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Old 01-31-2006, 08:54 AM   #1
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Dead Serial Port

I have a dead serial port; I'm 99.5% certain. I'm almost willing to consider the matter closed but that .5% of doubt is still tugging at me, so here goes.

I have a modem sitting on ttyS0. mgetty listens and responds. Or, at least it used to. Someone had called in, and was using the system, but either while they were using or shortly after, it stopped working, so when they called back, it wouldn't answer. As the modem wouldn't answer, I powercycled it and auto-answer returned. However, mgetty would no longer offer a prompt.

Using minicom, I tried issuing AT commands to the modem. No response.

Looking at the minicom log, I found this line:

01/27 16:55:24 yS0 check for lockfiles
01/27 16:55:24 yS0 checklock: stat failed, no file
01/27 16:55:24 yS0 locking the line
01/27 16:55:24 yS0 makelock(ttyS0) called
01/27 16:55:24 yS0 do_makelock: lock='/var/lock/LCK..ttyS0'
01/27 16:55:24 yS0 lock made
01/27 16:55:25 yS0 tio_get_rs232_lines: status: RTS DTR
01/27 16:55:25 yS0 WARNING: DSR is off - modem turned off or bad cable?
01/27 16:55:25 yS0 lowering DTR to reset Modem
01/27 16:55:25 yS0 tss: set speed to 38400 (017)
01/27 16:55:25 yS0 tio_set_flow_control( HARD )
01/27 16:55:25 yS0 waiting for line to clear (VTIME=1), read:
01/27 16:55:25 yS0 send: \dATQ0V1H0[0d]
01/27 16:55:26 yS0 waiting for ``OK''
01/27 16:55:26 yS0 got:
01/27 16:55:46 yS0 timeout in chat script, waiting for `OK'
01/27 16:55:46 yS0 init chat timed out, trying force-init-chat
01/27 16:55:46 yS0 send: \d[10][03]\d\d\d+++\d\d\d[0d]\dATQ0V1H0[0d]
01/27 16:55:50 yS0 waiting for ``OK''
01/27 16:55:50 yS0 got:
01/27 16:56:10 yS0 timeout in chat script, waiting for `OK'
01/27 16:56:10 yS0 init chat failed, exiting...: Interrupted system call
01/27 16:56:10 ##### failed in mg_init_data, dev=ttyS0, pid=9664

So, keying off the warning/suggestion, I replaced modems. Still, no luck. I replaced the cable, trying it with both old and new modems, no luck.

In a last ditch effort to simplify everything, I plugged my laptop into the serial port (via Cisco rollover cable), starting agetty on the laptop. Opening minicom (verifying 9600 8n1, exactly as the laptop was set), I got no characters whatever; a blank screen taunted me.

For the record, I had the new modem talking on the laptop, I'm sure it's good. I had both a windows and a linux machine talking to my laptop through the rollover cover before I plugged it into the buggy machine.

It appears I'm looking at dead hardware, however, the .5% of doubt I mentioned earlier tells me to ask here. Are there any further tests I can make? Is there any further information I can access to see the status of the serial port?

What'd'ya think guys? Anything I missed?
Old 01-31-2006, 01:23 PM   #2
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Test 1:
You can build or purchase a serial loopback test plug like:

When using minicom any characters typed on the keyboard should be displayed. This will work with hyperterminal or any other serial communications application.

Test 2:
If you have a floppy drive on this computer you can download a DOS diagnostic program to check the serial port.
Old 01-31-2006, 01:35 PM   #3
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I was dubious of a bad port until your last test. That should have worked.

I would look into something like michaelk is talking about, that would be a good start, but I have a feeling you would be getting the same results as you did with your test.

Do the boot messages not anything odd? Or not show the serial port being detected? Is there a possibility that your /dev/ file might have been corrupted, or the permissions are wrong? Perhaps you could generate a new /dev/ file for it?

Though I have to admit, from the description, it does sound a lot like a hardware failure of some type.


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