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Serial Communications via RS-232-C on HP 3000
Written by: Jerry Mills

The RS-232-C serial connection is one of the most common connections in the Commercial Computer system and also one of the least understood. It works so well, people frequently never get into how it works. The purpose of this paper is to shed enough light on the subject to make it a lot easier.

The RS-232-C Specification was designed to connect a DTE device (data terminal equipment) and a DCE device (data communication equipment) USING STRAIGHT THRU CONNECTIONS. All signals are referenced to DTE; meaning the DCE end of the cable will have the same signal name but opposite function (i.e. outs are ins and ins are outs). On a DTE device pin 2 is ‘TD’ transmit data therefore on the DCE end, pin 2 is also called TD but is an input (receiver). Confusing isn’t it!

What is DTE and DCE?

Who cares whether a device is DTE or DCE. YOU DO! Ever try to hook up a printer or terminal with cables of questionable origin or you don’t have a clue as to what a 40242G cable is. By knowing first what kind of devices you are trying to connect; you can quickly find a cable that will work.

A DTE device is a consumer or producer of data; computers, terminals and printers are examples.

A DCE device transfers the data; modems, multiplexers and data switches are examples.

To connect between two DTE’s or two DCE’s requires the crossing of many wires to work; add to this, inconsistency among manufacturers and you get a lot different cables types.

How to determine whether a device is DTE or DCE

Fortunately this is easily done by inspection. DTE devices transmit on pin two and DCE devices transmit on pin three. A breakout box will quickly show this by lighting the led on pin 2 or 3. If you don’t have a break out box use a volt meter and measure between pins 2/3 and pin 7 (signal ground). The transmit line will be approximately -12 volts.

If you find both devices are DCE or DTE you will need to use a cross wired cable. The classic example is the null modem eliminator / adapter we hear about frequently. It would be used when connecting say a laserjet (DTE) to a IBM PC Clone’s DBM25 serial port which is also a DTE device. This requires crossing pin 2 and pin 3. Printers don’t normally need control lines so a simple 3 wire connection with 2 crossed to 3 will do the job.

Surprisingly most connections actually require few wires to work. Even Modems will work on 3 wire connections! Just set the modems DTR high all the time and connect your 3 wires and go to work.

The real beauty of this method is it’s simplicity; you don’t worry about what the connector is wired for, DTE or DCE since you can determine it by simple inspection. And if the cable should be crossed you will see that quickly too. And oh yea, remember we never connect (TD) transmit data pin to a (TD) transmit data pin. Which brings us to:

RS-232-C Asynchronous Serial Data Format and voltage levels

Asynchronous character transmission is the most common type of Serial Data communications. We are only interested in this type of signal for our printer, terminal and modem communication’s which will comprise 95 percent of your need.

Everybody intuitively knows that an asynchronous serial character is an alternating voltage level. What most people don’t know is what the hell is a mark, space, start bit, stop bit or for that matter what the heck is a BAUD!

A mark is logically equivalent to a logical one. A space is equivalent to a logical zero. And of course ones and zeros are used to determine the ASCII code for any particular character. We refer to these marks and zeros as bits. The number bits or marks and zeros in a character determines the size of the Character Set.

The limitation as to the number of characters we can encode is limited to the number of bits used. In the printer world this is why we hear the terms; 7-bit character set, 8-bit character set etc. For instance: The Roman-8 character set uses 8 bits or marks and zeros when encoding a character. Of course 8 bits equal a byte and a byte can have a decimal value of 256. Therefor an 8-bit character set has 256 possible characters. A 7-bit character set has one half the binary weight or 128 characters available.

Mark and space also have additional functions. To alert a printer that a character is coming; the encoded character is preceded by a start bit (space). Upon seeing the start bit the printer starts building a character based on the marks and spaces transmitted following the start bit. The last piece sent is a stop bit (mark), which signals the end of a character. Therefor if we are using an 8-bit character set; we have the 8 bits for the character, a start bit and a stop bit giving us 10 bits in total.

Which brings us to the BAUD! The baud rate with which we are all so familiar; is the number of bits transmitted in one second. For example 19.2k baud is how many characters per second. Well if we are using an 8-bit character set, have one start bit and one stop bit; that equals 10 bits per character and at 19200 baud we get 1920 characters per second. Easy, huh! Whoops but what if the stop bit is 1.2 bits or 2 bits long. Now we get 10.2 bits per character or 11 bits per character which when divided into 19.2k baud equals less characters per second. So while baud rate appears to indicate the character rate it is not entirely true. It only indicates the maximum possible character rate depending on the character format used.

