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Cisco 837 and a serial modem? Special cable required?

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.

Reply to
dale-google
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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.

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.

Reply to
dale-google

Hi Dale,

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.

Reply to
Peter

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