Troubleshooting Communications

The SDK install scripts work well for those programmers who know which COM ports and IRQ levels their machines will use when communicating. However, when something goes wrong with this process, it can be very difficult to determine the correct combination of port and interrupt numbers. This chapter contains a systematic way to determine the correct configuration to use.

Most developers should not need to read this chapter. You should only read this chapter if you have been referred here from another part of this book.

The Cables

Your machines should be connected by a null-modem serial line. Lap-link cable works admirably for this purpose.

At some point as you try to set up communications between your machines, you may think that things should be working, but they aren’t. Under these circumstances, a judicious jiggle to the cable’s connections may prove helpful.

The Settings

Communications between machines are controlled by some settings on both machines. To get communication working, you will be working with these settings.

  • The COM port setting tells the pccom tool which serial port the serial communication cable is plugged into. This value can range from one to four.

  • The communication speed determines how quickly your machines will try to communicate. We suggest a speed of 19200 baud; if one of your machines (presumably your target machine) is slow, you may have to use 9600 baud instead.

  • Each COM port has an IRQ level associated with it. If you try to send a signal to a COM port using the wrong IRQ level, your machine tends to generate some bad interrupts and freeze. Thus, if you do not know the IRQ level associated with a COM port, but discover it through trial-and-error, you may want to note it down in case you ever need to use another COM port.

Setting and Overriding the PTTY variable

For the communication software to work correctly, both your target and host machines must have a correctly set PTTY variable. Type set on both machines to find out what the present value is for PTTY. The set command should return output that includes a line that looks something like the following (the numbers may be different):




If you are reading this chapter, then at least one of these values on at least one of your machines has probably been set incorrectly.

  • The first number of this variable signals which COM port this machine will use to communicate. This value may be different between machines. Your connection may be between your host’s port 1 and your target’s port 2. This number must be an integer between one and four.

  • The second number specifies the communication speed, measured in baud. This value must be the same on both machines.

  • The third number, if any, specifies the IRQ level to use when communicating. If there is no third number, then it is assumed that you are using the IRQ level normally associated with the given COM port. (I.e. if the first number of your PTTY variable is 1 or 3, the communication software will assume you want to use IRQ level 4. If the first number of your PTTY variable is 2 or 4, IRQ level 3 is assumed.)

To change your PTTY variable, issue the command

set ptty=1,19200,4


set ptty=1,19200

Note that there are no spaces in this command other than that between “set” and “ptty”. If you include a space before the equals sign, you are actually setting the value of a new variable, “PTTY “ (PTTY with a space after it). If you place a space after the equals sign, the communication software won’t be able to interpret the value. Nor should there be spaces after the commas.

When you have figured out the correct values to use when communicating, you will want to make sure there is a line in your AUTOEXEC.BAT file that sets the variable correctly.

When invoking the pccom tool, you can override the values set by the PTTY variable by passing the /c, /b, and /I flags. For instance, typing

pccom /c:2 /b:9600 /I:3

will invoke the pccom tool using COM port 2, IRQ level 3, and a 9600 baud communication rate, no matter what values have been set in the PTTY variable.

Step 1: Make Your Best Guess

When you troubleshoot your communication set-up, the most difficult problem to solve is the case where you don’t know which COM port either machine is using. If you are in this situation now, some research might be in order. The documentation of one or more of your machines should contain a clue as to which COM port number is associated with a particular serial socket. If you have another device which you know uses a serial port, then you can be sure that the cable is not hooked up to that port. Thus, if you know that your target machine has a mouse plugged into serial port two, you know that you will not be communicating with a COM port number of two.

If you narrow down the COM port choices now, you will save yourself a lot of time in the steps to follow.

Step 2: Invoke pccom on Both Machines

At the DOS prompt, type “pccom” on both machines. If you are passing flags to the pccom tool at either machine, now is the time to pass them. As you continue with troubleshooting, you will probably have to exit the pccom tool on one or both machines several times to change COM port and IRQ level settings. Always remember to start up the pccom tool again, as no communication will take place if it is only running on one machine.

Step 3+: Type “abc” On The Host Machine

Depending on what happens when you do this, you can narrow in on your communication problem.

“abc” appears on the Target (i.e. the other) machine-Success! You have found the correct communication settings. Edit your AUTOEXEC.BAT files so that your PTTY variables will be set correctly when you next restart your machines. If you have been experimenting with communications settings, you may wish to change your PTTY variables immediately, or just restart your machines after changing the AUTOEXEC.BAT files.

”???” appears on the Host (i.e. the same) machine-Incorrect cables. You do not have a null-modem connection in the serial line between your machines. The null-modem acts like a telephone line, making the host’s “mouth” talk to the target’s “ear,” and that the target’s “mouth” talks to the host’s “ear.” Without the null-modem connection, your machines will act as if they are talking on the phone, but one of them is holding their receiver upside down-listening at the mouthpiece and talking into the earpiece. Your host’s voice is being echoed back into its own “ears,” getting corrupted in the process. Use a null-modem connection.

The good news is that your signal is in fact reaching the cable, and thus your host machine is using the correct COM port and IRQ level settings.

“a” on Target, Host frozen-Everything’s correct except IRQ level. The host successfully transmitted the “a” to the target. The target machine listened at the right port and sent back a signal that it was ready for the next character. The host received this signal, but could not handle it and froze. Try a different IRQ level for your host (by using a different number for the third part of your host’s PTTY variable or by using the /I: flag when next invoking the pccom tool on the host machine). Before continuing with tests, “unfreeze” your host by restarting it. (You probably had to do this to change the IRQ level anyhow.)

No text appears, Host is frozen-Host IRQ level is wrong. The IRQ level you have chosen to use with your host’s COM port is not correct for that COM port. Note that there is no guarantee that you have chosen the correct COM port number either. Try using a different IRQ level, and when you find the correct one for that COM port, you might want to note it down. Before continuing with tests, “unfreeze” your host by restarting it. (You probably had to do this to change the IRQ level anyhow.)

No text appears, Host not frozen, Target Frozen-Target IRQ wrong. You’ve almost achieved the right settings; your host machine is sending the signal correctly, and your target machine is listening at the right port. However, your target machine is getting confused when the signal comes in. Change the IRQ level setting on your target machine, either by changing the PTTY variable, or by passing an /I: flag to the pccom tool. Before continuing with tests, “unfreeze” your target by restarting it. (You probably had to do this to change the IRQ level anyhow.)

No text appears, Neither machine frozen-Wrong COM port. The host machine successfully sent the signal to the COM port you selected. However, the signal never got to the target machine. Either the host’s COM port setting was wrong, so that the signal was sent to the wrong port and never reached the cable; or the target’s COM port was wrong, so that it was listening at the wrong port. It is possible that in fact both machines have the wrong COM port settings.

Try using different COM port settings on one or both machines. (Do this by changing the first part of your PTTY variable or by passing a /c: flag to the pccom tool.) Of course, if you are rather sure about which COM port one of the machines is using, you should try changing the other one’s settings first. Remember that the COM port value may be 1, 2, 3, or 4. If you know that some device is already using a port, than you need not test that port (assuming you haven’t somehow managed to plug a serial cable and some other device into the same serial port socket).

If you’re unsure about your target machine settings, you might try reversing these tests, typing “abc” on the target machine to see what happens. The pccom communication is bi-directional, so you may use the above tests just as well, substituting the word “host” for “target,” and vice versa.

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