Here is another thing to also remember about asynchronous transmission; you can have say a 56k baud rate but you may only be sending one character every 5 years. While it is true you have a fast baud rate, it will still take 25 years to print ‘HELLO’.

An asynchronous ASCII character data signal therefor varies between –12v (mark) and +12v (space). The lowest value this should ever be is –3v and +3v. So when we measure –12v on pin 2 of a DTE device (like a printer) we are actually looking at the stop bit. When a character starts you will see a start bit which will change the voltage level from –12v to +12v followed by the mark and space code for the ASCII character terminating with the stop bit. ASCII Character Format

ASCII Character format is the familiar, 8 data bits, no parity, 1 stop bit or 7 data bits, even parity and 1 stop bit. These different formats impact our mission so it is very important to understand the above explanation of 7-bit and 8-bit character sets. How does this effect us? For example:

Customer has connected a Laserjet to a DTC and is using 7 data bits w/even parity. Printer works ok but has trouble with some forms. You bet he does, he can not send any 8 bit characters to the printer, such as raster graphics until he changes to 8 data bits, no parity.

Customer logs into his HP Unix system over a modem, can’t run SAM. Normal connection on Unix is 7 data bits w/even parity. Whoops SAM has line drawing characters which requires the 8 bit character set.

Successful Serial Communications between 2 Devices always require:

1. BAUD RATE - The same baud rate
2. CHARACTER FORMAT - The same character format (type of parity in use and number of bits)

An important observation

Any device that transmits a signal; i.e. a mux panel port, supplies its own driving voltages (-12 & +12v). It is considered an active pin. That means whatever is connected to this active signal such as a modem or printer; that pin is a passive pin. Since the mux panel is supplying the voltage for the signal we wish to transmit, it is absolutely mandatory that the common, ground or signal reference wire be connected. Without this wire the passive device; i.e. the printer, cannot see the signal.

This is a law of physics that is frequently forgotten. Why the hype? Because many times people will not connect pin 7 of the RS-232-C, or the connection is resistive. Yet their mux panel connections or building wiring works! How can this be?

Simple, the printer has a chassis ground and the computer has a chassis ground; signal is developed by this alternate path. That being the case; you can actually connect a printer using only a one wire connection. Try it sometime. That is where people frequently get sloppy. Worse yet the printer setup has been working fine; the printer is moved or the computer is moved: suddenly it is not working any more. (spoolie i/o errors)

However if the cabling follows the standard, and the active pin and the signal ground go between both devices; it will always work! Period! Full Stop!

Now what about a signal coming from say a printer to a mux panel. Again the printer is now the transmitter and that makes it the active pin and the mux panel pin is the passive pin. Since the printer is now supplying the driving voltages you must provide a signal reference from it to the mux panel. Pin 7 has been designated as the common signal reference between any two devices. It is the only mandatory connection in the RS-232-C specification.

With what we have just discussed; we have a transmit line from a mux panel to a printer. Also a transmit line from the printer to the mux panel and a single common signal reference line (pin 7). This is a known as a serial channel and it only takes three wires to do it.

What is a Channel?

A channel is the expression used to describe a RS-232-C serial connection. For example when a program dials a modem to connect to a remote computer; this process is called ‘OPENING A CHANNEL’. This is why the RS-232-C standard refers to a primary channel. This primary channel consists of pins 2 & 3 plus the signal reference pin 7. The RS-232-C specification also has set aside 2 more pins out of the 25 pins as a secondary channel. It merely means you can transmit/receive data on to separate sets of pins; thus the RS-232-C specification has two data channels. Primary and the Secondary channel.

If you only need 3 wires (2,3 & &) for a serial channel; what the are all those other 22 pins for?

The answer is sweet and simple protocol control. How we transmit our serial channel. Lucky for us; 90% of all the different connections we use only require a few basic control signals.

BASIC RS-232-C Controls and Status - for typical applications; only 8 lines need to considered:

















SIGNAL COMMON (connection is mandatory!)





















































This table lists the most common signals required used in serial channel connections.

RS-232-C Signal Definitions - Check your modem or terminal handbook for local description:

DTR - Data Terminal Ready is used for call initiaiton, call termination, suppressing Autoanswer

DSR - Data Set Ready is an indicator that the modem is ready. Most modems have selectable behavior.

