- specification during the days of telephone system monopoly in the USA. The
- two standards, V.21 and Bell 103, differ slightly, but all of today's modems
- support both V.21 and Bell 103.
-
- V.22 and Bell 212A
- CCITT standard for 1200 bps full duplex modems. Specifies
- 1200 bps 2-wire full duplex communications using QPSK (Quadrature Phase
- Shift Keying) modulation at 600 baud. Again, these two standards differ
- slightly.
-
- V.22 bis
- CCITT standard for 2400 bps full duplex modems. Specifies 2400 bps
- 2-wire full duplex communications using a QAM (Quadrature Amplitude
- Modulation) scheme at 600 baud.
-
- V.23
- Specifies an asymmetrical communication scheme which implements 1200
- bps data transmission in one direction, and 75 baud data transmission in a
- back channel. This FKS-based standard is popularly used in Europe for
- applications which require high data rates in only one direction, eg CET 1 /
- 2 / 3 Teletext.
-
- V.24
- Specifies a serial interface; analogous to RS232.
-
- V.25 and V.25 bis
- CCITT standard for auto-dial commands for modems and other
- auto-dial devices. Not commonly used, because of the popularity of the Hayes
- command set.
-
- V.26, V.26 bis and V.26 ter
- Specifies half and full duplex leased-line
- communications at 1200 and 2400 bps. The specifications employ QPSK
- modulation at 1200 baud. V.26 ter was the first modem standard to specify
- echo cancellation.
-
- V.27, V.27 bis and V.27 ter
- Specifies 4800 bps communications requiring 2
- wires for half duplex and 4 wires for full duplex operation. The standards
- specify QAM modulation at 1600 baud. The Group 3 Fax standard references
- V.27 ter as the base requirement for 2-wire half duplex fax communications.
-
- V.29
- CCITT standard for 9600 bps half duplex modems. Specifies 9600 bps
- communications requiring 2 wires for half duplex operation, and 4 wires for
- full duplex. The standard specifies QAM modulation at 2400 baud. Group 3 Fax
- standard T.4 references V.29 as an option for fax transmissions faster than
- 4800 bps V.29 ter. A high percentage of fax transmissions rely on V.29, but
- virtually all fall back on V.29 ter.
-
- V.32
- CCITT standard for 9600 bps full duplex modems. Specifies 2-wire full
- duplex 9600 bps communications using QAM modulation at 2400 baud and echo
- cancellation. V.32 modems offer an upgrade path from V.22 bis for
- asynchronous dial-up modem applications. V.32 AUTOMODE was recently
- published as an annex to V.32, and defines an automatic fall-back capability
- which does not support 9600 bps communications.
-
- V.33
- Specifies 4-wire full duplex and 2-wire half duplex communications at
- 14400 bps. V.33 employs TCM (Trellis-Coded Modulation). The Group 3 Fax
- study group has modified T.4 to include this.
-
- V.42
- Specifies error correction techniques which can be implemented in
- modems independently of transmission speed and modulation system. The
- recommendation includes LAPM (Link Access Procedures for Modems) and MNP
- (Microcom Networking Protocol) 2 to 4 error correction. The CCITT standard
- (which is the V.42 specification) utilises MNP4 and LAPM. When a modem makes
- a connection, it tries to use LAPM; if the receiving modem does not support
- LAPM the connecting modem tries MNP4, and if the receiving modem does not
- support MNP4 the system falls back on asynchronous non-error-correcting
- communication. LAPM and MNP4 protocol querying and detection is entirely
- transparent to the user.
-
- V.42 bis and MNP5
- This specifies compression algorithms which can be
- implemented in modems independently of transmission speed and modulation
- system. V.42 bis provides a 4:1 compression ratio, using the Lempel-Ziv
- algorithm (as used in !Squash under RISC OS 3). MNP5 uses a combination of
- dynamic Huffman and run-length encoding. MNP5 is not a standard as defined
- by a specific organisation, but has become a standard in its own right for
- 2:1 data compression.
