An RS232 cable question or two

I'm just a hobbyist, not an expert. Others can and should chip in. But I have the RS-232 spec handy and figured I'd give it a shot.

I can't answer about how good your capacitance measurement is. Probably should be done with an LCR meter set to 10kHz and 100kHz. Good twisted pair runs 15pF/ft or less... but cable for RS-232 would be roughly 40pF-50pF/ft. Your 300' of cable measurement comes out at 7.7pF/ft. Which doesn't seem likely.

RS-232D specifies at most 2500pF for data rates up to 9600 and receiver impedance between 3k and 7k Ohm. The bit times for 9600 is 104us. So the tau=RC=7k*2500pF=17.5us. That's 6 tau per bit time. At 19200, the bit time is 52us, so the tau should be half of 17.5us or 8.75us and the capacitance should be 1250pF or less. If the cable were older RS-232 cable at

Also, actual testing of cables shows that the RS-232 driver and receiver pairs actually do better than the standard says. Receivers usually aren't 7k Ohm, for example, but less. So that's good.

Since these were based on worst case 7k ohm receivers... just guessing at this... I'd say you've got a good shot at it working okay at 60' and 19200. Of course, none of this takes into account noise and nearby machinery effects. Or ground loop currents, if any. But I'd say it's worth a shot and that you will probably be okay. (I know I've run more than 60' of non-twisted pair cable at 19200 without any trouble --- from a VAX 11/780 computer to consoles placed outside the raised floor, air-conditioned room.)

But maybe an expert will chip in and set me straight.

Jon

You could build your own driver with "pre-comp"... boost the leading edge dramatically. I've done that for ~1000' of cable. That's so old a design (pre-CAD) it's own paper (pre-CAD). I'll see if I can find it in my archives. ...Jim Thompson

Really, obtain a couple of 422 or 485 to 232 converters and do that. Even if it works while you're looking at it*, the additional CMRR of 485/422 is vastly worth it, and the adapters don't have to cost much.

Thanks for the reply Jon. I was dubious about how accurately my meter was measuring the capacitance. But since the cable is pretty new, and I'm running less than 75 feet, I think I'll be OK. I do have the option of slowing down the data rate and programs now transfer between machines in less than 30 seconds usually so slowing the data rate down won't really slow me down much. Especially since I'm only transferring programs about once a day. Thanks, Eric

schreef in bericht news: snipped-for-privacy@4ax.com...

Cables - all cables - have resistance, conductance, capacitance and inductance. Measuring the capacitance of long cables will be influenced by the inductance. The rate of that influence depends on the cable, its length and the measuring method. To get an indication of the capacitance you'd better take a shorter piece of cable. Let's say a 5ft and a 10ft piece. Inductance will have hardly influence as capacitance is (almost) always measured at relative low frequencies. If a double length measures double the capacitance you know you're on the right track.

Whether or not an unshielded cable will do, depends mainly on the disturbing EM-fields in the environment. Engines, switchers, triac controls and the like. I've used longer cat 5 cable in an office environment without problems. But I also had to provide a solution for the same length of cables - though at higher speed - in a factory hall. So if it's no problem to try, give it a try. Otherwise you'd better go for the RS232 to RS458 converters. (If even they fail, you can go for glassfibre. I once had to do for a very long cable.)

petrus bitbyter

I don't think 60 feet at 19600 will be too much problem.

The real issue in machine shops is stray (big) voltages between local "grounds". I've seen no end of fried interface chips, and worse, in industrial,environments. You need galvanic isolation.

Try it! I've run RS232 over a city block at 9600.

I've done similar without any problem.

But I also had some customer leading a cable along a gutter for maybe 25m happily paying the bill 4 or 5 times a year when his equipment was blown and had to be replaced. Relative cheap office equipment but nevertheless. He kept saying it was only temporarily and he was right off course. Everything in this life is temporarily isn't it? Though this time it took over 2 years if memory serves.

On another location a double shielded ground cable of about 100m did 9600 RS458 very well until the next thunderstorm. Eventually I used glass fiber to solve the problem.

petrus bitbyter

In a machine-shop environment, it might be better to use a differential protocol for long cables. One could consider Ethernet twisted-pair cable, and a little Ethernet/serial converter near the machine tool. It's probably going to be useful to have Ethernet wires later, anyhow.

the problem with rs-232 is isn't not even a real standard. If it works with 6 foot cables, great, and most stuff does.

For long distances, using fiber to serial converters works great. It's not cheap though. It does eliminate grouding problems and noise issues though.

It certainly is a standard, or if you're a pedant, a series of standards. It's defined by the EIA (and probably a dozen others). It works with cables a *lot* longer than 6', but there is a length-data rate tradeoff.

Where is fiber in the RS-232 standards? RS422 or RS485 makes a lot more sense than fiber, here, but RS-232 should work fine.

When I designed the very first 1488/1489 chips in the mid-60's there WAS a standard. Now almost anything that switches between two levels gets called RS-232. ...Jim Thompson

a series of old standards nobody really follows. Some of the worst offenders are those usb to serial dongles which struggle to work with anything.

notice they keyword "converter". Since you'd locate them just a few feet away from your equipment you mostly eliminate problems with cables and what kludged, random version of rs-232 somebody tried to implement in your devices.

The original poster mentioned machine tools, not a dumb terminal attached to a minicomputer in the other room. Complete electrical isolation may be a good thing to have in this case.

It's also likely some ebay optical converers could be cheaper than blown out serial ports in the long run too.

bingo.

I was fiddling with a BASIC stamp recently and there's some hack mode to fake serial output using just +5 and 0V. It will run a "rs-232" interface vacuum flourescent display with no errors, so far, but it's such a sketchy hack, like most serial stuff these days.

I'd like to see how the +5 to 0 serial plays with a classic terminal like an ADM5a or ADDS Regent 100.