DCD - modem indicates it is receiving carrier from the remote modem

RTS - terminal requests permission to send data to the modem

CTS - Clear to Send grants permission for the terminal to send data for retransmission to the remote modem.

RI - modem indicates that the phone is ringing

3 Wire connections don’t need control or status signals.

Controlling data between say a DTC port or mux panel and say a printer is done using

In-Band Control.

HP uses two types of In-Band Control. Flow Control and Status Checking.

What is Flow Control?

Flow control is the ability to start and stop the flow of ASCii characters over a serial channel. This is a critical process needed to ensure characters are not sent at the wrong time and subsequently loss. There are two basic types of flow control used in RS-232-C channel communications; in-band and out-of-band flow control.

In-Band Flow Control (Software Handshaking)

XON/XOFF - flow control is the most common type of in-band flow control. The ASCii control characters for XON/XOFF are mixed in with the data being sent over the channel for the purpose of starting or stopping data flow. A XON sent from a printer to the computer tells the computer the printer is ready for more data. XOFF performs the opposite function and tells the computer to stop sending data.

A classical example of loss of in-band control to a printer is that the first few pages printed are fine. But suddenly there is a loss of printout then the printing picks up further on. This is usually the result of the computer not seeing an XOFF from the printer to stop the data flow. The problem can be a bad connection on pin 2, broken or misconfigured printer, bad serial port on the CPU or frequently the printer is configured for 7 data bits with even parity but the HP3000 is configured for 8 bits no parity. The HP3000 won’t recognize the XOFF!

ENQ/ACK - flow control is the same as XON/XOFF but are 2 different control characters. Older versions of MPE used ENQ/ACK as well as HP1000 computers.

ETX/ACK - flow control is the IBM version of XON/XOFF but are 2 different control characters

Status Checking (Software Handshaking)

Hewlett Packard has developed it’s own strategy of In-Band Control that it uses in addition to XON/XOFF flow control. The purpose of this was to improve the integrity of remote printing.

HP Term types 19,22 and 26 use status checking. Before a print job is sent the software sends the 3 following characters: escape, XON and a question mark (?). A printer upon receiving the (?) question mark transmits a single decimal character to the computer. If the printer is ready and on-line the printer will send the ASCII character for a zero. The HP computer sees this character; determines the printer is ready and starts transmitting the print job. If the Computer does not receive the character or the character has a value other than zero; then the computer assumes there is a problem and will shut down the print job (frequently seen as a spoolie i/o error).

There is nothing to indicate that this handshake failed. The printer just sits there and looks like it can’t be printed to. Review the part about transmit voltages (An Important Observation).

If you think you may be experiencing Status Checking failure; change the term type to 18. This is known as the dumb term type and does not use Status Checking. Use term type 18 only for printers that don’t support status checking.

Out-of-Band Flow Control (Hardware handshaking)

Out-of-band flow control; frequently called hardware handshaking use the RS-232-C control line or lines to control data flow over the channel. This is most frequently needed for binary transfers (8 bits per character) where embedding XON/XOFF control characters would interfere with the binary data.

RTS/CTS - is the most commonly used control lines for flow control. Using a computer modem hookup as an example; when the computer is ready to send data it raises or asserts the RTS request to send control line. The modem is monitoring this line; upon seeing the line asserted and the modem is able to receive and retransmit a character; the modem will assert CTS clear to send which grants the computer the right to send. The computer will then send data until CTS is lowered or the data flow is complete. With this arrangement data can be paced i.e. start..stop..start..stop..etc.

DTR - data terninal ready can also be used to control flow. When DTR is asserted the computer will send data. When de-asserted the data flow stops. HP3000.s use this signal to enable the modem and force the modem to hang up by lowering of DTR. The old MPE spooler would assert DTR upon receipt of a print job. When the print job was complete the spooler would de-assert or lower DTR. DTR can enable flow or even pace the flow i.e. start..stop..start..stop..etc. Printers can control flow by de-asserting DTR when the buffer is full and asserting DTR when the buffer is ready for more data.

DCD - data carrier detect will also work for flow control. If DCD is asserted then the modem has established a connection with the remote modem’s carrier which shows the channel is open. DCD can enable flow but does not pace the flow i.e. start..stop..start..stop..etc.

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