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Using an Old Modem to Communicate with Modern Computer

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Bill_Loguidice

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OK, this is a silly question, I know, but is it somehow possible to communicate with another computer, say a modern PC running Hyperterminal, by using a modem of any speed (say 300 or 1200 baud) on a vintage system. In other words, I'd have a modem of some type on the vintage computer and something on the PC on the other end, perhaps another modem for direct phone line connection, so this way the two modems were connected directly to each other. Is something like that even possible? The reason why I ask is that there are certain systems that have no other means of communication (or storage) other than via modem. Hopefully that's clear enough... Thanks!
 
Hi Bill,

Modems can generally be told to ignore the presence of a dial tone, but they do expect the line to be 'live', ie: have some voltage on them. The voltage is provided by the phone network. You can simulate this condition with a simple circuit:

http://www.jagshouse.com/modem.html


The other way to do this is to eliminate the modems all together and to use a null modem connection. If you can do this (ie: the machines have serial ports, not just built in modems) this is the preferred way .. The machines can think that they are still talking to each other through modems, but there is less equipment and no 'line simulator' circuit in the way. I was using this technique to use a PCjr as a terminal to a Unix box, and for file transfer before I started using TCP/IP over Ethernet. It is generic and will work on any system with a serial port.


Mike
 
It might be useful to know what you're trying to accomplish.
Data transfer? Simulate a BBS?
And perhaps the machine(s) involved.
 
I have no personal experience with these things, but you are on the right track.

That is a seriously professional piece of equipment .. overkill for what you are doing. Toward the more simpler end, you are just providing enough current/signal to trick the modems into thinking the line is live .. simulating a PBX like this unit does is not necessary.
 
ahm said:
It might be useful to know what you're trying to accomplish.
Data transfer? Simulate a BBS?
And perhaps the machine(s) involved.
Click to expand...

Data transfer. Hypothetical machines: A TI-CC-40 with a 300 baud Hexbus modem to a modernish PC (assuming the latter would be the easiest). The machines should be irrelevant though I'm assuming, as if there was a way to reliably do modem-to-modem via phone line without actually dialing a real number, it should theoretically work for anything.

This would be useful in general for a few different systems, but in the CC-40's case, it would be a possible way to get around the inability to save any data externally.
 
mbbrutman said:
I have no personal experience with these things, but you are on the right track.

That is a seriously professional piece of equipment .. overkill for what you are doing. Toward the more simpler end, you are just providing enough current/signal to trick the modems into thinking the line is live .. simulating a PBX like this unit does is not necessary.
Click to expand...

While I'm trying to bring my electronics skills up to a reasonable level, building something like that would be well beyond my abilities foreseeably for the next few years... At the same time I wouldn't want to spend more than $30 - $50 for a turnkey solution that would solve the "problem". Also, I'm in no particular rush, but it's something that's been bothering me on and off from a thought standpoint. I have a million other projects/things in this regard to keep me more than busy...
 
Well, if it works out let me know. The other big uses for circuits like this is using two old telephones as intercoms, something for the kids to play with. (This summer it might be worth doing some digging to put buried cabling out to the play house so that I can call the kids instead of yelling out the window.)
 
Bill,

Modem to modem can be done, I have used that method before, but only PC to PC. I have no experience with dis-similar platforms, so although I believe it's possible, I wouldn't know how to go about it. I do recall that I had more trouble with some machines than others, so there probably are some hardware issues with some modems, as Mike pointed out. If you'd like to experiment for educational purposes, one setup that comes to mind is a Zenith SuperSPORT laptop with internal modem jacked into an Everex 2400 baud (Rockwell chipset) modem in a generic-type '386, running Telix on both ends. Telix does have the ability to fake a telephone carrier so the two modems can talk, but I'm klewless which modems this is possible with. The software does make a difference, IIRC, as Procomm+ would not do the trick for me.

--T
 
I don't understand the bit about faking a carrier - that doesn't make sense at all. There is no 'faking' a carrier signal. Changes in the carrier signal are what convey the data. Without the carrier and the changes to it, there is no data transmission.

You can't connect two modems to a 'dead' piece of copper wire and get anything to work. The whole principle of the system is that a change one one end (a noise being made) causes a change in the current on the line, which produces a result on the other end. If there is no current on the line, the modems can squeal all they want - no electrons will move on the wire.
 
mbbrutman said:
I don't understand the bit about faking a carrier - that doesn't make sense at all. There is no 'faking' a carrier signal. Changes in the carrier signal are what convey the data. Without the carrier and the changes to it, there is no data transmission.

You can't connect two modems to a 'dead' piece of copper wire and get anything to work. The whole principle of the system is that a change one one end (a noise being made) causes a change in the current on the line, which produces a result on the other end. If there is no current on the line, the modems can squeal all they want - no electrons will move on the wire.
Click to expand...

Makes absolute sense, but how did it work under Terry's scenario? Could that have been a function of those particular modems?
 
Wrong wording, my bad. I s'poze that it doesn't "fake" the carrier, but actually sends a real signal so the modem knows it's connected. I don't know how much is dependent on hardware, software, phase of the moon, or whatever, just that YMMV. I do recall spending a lot of time trying to get a Kaypro to talk to a PC via modem, but was never successful.

--T
 
mbbrutman said:
You can't connect two modems to a 'dead' piece of copper wire and get anything to work.
Click to expand...

I'm pretty sure I did it using SmartModem 1200s many years ago. Used ATD on one end and hit ATA or ATO at the other.

I also did it using two Zoom v.32bis modems. Same way...
 
The challenge will be trying it with two different makes of modems from different eras using different communications packages. I doubt I'll have time tonight, but if I do, I'll do some tests. I think MD's suggestion is probably the only one that would be work for my desired situation though...
 
Many of the "newer" modems don't talk 300, and some don't do 1200. If you're older modem is slow, try finding a 2400 baud or slower modem for the "newer" end.
 
Many machines have built in modems, and the serial port is not accessible. There would be no way to eliminate the modem on a machine like that.
 
Thank you Mike!!

I am very interested to find out whether having two different
communication softwares on two ends would work flawlessly. On the
other hand, if it would be necessary to have the same communication
software on both ends where one system is under DOS/Win and another
system is under, let's say cp/m, then one alternative might be a
communication software such as Kermit. I am throwing this to get a
feedback from much more experienced comrades. I have been thinking
along the same ideas for doing serial communication between a PC and
a vintage computer. What are the alternative comm softwares?

Thank you

ziloo
 
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