Modem to Modem networking revisited

[NathanAllan]

A while back I asked about networking via modems only, and really didn't get a satisfactory response. The idea was to use all these modems in these computers to create a local network without using phone lines. One suggestion was use the parallel ports and a kit not widely available. Another was to use a telephone test kit.

At the flea market today I got a telephone test kit *part,* a tone generator! I am hoping to use this instead of swapping the wire from the wall to get the modem to listen to the tone right before it would start to send its data. Now all that I have to do is switch it off. I scored for $5, I think, well worth it for just these projects!

ALL of my computers here have modems or compatible modems, while hardly any of them have networking hardware or at least easily networked ports. With this cool little device I'll be able to produce that signal and get the modems to talk (even though I would have been able to force them, that has been unsuccessful in the past).

Not meaning any kind of windows machines, but my Commodores and Ataris. Now they will be able to trade data along a common language.

A lot of you guys probably have done this already, this is an old task I know. But I've never been able to get it done before, and a telephone test kit is expensive and so is a signal generator (at least a small one, I have a really big one but I'm packed enough as it is).

I didn't find too many things today at the market, but did find a few small gems, like that tone generator, specifically for phone service. Loved finding that one.

 

[Ole Juul]

AFAIR you just reverse the wires (null cable) and you're on. I used to put two terminals back to back like that. That, however is not a network, only a p2p. (Hopefully the RIAA won't find out.. hehe) I am at a loss on how to do some kind of hub. Perhaps a couple of rotary step switches like the exchanges used to have - they're really cool! Still, how do you connect 3 computers at once?

What kind of tone generator did you get and what do you intend to do with it? I don't understand the relevance here. The ones I usually see are just for putting a tone on the line so you can see what wire goes where. Nothing to do with communication, just a simple way to do continuity testing. They're especially handy from floor to floor or room to room where you can't always easily confirm which wire goes where. If you wire things yourself you don't usually need it because you know what colour is what.

 

[MikeS]

I'm not sure what tone he's going to generate either unless it's dial tone. Direct modem to modem can be tricky with the old modems and many of them did need dial tone; it might be easier to have something like a DOS system with a modern modem or two in between. I guess he can just connect them all up in parallel and only turn on the ones he wants to use when the time comes.

I suppose he doesn't have serial ports available on all his machines; that might simplify things and perhaps give him more flexibility and maybe even more speed; that sounds like what you're talking about with your reversed 'null' cable.

Oh well, sounds like fun; good luck!

 

[glitch]

One of my old issues of Electronics Illustrated has plans for building a small supply that provides ring and hook voltages for a home telephone network. You ring other phones via the rotary dial -- most old modems could dial pulse, so that shouldn't be a problem. I haven't built it, or tried it with modems, but it might work. I've got all of the parts for it, so I could throw it together one of these days and test it out.

I'd been considering setting up some manner of internal network for modem connections at home as well. One thought I'd had was in setting up a small Asterisk PBX with real Digium boards. Those boads provide true ring and hook voltages. I've seen them used with old rotary phones, as well. I'm not sure of the potential data throughput you'd get from SIP encoded telephone connections with a modem, though.

When it comes down to it, all you really need is a supply to furnish the off-hook voltage between the two modems, and a method of inducing ring voltage to one of them. You won't have a dial tone or actual dial logic, so you set the sending modem to ignore a lack of dial tone. Setting the receiving modem to answer on the first ring, then applying your ring voltage (140 VAC, 20 Hz, I think) would get it to pick up. After that, the hook voltage should allow the two modems to communicate.

 

[Dutch]

I've done this in the past as an experiment just to see if I could get it to work, and was successful. I used this method:

http://www.jagshouse.com/modem.html, but modified it to use a 48 VDC source, which AFAIK is the correct off-hook voltage, instead. The modems I used were both 33.6bps, but would only reliably communicate at around 28.8. I was able to allow the sending side to ingnore the lack of a dial tone and force the reciver to go off-hook without a ring signal. IIRC this is a normal function, as my first 300bps modem from the early '80s didn't have auto-answer. This seemed simple, as it was just ATD on one end and ATA one the other

Dutch

 

[MikeS]

I've done this in the past as an experiment just to see if I could get it to work, and was successful. I used this method:

http://www.jagshouse.com/modem.html, but modified it to use a 48 VDC source, which AFAIK is the correct off-hook voltage, instead. The modems I used were both 33.6bps, but would only reliably communicate at around 28.8. I was able to allow the sending side to ingnore the lack of a dial tone and force the reciver to go off-hook without a ring signal. IIRC this is a normal function, as my first 300bps modem from the early '80s didn't have auto-answer. This seemed simple, as it was just ATD on one end and ATA one the other

Dutch

Yes, it's fairly straightforward if the modems are Hayes-compatible, but Commodore and Atari modems were generally dumb and wouldn't know what to do with an AT command (or any other command for that matter).