-
- The Programmer's Interface to the Serial Port
- ---------------------------------------------
- There is
- a "raw" device which corresponds to the serial port (this is serial:);
- however, writing to the serial port by directly using this device is
- deprecated. The approved programmer interface is via the SWI"OS_SerialOp"
- call (SWI &57), which is described starting on page 2-459 of the RISC OS 3
- Programmer's Reference Manual. Of particular interest is the provision for
- implementation of flow control; the serial status word, accessible via
- SWI"OS_SerialOp",0 allows XON/XOFF with CTS handshaking, use of DCD or use
- of DSR to implement signalling of intent to transmit / receive data.
-
- Unlike IBM compatibles fitted with the 82710 / 82711, Acorn computers
- default to the behaviour associated with the 6551 serial controller fitted
- in the original Archimedes range. In order to allow an IBM type lead to be
- used, bit 1 (ignore DCD) of the serial port status word must be set using
- the appropriate SYS"OS_SerialOp",0 mask. However, to produce a robust
- program capable of coping with a noisy connection, it is necessary to
- periodically reset the status word so that DCD can be checked. The type of
- cable which is connected may either be selected by a user menu option, or it
- is acceptable to check the state of DCD on program startup.
-
- Full details of this status word are listed on Page 2-462 of the RISC OS 3
- Programmer's Reference Manual.
-
- Connecting Printers to the Serial Port
- --------------------------------------
- !Printers is capable of sending data
- to a suitable printer via a serial link; the printer usually has to be
- specially configured to receive data in this manner, and it is suggested
- that you refer to your printer manual for information on how the printer's
- DIP switches must be set and which formats of serial word it will accept.
- !Printers must itself be configured according to the information supplied in
- the User Guide, and some suitable cable wiring diagrams are shown in
- Appendix A. Printers tend to be configured to the DTE standard, however you
- should check this with your printer supplier. It is also worth noting that a
- cable for this purpose usually has fewer wires to be soldered, as data is
- generally only sent in one direction; from computer to printer.
-
- Troubleshooting
- ---------------
- First of all, make sure that your serial lead and your
- communications package are compatible; if you are using an A5000 or later
- with an IBM type lead, make sure that your communications package supports
- reprogramming of the serial controller.
-
- On some modems, the modem takes control of the RI line, and this may cause
- an Acorn computer to hang up. The solution to this is to leave Pin 9 (RI) of
- the serial port disconnected if such a modem is in use.
-
- Sometimes, when handshaking signals become corrupted (this only occasionally
- happens over long, unshielded cables in areas where a lot of electrical
- equipment is operating), communications will "hang up" in a deadlocked
- state. Rather than reset the computer or the peripheral, it is often
- possible to "wake up" the peripheral by sending a Break Level. Not to be
- confused with an operation involving the Break key on the keyboard, a Break
- Level sends a 0V pulse to the RxD pin on the peripheral. A Break Level can
- be sent, if your communications package does not already implement it, by
- entering BASIC and issuing
-
- SYS"OS_SerialOp",2,<duration in centiseconds>
-
- eg SYS"OS_SerialOp",2,20 would send a "short break."
-
- A "short break" is generally about 0.2 seconds long, and a "long break" can
- be as much as 1.5 seconds. It is suggested that a "long break" only be tried
- as a last resort before a device reset. Any characters being sent when the
- break is issued may be garbled; however, if the break succeeds in waking the
- peripheral, a robust communication protocol would simply ask for the last
- data packet to be re-transmitted.
-
- On computers prior to the Risc PC running RISC OS 3.10, communications may
- be unreliable above 9600 baud, depending on the length and impedance of
- cable connected. The unreliability appears in the form of occasional "missed
- words" when receiving data.
