[mdlug] Semi-OT: Serial modem behavior

[Aaron Kulkis]

Ingles, Raymond wrote:
> I have access to three serial modems, all different models. One of them works just
 > fine - a USR Sportster. The other two (a Zoom 2400 baud, and another 14.4 modem
 > whose manufacturer escapes me for the moment) display the same problem - no data
 > coming back from the modem to the computer.
>
> Cabling doesn't seem to be the issue; the same cables work with one modem, but
 > not the other two. I've used a couple different cables, and used both USB-to-serial
 > adapters and on-motherboard serial ports.

Do they work on a normal 9- or 25-pin Serial Port
(typically driven by the standard 16551 UART chip)?

>
> The behavior on the two problem modems is the same. Using a program like 'minicom'
 > or 'gtkterm', I can see the modem notice things like DTR and such - the appropriate
 > lights turn on. When I send AT commands, I can see the RD and even SD lights blink.
 > Yet absolutely no response shows up in the terminal. On the working modem, I get
 > responses without issue. But on the failing ones, nada.

Make sure that both the communications port and the modem are in duplex communication mode.

>
> I've tried bauds from 300 to 115200. (At higher bauds, the non-working modems RD
 > and SD lights stop blinking.) I've pretty much stuck with 8N1 parity, but if
 > anyone has suggestions for alternatives there I'm curious.
>
> Obviously the use cases for serial modems are... pretty rare these days. But
 > I've got this 'dream machine from 1993' that I occasionally bring out, and a
 > functional serial modem would be a lovely thing.
>

Keeping it working is a good thing.

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LOL