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Posted by Joe Morris on May 31, 2006, 9:39 am
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>Thanks for the suggestion Joe. Not real clear what you mean by null
>modem.
A null modem cable swaps the input and output pins between ends. Typically
this involves (from "A" end to "B" end):
DCD (1) --> DTR (4) and also to "A" end DSR (6)
Receive (2) --> Transmit (3)
Transmit (3) --> Recieve (2)
DTR (4) --> DCD (1) and DSR (6)
Ground (5) --> Ground (5)
DSR (6) --> DTR (4) and also to "A" end DCD (1)
RTS (7) --> CTS (8)
CTS (8) --> RTS (7)
RI (9) not connected
This allows two DTE devices to talk directly to each other.
> I was using a DB-9 to DB-9 cable that came with an HP switch. I
>also tried a cable set we bought that comes as RJ-45 to RJ-45 with
>adapters to DB-9 and DB-25 (although the DB-25 doesnt apply here). Its
>a flat cable similar to the DB-9 to RJ-45 that comes standard with
>Cisco routers. According to the CISCO docs, it is a "standard" RS-232C
>cable that plugs from the console port to a PC. This is the purpose of
>the HP cable, so I dont think it is a DTE/DCE issue, but I cant say
>that with 100% confidence.
Documentation has been known to occasionally <grin> have errors. OTOH,
it might be right, and there isn't a DCE/DTE problem. OTOH, whenever
I see an async interface in network server-side equipment I always
suspect DCE/DTE problems if the first attempts through a standard cable
(with various speed settings checked) fail.
Good luck.
Joe Morris
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