I use for such tests a simple ISDN annex where one analog phone line can call another internally. On each line you can have a PC with modem and call each other. Since a typical ISDN box supports eight analog lines, you could plug theoretically 8 PCs with a modem each.

Such a unit may not be readily available in the USA. ISDN wasn't really popular here as it was hideously expensive. Most places needing multiple voice lines just got POTS copper loops or jumped straight to T1. I have a 3Com ISDN modem here, and it only supports 2 analog lines since the BRI systems here only supported two B channels in residential applications. I don't think there is any sort of loop back in it either.

You don't need a 'hot' line to connect most modems together, just a regular telephone cable, set the modems to ignore dial tone, dial a bogus number on one side and answer at the other - they should connect just fine.

If for some reason they need some voltage present on the line you can do it with a 9V battery and a ceramic capacitor.

I have a cheap Dell/Conexant USB 56K modem for a few uses, and when connected directly to the other system's US Robotics ISA modem (which is actually a fairly nice unit, not OEM garbage like most), neither end can register any connection, even when set to ignore a dial tone. I was under the impression that an internal modem won't actually drive a data line, but I could be wrong. I'll give it another try shortly.

Agreed - from what little I know, I never recognized ISDN as being widely available, and it doesn't look that's really changed all that much... I'm flipping through the internet right now looking for a unit that will work, but I'm not finding a whole lot at a reasonable price. It's like people don't even want to sell things... I stumbled across an unrelated 486DX2 motherboard that someone was asking a little over $500 for. Seriously?... 😒 I just picked mine up a few months ago, as-new in the original box, for $30 with free shipping, and another $20 for the Cx486.

Here's a question. If you ran a length of coax cable between two cable modems, would they be able to communicate with each other or would more hardware be required?

sliderider wrote:

Here's a question. If you ran a length of coax cable between two cable modems, would they be able to communicate with each other or would more hardware be required?

Unfortunately not. The cable modem is designed only to speak to a piece of equipment at the cable company called the "Cable Modem Termination System". When you turn the modem on, it will be looking for the signal that the CMTS is broadcasting, which it then locks on to and uses to transfer data between the CMTS/the internet and the modem (there is a separate upstream signal).

So if you connected two modems together, all they'd be doing is trying to find a signal that is never there, it's not like Ethernet or analog modems where they can find each other. If you connected the two modems together and to a cable network, they'd both connect to the cable company's CMTS but you would still not be able to exchange data between them (unless they were both authorized to be on the network e.g. you had two internet subscriptions from the cable ISP and one modem was on each). If you wanted to go so far as to buy a CMTS, they're not exactly readily available and aren't cheap when you can get them. I found one at work and still couldn't use it because it needed some special hardware to convert its signal into one that a modem would accept.

This is almost true for DSL modems too (however a few specialist modems have a mode where they can emulate the central office/telephone exchange). DSLAMs (the equipment in the central office that a DSL modem talks to) can be quite easy to find though. I have one, it's quite interesting to play with sometimes.