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Connecting a T440 terminal to a distant computer - modems are needed?

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blabsxar

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May 9, 2017
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Sao Paulo, Brazil
Hi!

I'm born on 80's and started to use computers around 2000 (Windows 98) but since I became a Unix user, I felt in love with Unix history, computer history and vintage computer in general. I recently acquired a DEC T440 terminal and, more recently, a SPARCClassic workstation. I want to increase my collection! :)

Since I don't know how things worked in the pre-internet era, I'm going to ask you a thing that may sound silly for most of you.

I successfully connected my T440 terminal to a modern amd64 machine running NetBSD via serial line (with a null modem cable). The question is that I want to have my terminal in another distant room. I couldn't, then, use a long cable (the limit for RS232 lines is 15 feet, isn't it?).

Is the correct solution for that use two modems that connect to the DTEs (T440 and the computer) via serial line and communicate each other using a telephone cable? I've heard about that, but I've never seen such setup.

If yes, are there any [cheap] external modems you'd recommend? Is it vintage computing either or are they still factored nowadays?

Thanks!
 
FWIW, the setup were I worked during my first job had a centralized computer system (minicomputers from Norsk Data), with about 100 (yes, one hundred) serial terminals connected to it. And those terminals were not only in the same building, some were in buildings a few kilometers away.
So how did it work? Mostly with what we called "current loop modems", boxes that converted to / from serial (RS-232C) to current loop on each end. The terminals ran on 9600 baud, IIRC.
 
As Tingo says, over long distances you need a pair of line drivers of some kind, usually but not exclusively current loop 'modems'; using 'normal' telephone line modems is possible but a little more complicated because you usually have to emulate at least partially a phone line, ringing, answering etc.

But 'true' RS-232 is pretty forgiving and the 15 foot limit is extremely conservative; unless you have unusual noise or ground loops you should be able to go much farther than 15 feet with only a null modem cable; distance is also related to speed so if you do have problems at a certain speed you can always drop down one.

There are also wireless solutions out there, but they usually require some tinkering.
 
I've run hundreds of regular rs232 lines in the 500ft-1000ft range. In almost all cases they work fine as null modem cables over that distance, although if they go outside or near heavy-duty electrical interference items (big electric motors, etc) they may not work. Up to a couple hundred feet generally always works.

If you do want to use a modem over a dialup line, you have a couple considerations
- You need two phone lines (one at each end) and two modems (one at each end)
- You need your software to support being able to dial/answer the modems. Lots of older software supports dial-up. Also if you know the standard Hayes AT codes for your modem, you can usually make your own settings for accessing the modem. I've used a dumb terminal to "talk" to the modem and dial another number and connect.
- You don't want a null modem cable between your computer and mode, just a standard straight-through rs232 cable.
- You need modems. I used tons of external US Robotics modems over the years with good luck, used ones are readily available.
- You should have a basic understanding of baud, flow control, DTE/DCE handshaking, etc, to help resolve the inevitable glitches.
 
No problem on the distance if it's less than a couple hundred feet. Use cat 5 or cat 6 cable, you can get serial to RJ45 adapters for each end, and then you have an Ethernet cable if you want to use it in the future. :) Heck, run two cables while you're at it, so you can have one for future use.
 
Like Doug, I've run very long cables >30m for RS232 back in the day. Distance over RS232C depends, among other things, the speed you'll be trying to push over it. At 9600 bps, you can probably go 100m safely, but use shielded cable to keep EMI down.

For very long haul, (i.e. up to 1000m) you can use special "short haul" modems, which are little more than RS232C-to-current loop adapters.

Some protocols, such as RS422, are differential and less susceptible to external noise.
 
Running any long rs232 cables outside (between buildings, for example) can be a big problem in lightning country. rs232 driver/receiver chips are very sensitive to voltage spikes that bad weather can induct into the cable. On occasion we used rs232-fiber optic converters for outside runs for EMI immunity, but this was back in the late 80's and in most cases fiber was cost-prohibitive.

But I just did a quick google look, it looks like there are plenty of affordable fiber converter units available these days. Fiber could also extend your usable distance considerably.
 
Doug G said:
Running any long rs232 cables outside (between buildings, for example) can be a big problem in lightning country. rs232 driver/receiver chips are very sensitive to voltage spikes that bad weather can induct into the cable.
Click to expand...

That's why they're usually in sockets ;-)
 
I've used RS-232 in very electrically noisy environments, in the 100-500' range with great success. As Chuck(G) points out, the speed you run will be affected. Don't expect to run 57600 at 500', but 19200 works on some and 9600 usually works on the rest.

Even indoors, with lengths like that, lightning is a big problem. You can buy a ton of 1488/1489s for very cheap and just replace them every time there's a storm. (When you're getting paid to do it, you don't mind, even if you have to desloder them.) Or, you can put optical isolators on each end of the line and never worry about it.


15'? Heck I've run SCSI-2 farther than that...
 
As others have stated, 232 *should* be good for 100 feet or so. If you have a lot of EMF or noise, the easiest way is to buy a pair of 232 to 422 (or 485) converters, and run a standard 2 pair line between them. 422/485 can easily run 500 feet or more, even in noisy environments. We used them on the ships I worked on before to run high speed serial from the nav suite to equipment all over the vessel.
 
You can buy a ton of 1488/1489s for very cheap and just replace them every time there's a storm.
Click to expand...
That's easy to say if it's your own equipment close to home. But replacing chips even if socketed is not exactly on the list of skills of non-technical business users half-way across the country :)
 
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