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You need a terminal server: it has multiple serial ports on one side, ethernet on the other. You will need serial cables going from client devices to the terminal server. The serial cables should be the "null modem" variety, so no actual modem is needed. Hopefully you have spare serial ports, or can replace the modems on the clients with serial cards.

Take a look on ebay for "terminal server", "console server", etc.

I planned to use a lot of cheap usb to rs232 converter. I didn't know about console server. They seem very neat! I have only two question:

1- They seem to all use RJ45 i'd imagine that i would use a cable RJ45<->RS232. I found some on amazon, but i don't know if they are standard or i need to look for some specific type of cable.

2- Do they allow the "master" ethernet port to talk to all serial port at the same time? I found some conflitting information about it. If not the usb<->rs232 seem a better solution.

Thank you a lot!

1) Yes, you will need those cables... The RJ45<->RS232 cables are not standardized. You will need to buy the exact cable, or do some research to make your own: Example: http://pages.cs.wisc.edu/~ajkunen/wiring.html

2) The ethernet port can communicate with all serial ports. Depending on the model, you can program the serial port for PPP, SLIP, telnet, SSH, etc. Back in the old dialup ISP days, we connected modems to these terminal servers and users could either telnet to a host, or initiate a PPP session.

And yes, you can basically build your own terminal server with a bunch of those USB<->RS232 adapters.

Thank you, i will look into it. I found some on ebay, now with a bit of researchi should be able to find one that fit my need!

also important that you can configure these terminal servers to have different TCP ports corresponding with each serial port. this would enable you to set up a router for the console server and NAT so that each device can have its own IP

What are you trying to accomplish by using or emulating modems vs just a serial line?

Your best bet would probably be getting a terminal server style multiport serial card and just do PPP over serial from the client devices. The modem is an extra layer you typically don't need at all for local connections. You can get relatively inexpensive 16+ port cards and I'd expect them to be far more reliable and manageable than a wad of usb dongles.

When i started testing this, i used a "master" computer as serial "router" with 6 serial port, so that the "slave" computer could talk to each other. I quickly found that i need some sort of management protocol to specify which computer could talk to. I decided to emulate Hayes since it's a simple protocol very well documented and supported.

Also some software expect to talk to a modem and don't play well with raw serial.

I searched for multi serial port card, but i had trouble finding one. Maybe i'm using the wrong keyword but i only found pci/pcie card with up to 4 serial port.

Usb seem to work decently but i admit that sometime the os (linux) seem to become confused and some serial port change name, causing some trouble. Having one card (or two) will make everything easier. I'm also considering "console server" as others have suggested.

Thank you!

edit: correction

Ok, it sounds like what you were trying to do wasn't routing, but circuit-switching the rs232 lines.

As an alternative, PPP gives you a routed TCP/IP connection over a serial line...you offload routing onto the network layer and you can potentially have _all_ your client machines communicate with each other or even externally via standard TCP networking, and don't deal with circuit-switching the serial connections at all. For things that are TCP capable this is probably the cleanest solution.

If you want to support software that requires direct serial/modem links and not TCP, you will need something else in the mix. On the software side, I'd check the software to make sure you can't just turn off the AT commands altogether and use raw serial. Even where it wasn't a supported/intended setup, most software that explicitly handled modems allowed specifying alternate AT command strings, so you _might_ be able to just blank them out and go straight to serial. As far as circuit-switching...I know I've seen hardware devices that did this, but not in decades, so likely not easy to find any more. A quick google only turned up a bunch of datasheets for chips that could do it, but seems like you'd have to build your own device or track down a vintage device.

For multiport cards, an amazon search turned this up as the first result: https://www.amazon.com/Port-Rs232-Multiport-Serial-Card/dp/B...

Yes, it was actuality circuit switching, every serial port has it's number assigned and each computer could "dial-up" each other, but there is also a "virtual" number that anyone could dial up to (even at the same time) that will connect to a PPP session to connect via IP/TCP to the outside word. This last part was the one that still need a bit of work since it doesn't always work as i expect.

So i, basically, do both. packet and circuit network according to need.

As far as circuit-switching...I know I've seen hardware devices that did this, but not in decades, so likely not easy to find any more

software based circuit switching seem to work fine for what i need, is really not that complicated, it also make me do cool thing like wiresharking!

Like others said, if you can do rs-232 instead of modems, use a multiport serial card on your terminal server. With your host count, you should look into cable length and ground loop issues.

For those systems without a serial port, for whatever reason, you can find a matching modem. If you get external serial modems, you'll be able to hook it to your multiport card, but USB modems will work too (as long as they're hardware modems). You might need to apply a DC voltage to your line to convince the modems it's working, but you probably don't need a dialtone, most modems will be fine with blind dialing. Caveat: you won't be able to get 56k without a lot of work.

If you really want, you should be able to make a back to back ATA adapters work, make sure your network isn't congested, and use PCM codec and it should work ok, but probably not great.

I currently tested the system with 6 serial client plus the server. it seem to work quite well but the cable were rather short. Is rs232 more prone to ground loop than ethernet?

i looked into using serial modem, but it seemed quite complicated, and very fragile. I didn't find a lot of guide to connecting directly two modem. I tried doing some experiment with a couple of modem i had laying around but without success. Pots and modem aren't really something a know a lot of, (that's the reason of the plan B, a lot simpler and something i understand)

I also look into ATA to modem and i will probably use that for the device i cannot connect via serial, but it seem that a lot of them don't support the pcm codec (or at least i didn't found info about it), i never used one so my knowledge about them is limited, but the article should help me. also they will stay in a dedicated network so congestion shouldn't be a problem.

Thank you a lot!

rs232 is simple singalling with high and low voltages and a ground pin. Ethernet is differential signalling with no ground. That makes ethernet less susceptible to ground loop issues, and interference issues from long distances. I think it's just something to be aware of, and you likely won't have to do much to adress. But don't plan for running an rs232 to your neighbor's house without isolation.

Many modems (notably Dreamcast modems) require line voltage to work. A normal POTS line provides a good amount of DC voltage when the line is off hook for various reasons, and a modem may use that for powering parts of itself or as a check to confirm it's properly connected. Something like this could help supply that voltage for picky modems http://dreamcast.onlineconsoles.com/phpBB2/guides_pcdcwin98....

Many modems will also default to waiting for a dial tone before dialing, but that should be configurable via the dialstring.

As with all things Telco, the terminology is strange. You're looking for the g.711 codec. It's basically what was used for T1s, so it's just right to fit a modem, because actually most ISPs were receiving calls via T1 --- that's how 56k works.

Scour Ebay for an old enterprise grade pbx?

I'd expect you to get them for nearly $0 as long as you dismantle and move them yourself. Be prepared to bring a good sized truck.

An old Partner ACS should get the job done on the cheap.