 

[NeXT]

I have always assumed a phone line simulator would do the trick (actually, I could see it being very useful at a computer convention when you want to show how acoustic couplers worked). I ahve seen them in a 1999 Black Box catalog but even to this day they are mad expensive. I wonder if a schematic for one exists and if it is easy to make?

 

[Lord Moz]

Other than having lots of modems, is there any other reason you want to use modems specifically?

Wouldn't nearly any system with a modem also have some form of serial communication available as well? I mean, aren't most modems interfaced serially, at least internally, anyway?

I would think going serial would be much more practical as it is much less complicated, and even more universal, as some system's modems can be difficult to find.

For connecting modems, or serial ports, together you need a terminal or console server of some sort. eBay has loads of these as dialup ISPs unload their old equipment. Many are available in TCP/IP connected versions, and some have matching "virtual serial port" drivers that will map a link to one of the ports of these terminal servers to a serial port on a modern Windows machine. Like this 16 port model here, or most of these.

By using a serial terminal server, you can connect machines without modems directly, and attach a matching modem to a port on the server for machines with modems.
After that, it's all a matter of software.

I purchased a number of Lantronix serial-tcp/ip adapters previously for a similar idea... many machines predate ethernet by a fair bit, but just about everything has serial. So serial to TCP/IP converters were developed to allow businesses to continue to use legacy equipment.
These converters can be had for very good prices if you keep your eye out. I purchased 4x Lantronix UDS-10 adapters for ~$10/each (no PS, but just use 9-30vdc).

The UDS-10 is an older model, no longer sold, but supports 10base-T at half duplex, with up to a 115Kbps serial link according to the manufacturer. This is much faster than modems were ever made, and these devices in fact will emulate a modem and respond to AT commands.
You can tell them to ATDT a url with target machine and port number if you want, and the target could be another of the same device, located in the same room, or across the planet.

So, the hardware is already available, and much of it can be had for very little money. Most of the rest is all about software and what you want to do once two machines are connected.

My idea for the UDS-10s was to get a SLIP/PPP connection going on a server machine, and have the client use a SLIP/PPP packet drive on the ethernet-less client. For machines even older that have no use for "full" internet connectivity, you could at least use LYNX to browse the text internet, or shell into a unix machine and still send files via x/y/z-modem to the client machine.

Serial port are the "universal translators" of the digital world. I am willing to bet that whatever interface standard our desktop machines are using 20 years from now will still have adapters available for it to connect with RS-232 devices :D
And with devices like the Lantronix UDS-10 (or equiv, no loyalty to them in particular), even after RS-232 is dead, ethernet is likely to be kicking for a good long while after that.
__
Trevor

 

[NathanAllan]

Yes, I have lots of modems, lots and lots of them. With more available if I ever wanted more. This is mainly the real reason, and why I don't use terminal emulation on say those Ataris and C64's is because of making disks is a real pain with what I have hooked up all the time (modern computer).

UDS-10's for $10 each?? If I ever see those at a price less than $50 I'm usually so short of funds (and rich in modems) that I have to pass on them.

So I still haven't gotten to this project yet (poor personal projects are all on the wayside right now to make a living fixing newer computers [funny all the old ones still work and never need me to fix them]) but one of these days I will.

I might have to take a vacation to get just my personal projects working! :D At least I'll have all the components on-hand.

 

[Ole Juul]

I wonder just how many modems you could get going at once. I mean, forget the wires, just get them answering. What I'm thinking is, this could be a musical project. /evil grin

 

[Chuck(G)]

What Lord Moz said, basically. If you want to interface a bunch of computers, why not multi-drop RS-422? That was (and is still in factory settings) a popular serial networking method. The first Novell networks used it and I have at least one old 286 motherboard that supports it directly.

 

[Lord Moz]

UDS-10's for $10 each?? If I ever see those at a price less than $50 I'm usually so short of funds (and rich in modems) that I have to pass on them.