-
- A pair of soft-loadable patch modules, SerialDev and SerialUtil, are
- available via Acorn dealers, Acorn Education Centres and bulletin boards,
- and may be downloaded via ftp over Internet from ftp.acorn.co.uk. SerialUtil
- is applicable for use under RISC OS 3.1 in conjunction with communications
- packages written under RISC OS 2, and provides enhanced arbitration of the
- claiming of the serial interrupt vector. SerialDev is a modified version of
- the serial port device driver which forms part of RISC OS 3.10, and improves
- the interrupt latency of the serial port to give improved communications
- reliability at high baud rates (eg 9600 baud). This improved driver was
- included as part of RISC OS 3.11.
-
- If you think that you may have a hardware fault with your serial port, you
- may find it useful to make up a loopback testing plug; using this plug, in
- conjunction with serial loopback test software, should pass data straight
- from the "transmit" pins to the "receive" pins.
-
- Insert Figure 4: Serial Port Loopback Test Plug
-
- Appendix A Cable Configurations
- -------------------------------
- The pinout of the Serial Port is as follows:
-
- Insert Figure 5: Acorn Serial Port Pinout
-
- 9-way serial connectors on other systems generally have the same pinout;
- however, it is suggested that you consult the appropriate documentation for
- the other system. Some suitable cable configurations for Acorn machines are
- shown below; if you make your own cables, ensure that the cable you use is
- adequately screened.
-
- Insert Figure 6: DTE to DTE (Acorn computer to Acorn Computer
-
- Insert Figure 7: DTE to DCE (Acorn computer to 25-pin Modem)
-
- Insert Figure 8: Acorn Computer to IBM PC compatible
- Insert Figure 9:Acorn 32 bit computer to BBC Model B / Master series.
-
- Insert Figure 10: Acorn Computer to Apple LaserWriter
-
- Risc PCs and A5000s use an 82710 or 82711 peripheral controller, which
- provides the driver hardware for the serial port; as this IC is used in many
- IBM PC compatibles, it is possible to software-configure the computer to use
- the serial port in exactly the same way as an IBM PC compatible does.
- However, the system is pre-configured to behave as an Acorn machine, so
- Archimedes cables which require DCD and have flow control via DSR will work
- correctly without any reconfiguration.
-
- Some software communications packages have an option to reconfigure the
- serial port on A5000s and Risc PC so that IBM-type cables can be used.
-
- Appendix B:Useful Contacts
- --------------------------
- For SLIP modules, which can be used in
- conjunction with the Acorn TCP / IP suite on all Acorn 32 bit machines,
- including the Risc PC:
-
- Gnome Computers Limited 25a Huntingdon Street St Neots Cambridgeshire PE19
- 1BG
-
- Tel: 0480 406164 email: chris@gnome.co.uk
-
- For copies of the RS232, RS423 and CCITT standards documents:
-
- BSI Standards Linford Wood Milton Keynes MK14 6LE
-
- Tel: 0908 221166
-
- For Internet RFCs: Anonymous FTP to
-
- doc.ic.ac.uk or ota.ox.ac.uk
-
- These two sites are given as example only, and are convenient owing to their
- UK location; RFCs are available on many more Internet sites.
-
- Appendix C: RS232 Signals
- -------------------------
- For completeness, and for use when interfacing
- Acorn systems to hardware which uses 25 way serial connectors, there follows
- a list of the signals present in the full RS232 serial port specification.
-
- Insert Figure 11: 25 Way Serial Pinout
-
- Pin 1. Prot (Protective Ground) This will usually form a connection between
- any metal screening on the cable and the metal chassis of the computer and
- peripheral.
-
- Pin 2. TXD (Transmitted Data) This is the line all data is transmitted on.
- Transmission will only occur if line 5, CTS, is active.
-
- Pin 3. RXD (Received Data) This is the line all data is received on.
-
- Pin 4. RTS (Request To Send) This is the line which indicates that an RS232
- device is ready to transmit data. In order to find out whether data is
- expected, a receiving device tests this line.
