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Cisco 837 and a serial modem? Special cable required?

 
 
dale-google@monsya.net
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      07-25-2005
Hi,

I've been playing around with a Cisco 837 and connecting a modem to the
console port, which also works as a virtual auxiliary port. Somtimes
the connection works, sometimes it doesn't.

I'm using a standard cisco modem DB25 male connector to a DB9 adapter,
not the special Cisco cable for this router.

The problem is, is that the router doesn't seem to drive the signals on
the serial port to the modem, unless it already senses that the modem
is there. Unfortunately, the modem seems to do the same thing, and wont
respond unless it 'sees' the router!

An RS-232 break-out box shows this clearly, with the router being
prompted into action when it 'sees' a signal from the modem. When I
connect the exact same cable to the aux port of a 2500 series router,
everything works fine, repeatedly, and never fails.

Is the modem cable for the 837 special? Does it somehow behave
differently to an older rollover cable and modem connector?

Thanks,
Dale.

 
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Peter
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      07-25-2005
Hi Dale,

> The problem is, is that the router doesn't seem to drive the signals on
> the serial port to the modem, unless it already senses that the modem
> is there. Unfortunately, the modem seems to do the same thing, and wont
> respond unless it 'sees' the router!


I have never tried it with an 827/837, however the main difference
between using a Cisco CON port and an AUX port for a Modem is that the
AUX port does handle most of the RS-232 control signals correctly, but
the CON port does not, it ignores ALL control signal states. You also
need to configure the Modem to operate in a pretty "dumb" manner, IE
to not look for DTR, or RTS signals as input (IE to assume they are
always HIGH), and to always drive CTS, DSR & LSD high. This also means
you cannot use H/W Flow control on the Modem (because the CON port
doesn't do H/W flow control) and any scripting may need specific
pauses at various places to drive things correctly.

Well that's the theory anyway, I hope it helps................pk.

---
Peter from Auckland.

 
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dale-google@monsya.net
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      07-25-2005
Hi Peter,

thanks for the response.

Yeah, I know that the console port doesn't support DCD or handshaking,
at least on a conventional Cisco router.

Just to confuse us, Cisco have implemented a 'virtual aux port' on the
837 ADSL routers that uses the console port for both a console and a
aux port. It does have complete signals for handshaking and carrier
detect, and uses the same physical port for both.

I have been able to connect this virtual aux port to a modem and get it
to answer a call, and it does see CD and RTS/CTS. The problem is that
it seems the router autodetects that its in 'aux port' mode and wont
see my modem that also seems to autodetect if something is connected.
If you fool the router by unplugging the cable into the 'console/aux'
port and plugging it back in, it seems to wake up. You can also do this
by strapping some of the RS-232 signals.

The cisco doco suggests there is a thing called a SOHO/800 series
router modem cable, which funnily enough doesn't seem to show up in the
pricing lists. I need to find out if its 'special' or if its just the
equivalent of the old rollover cable and a normal 'modem' RJ-45
adapter.

>From what I have been seeing with a breakout box, I wouldn't be

surprised if there's something clever inside the special cable so that
it knows there's a modem connected and switches from console mode to
modem mode.

Anyhow, thanks for the response. I guess I should try a 'conventional'
(no autodetect function) modem and see if the 837 recognises it. At
least that will tell me if its the router being clever or a combination
of the router and the modem.

Dale.

 
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