[wplug] Serial killer

William N. Powell billpwl1 at verizon.net
Fri May 13 09:30:41 EDT 2005


Vance Kochenderfer wrote:

>My parents have been having trouble with their external serial modem.
>It reportedly worked fine last Friday, but when they went to go online
>Saturday morning, no go.  I've spent a bunch of time with them on the
>phone (from a couple hundred miles away) trying to diagnose the trouble,
>but have hit a brick wall.  They're using Mandrake 10.1 (kernel 2.6.8).
>
>The modem simply won't respond to commands.  KPPP has a nifty "Query
>Modem" function that sends a series of ATI commands to the modem.
>This causes the modem lights to flicker, but no response comes back.
>Trying an ATZ or any other AT command in a terminal likewise gets no
>reply.  I suspected the modem could have died, but I had them put on
>an old modem I had left at their house, and we get the same results.
>
>My next suspicion was a serial port problem, but dmesg and a
>setserial -a /dev/ttyS0 both show normal values.  Permissions on
>/dev/ttyS0 are correct.  There's only one serial port, so I know
>it's the right device.  It's properly enabled in the BIOS.
>
>Anyone have any ideas for further diagnosis I can do remotely (with
>non-computer literate people on the other end)?  Or alternatively, if
>it turns out the serial port is bad, anyone have a PCI modem that
>works under Linux they'd like to unload?  Thanks a bunch.
>
>Vance Kochenderfer        |  "Get me out of these ropes and into a
>vkochend at nyx.net          |   good belt of Scotch"    -Nick Danger
>_______________________________________________
>wplug mailing list
>wplug at wplug.org
>http://www.wplug.org/mailman/listinfo/wplug
>
>  
>
Well, with non-technical people on the other end, this might not help, 
but,  my next test would be a loopback test. 

Open minicom and configure for correct serial port w/ no flow control. 
(Any baud rate will work)
Use a small piece of wire(a paper clip will do) to jumper pins 2 and 3 
on the modem end of the serial cable.
Any key pressed on the keyboard should be echoed back to the screen. If 
this works cable and serial port is probably good.
If this fails, repeat at the serial connector on the computer. If this 
works the cable is bad. if not, Probably serial port problem or minicom 
is not configured correctly.

Note: this does not test handshaking lines (DTR, DSR, RTS, CTS). A full 
loopback connector will test, but we will keep this simple for now.

External modems may have switches for baud rate selection, or it may be 
automatic, verify baud rate is set correctly.

Google RS-232 handshaking or loopback for more info.

Bill


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