-
- Pin 5. CTS (Clear To Send) The state of this line is used to indicate that a
- device is ready to receive data transmitted to it by another RS232 device.It
- is used to inhibit data transfer until the receiving device is ready to
- accept it.
-
- Pin 6. DSR (Data Set Ready) This line is used to indicate that a connected
- RS232 receiving device is switched on.
-
- Pin 7. GND (Signal Ground) This provides a common reference for both input
- and output signals on both systems.
-
- Pin 8. DCD (Data Carrier Detect) This line is used for hardware flow control
- on Acorn systems, instead of CTS.
-
- Pin 9. Not connected
-
- Pin 10. Not connected
-
- Pin 11. STF (Select XMT Frequency)
-
- Pin 12. dcd (Secondary DCD)
-
- Pin 13. cts (Secondary CTS)
-
- Pin 14. xmt (Secondary XMT)
-
- Pin 15. Xclk (Transmit Clock)
-
- Pin 16. rcv (Secondary RCV)
-
- Pin 17. Rclk (Receive Clock)
-
- Pin 18. Not connected
-
- Pin 19. rts (Secondary RTS)
-
- Pin 20. DTR (Data Terminal Ready) This line is used to indicate whether the
- RS232 transmitting device is switched on.
-
- Pin 21. SQ1 (Signal Quality)
-
- Pin 22. RI (Ring Indicator)
-
- Pin 23. DRS (Data Rate Select)
-
- Pin 24. (External Transmit Clock)
-
- Pin 25. BY (Busy; Standby)
-
- Glossary
- --------
- Asynchronous communication: Communication over a serial line where
- one device has to "sit and listen" while the other sends data; the two
- devices are not able to send data simultaneously.
-
- Baud: See "A Note on Baud Rates"
-
- DCE: Data Communications Equipment. A piece of equipment which obeys the DCE
- specification as laid down in the RS232 standard. Usually a modem, or other
- device which passes information through it to other devices.
-
- DTE: Data Terminal Equipment. A piece of hardware such as a computer or a
- printer, which obeys the DTE specification as laid down in the RS232
- standard, and which generally does not relay data on electronically.
-
- FTP: Internet File Transfer Protocol. Used to download data from an FTP
- server (usually a UNIX or VMS host), via an FTP client such as the one
- supplied as part of the Acorn TCP/IP suite.
-
- Hayes Command Set: This is a command protocol used for software control of
- many of today's high-specification modems. A Hayes command sequence starts
- with the letters AT (short for ATtention), and can be followed by a number
- of parameters. A list of Hayes commands is usually supplied with a modem
- supporting the command set. Modem: Short for MODulator / DEModulator. A
- piece of equipment which converts serial data into audio-ftrequency tones
- suitable for sending over the Public Switched Telephone Network (PSTN), and
- which can take audio data back in from a remote system and convert it to
- serial data.
-
- Packet: A standard "chunk" of data sent over a serial link. A basic packet
- comprises a start bit, a single data word, a parity bit and either one or
- two stop bits. Acorn systems default to two stop bits. Communication
- protocols generally use larger packets, containing many words per packet.
-
- Parity: A bit added onto the end of a word in a serial data packet, which
- allows a simple measure of detecting whether a word was received without
- being corrupted. Parity comes in two flavours, even and odd; this determines
- the type of checksum calculation carried out.
-
- RFC: A "Request For Comment" document. These are usually available over the
- Internet, and are the standards documents which define Internet protocols.
- Two sites which carry many of the RFCs as plain text documents are listed in
- the Useful Contacts section.
-
- Synchronous Communication: Communication over a serial line where both
- parties in the transaction can be sending and receiving data simultaneously.
-
- Word: A sequence of bits in a basic serial packet which contains the
- "useful" data being transferred. In serial communication, a word is normally
- seven or eight bits long.
-