My memory is getting fuzzy I guess, I looked through my records and see I spent $20/each on the UDS-10s.
There are a few on eBay at the moment in auction format with low starting prices. You could watch them and see how they finish. Looking at the completed listings, it looks like ~$25 + s&h is fairly normal. The $35+ listings were unsold.

I would love to get some kind of packet driver interface running on my Kaypro IV or Commodore 128D :D
I had also thought of using them on my Tandy 1000RL since it has only 1 ISA slot and it's parallel port won't support the parallel-ethernet adapters.

I've heard about Contiki running on the C64 & 128 & other older 8 bit machines, but I really can't figure out their site as to how to get any install disks or even read about various platform ports.

__
Trevor

 

[MikeS]

My memory is getting fuzzy I guess, I looked through my records and see I spent $20/each on the UDS-10s.
There are a few on eBay at the moment in auction format with low starting prices. You could watch them and see how they finish. Looking at the completed listings, it looks like ~$25 + s&h is fairly normal. The $35+ listings were unsold.

I would love to get some kind of packet driver interface running on my Kaypro IV or Commodore 128D :D
I had also thought of using them on my Tandy 1000RL since it has only 1 ISA slot and it's parallel port won't support the parallel-ethernet adapters.

I've heard about Contiki running on the C64 & 128 & other older 8 bit machines, but I really can't figure out their site as to how to get any install disks or even read about various platform ports.

__
Trevor

There are system-specific Ethernet adapters available for several of the classics including the C64; for most others if they have a serial port you can use a dedicated RS-232 <> TCP/IP bridge like the UDS-10, or use one of the free bridge programs that let you connect to the Internet through a PC.

As to Chuck's RS-422 idea, I think you could get an RS-232 terminal controller more easily and cheaply than a bunch of RS-232<>RS-422 line drivers; in fact I have a few 16-port RS-232 only units that I could let go for a pittance if anyone's interested; any unit can talk to any unit, ad hoc on demand or according to a fixed routing table, with baud rate and protocol conversion, etc. You'd still need one bridge modem or computer to get on the Internet though.

 

[Dwight Elvey]

I wonder just how many modems you could get going at once. I mean, forget the wires, just get them answering. What I'm thinking is, this could be a musical project. /evil grin

Hi
You can only have two connected at any one time to one POTs line.
There are only two channels, one for the transmit modem and one for
the answer modem. To connect multiple systems together, you'd need
to have a hub with multiple modems. One for each remote modem.
The modem circuits I've seen don't require the line voltage, tone
or ring signal to work. These just need the AT commands.
Dwight

 

[Dwight Elvey]

---snip---
Setting the receiving modem to answer on the first ring, then applying your ring voltage (140 VAC, 20 Hz, I think) would get it to pick up. After that, the hook voltage should allow the two modems to communicate.

90V and 30 Hz.
dwight

 

[MikeS]

When it comes down to it, all you really need is a supply to furnish the off-hook voltage between the two modems, and a method of inducing ring voltage to one of them. You won't have a dial tone or actual dial logic, so you set the sending modem to ignore a lack of dial tone. Setting the receiving modem to answer on the first ring, then applying your ring voltage (140 VAC, 20 Hz, I think) would get it to pick up. After that, the hook voltage should allow the two modems to communicate.

If it has an option to ignore dial tone, then it probably won't need line or ring voltages either; just hook 'em together (no need to reverse polarity either), manually answer one, and away you go.

 

[Dwight Elvey]

If it has an option to ignore dial tone, then it probably won't need line or ring voltages either; just hook 'em together (no need to reverse polarity either), manually answer one, and away you go.

I've never seen one that needed the line voltage. I also doubt the polarity makes any
difference either. Like you say, it is just a matter of if it has the commands to ignore
dial tone and to pick up without ring.
Dwight

 

[Ole Juul]

I've never seen one that needed the line voltage. I also doubt the polarity makes any difference either. Like you say, it is just a matter of if it has the commands to ignore dial tone and to pick up without ring.
Dwight

That makes sense to me. Like I mentioned above, you can certainly connect two terminals back to back - I've done it.

 

[MikeS]

That makes sense to me. Like I mentioned above, you can certainly connect two terminals back to back - I've done it.

Sounds like you're talking about direct connect; not quite the same thing, digital vs. analog, different cable, handshaking, etc.
But that's how I'd do it as long as there are serial ports on all the systems; BTW, most home computers are electrically equivalent to terminals.