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12. Troubleshooting

See Modem-HOWTO for troubleshooting related to modems or getty for modems.

12.1 Serial Electrical Test Equipment

Breakout Gadgets, etc.

While a multimeter (used as a voltmeter) may be all that you need for just a few terminals, simple special test equipment has been made for testing serial port lines. Some are called "breakout ... " where breakout means to break out conductors from a cable. These gadgets have a couple of connectors on them and insert into the serial cable. Some have test points for connecting a voltmeter. Others have LED lamps which light when certain modem control lines are asserted (turned on). Still others have jumpers so that you can connect any wire to any wire. Some have switches.

Radio Shack sells (in 1998) a "RS-232 Troubleshooter" or "RS-232 Line Tester" which checks TD, RD, CD, RTS, CTS, DTR, and DSR. A green light means on (+12 v) while red means off (-12 v). They also sell a "RS-232 Serial Jumper Box" which permits connecting the pins anyway you choose.

Measuring Voltages

Any voltmeter or multimeter, even the cheapest that sells for about $10, should work fine. Trying to use other methods for checking voltage is tricky. Don't use a LED unless it has a series resistor to reduce the voltage across the LED. A 470 ohm resistor is used for a 20 ma LED (but not all LED's are 20 ma). The LED will only light for a certain polarity so you may test for + or - voltages. Does anyone make such a gadget for automotive circuit testing?? Logic probes may be damaged if you try to use them since the TTL voltages for which they are designed are only 5 volts. Trying to use a 12 V incandescent light bulb is not a good idea. It won't show polarity and due to limited output current of the UART it probably will not even light up.

To measure voltage on a female connector you may plug in a bent paper clip into the desired opening. The paper clip's diameter should be no larger than the pins so that it doesn't damage the contact. Clip an alligator clip (or the like) to the paper clip to connect up.

Taste Voltage

As a last resort, if you have no test equipment and are willing to risk getting shocked (or even electrocuted) you can always taste the voltage. Before touching one of the test leads with your tongue, test them to make sure that there is no high voltage on them. Touch both leads (at the same time) to one hand to see if they shock you. Then if no shock, wet the skin contact points by licking and repeat. If this test gives you a shock, you certainly don't want to use your tongue.

For the test for 12 V, Lick a finger and hold one test lead in it. Put the other test lead on your tongue. If the lead on your tongue is positive, there will be a noticeable taste. You might try this with flashlight batteries first so you will know what taste to expect.

12.2 Serial Monitoring/Diagnostics

A few Linux programs will monitor the modem control lines and indicate if they are positive (1) or negative (0). See section Serial Monitoring/Diagnostics

12.3 The following subsections are in both the Serial and Modem HOWTOs:

12.4 My Serial Port is Physically There but Can't be Found

Check the BIOS menus and BIOS messages. If it's an ISA bus PnP serial port, try "pnpdump --dumpregs" and/or see Plug-and-Play-HOWTO. For the PCI bus look at /proc/pci. You may try probing with setserial. See Probing If nothing seems to get thru the port it may be there but have a bad interrupt. See Slow: Text appears on the screen slowly after long delays

12.5 Slow: Text appears on the screen slowly after long delays

It's likely mis-set/conflicting interrupts. Here are some of the symptoms which will happen the first time you try to use a modem, terminal, or printer. In some cases you type something but nothing appears on the screen until many seconds later. Only the last character typed may show up (and it may be just an invisible <return> character but you notice the cursor jumps down one line). In other cases where a lot of data should appear on the screen, only a batch of about 16 characters appear. Then there is a long wait of many seconds for the next batch of characters. You might also get "input overrun" error messages (or find them in logs).

For more details on the symptoms and why this happens see

Interrupt Problem Details and/or Interrupt Conflicts and/or Mis-set Interrupts.

As a final check to see if it really is an interrupt problem, set the IRQ to zero with "setserial". This will tell the driver not to use interrupts but to use inefficient (but faster) polling methods. If this seemingly fixes the "slow" problem then you had an interrupt problem but should still try to fix it since polling uses excessive computer resources.

Checking to find the interrupt conflict may not be easy since looking in the /proc directory may be misleading. Make sure there are no IRQs being illegally shared. Check all your boards (serial, ethernet, SCSI, etc...). Make sure the jumper (or PnP) settings, and the setserial parameters are correct for all your serial devices. Also check /proc/ioports and /proc/interrupts and /proc/pci for conflicts. For more details see Serial-HOWTO: Interrupt Problem Details. If it involves Plug-and-Play devices, see also Plug-and-Play-HOWTO.

12.6 The Startup Screen Show Wrong IRQs for the Serial Ports.

Linux does not do any IRQ detection on startup. When the serial module loads it only does serial device detection. Thus, disregard what it says about the IRQ, because it's just assuming the standard IRQs. This is done, because IRQ detection is unreliable, and can be fooled. But if and when setserial runs from a start-up script, it changes the IRQ's and displays the new (and hopefully correct) state on on the startup screen. If the wrong IRQ is not corrected by a later display on the screen, then you've got a problem.

So, even though I have my ttyS2 set at IRQ 5, I still see

ttyS02 at 0x03e8 (irq = 4) is a 16550A
at first when Linux boots. (Older kernels may show "ttyS02" as "tty02") You have to use setserial to tell Linux the IRQ you are using.

12.7 "Cannot open /dev/ttyS?: Permission denied"

Check the file permissions on this port with "ls -l /dev/ttyS?"_ If you own the ttyS? then you need read and write permissions: crw with the c (Character device) in col. 1. It you don't own it then it should show rw- in cols. 8 & 9 which means that everyone has read and write permission on it. Use "chmod" to change permissions. There are more complicated ways to get access like belonging to a "group" that has group permission.

12.8 "Operation not supported by device" (error message) for ttySx

This means that an operation requested by setserial, stty, etc. couldn't be done because the kernel doesn't support doing it. Formerly this was often due to the "serial" module not being loaded. But with the advent of PnP, it may likely mean that there is no modem (or other serial device) at the address where the driver (and setserial) thinks it is. If there is no modem there, commands (for operations) sent to that address obviously don't get done. See What is set in my serial port hardware?

If the "serial" module wasn't loaded but "lsmod" shows you it's now loaded it might be the case that it's loaded now but wasn't loaded when you got the error message. In many cases the module will automatically loaded when needed (if it can be found). To force loading of the "serial" module it may be listed in the file: /etc/modules.conf or /etc/modules. The actual module should reside in: /lib/modules/.../misc/serial.o.

12.9 "Cannot create lockfile. Sorry" (error message)

When a port is "opened" by a program a lockfile is created in /var/lock/. Wrong permissions for the lock directory will not allow a lockfile to be created there. Use "ls -ld /var/lock" to see if the permissions are OK: usually rwx for everyone (repeated 3 times). If it's wrong, use "chmod" to fix it. Of course, if there is no "lock" directory no lockfile can be created there. For more info on lockfiles see What Are Lock Files

12.10 Software which may help


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