henning@thor.acc.stolaf.edu (Mark D. Henning) (10/09/89)
Here is my promised posting of the solution to my 2 wire serial problem. I got two answers (one of them by several people). The first answer would be very expensive and unprofitable. This was to get a pair of short-haul modems (about $300) at a fixed baud rate( 2400 I believe). Anyway, This was not viable, because a network at 2400 would be unbearable. The other solution I have posted below. It is very good, however, I am unsure whether or not it works, because I finally convinced telecommunications to let me rewire the jacks (the 4 wire had been run, but only 2 connected.) Anyway, Thanks again to all who sent me the ideas. ^Mark, ^You actually have three wires available. The two in your room and ^ground. The best option of all would be to use the phone ground. If ^the phone system is hooked up in a standard way, one of the leads in ^the phone box will be ground. You can use this ground for both the ^phone AND the RS-232 ground line since the potential of the dirt is ^the same in both cases (phone ground should be an actual ground -- if ^it's not, see the water pipe option). This works less well with ^farther distances, but the phone ground is probably identical (within ^50 mV or so) in both rooms anyway. To check if the phone ground is ^a real ground, measure the resistance between it and a water pipe ^(metal - metal pipes go into the dirt), though it actually doesn't ^matter as long as both phones are on the same ground circuit. Another ^way to check the line would be as follows: ^ ^ Room 1 Room 2 ^ ------ ------ ^ ^ ---> A1 -------[ ] [ ]------- A2 <----[VOM] ^ | RS-232 [wall] [wall] RS-232 | | ^ |--> B1 -------[ ] [ ]------- B2 <------ | ^ | | ^ | | ^ ---> G1 -------[ ] [ ]------- G2 <-------- ^ ^Tie all three wires together in 1 room. Measure resistance between ^phone-net wire 1 (A2) and phone-net wire 2 (B2) -- it should be ^low (like 100, 200 ohms) -- if so, that means that the 2 wire setup ^is hooked up OK, no measure either of the phone-net lines in room 2 ^(for ex. A2) to the ground in room 2 (G2). If that is low too, it ^should all work fine. ^ ^Note: Make SURE that the ground wire is used ONLY for the RS-232 ^ground, NOT Tx or Rx -- hooking either of these to ground might ^fry the computer (the computer can't drive the dirt to +/- 12 VDC, ^and if it tries, it will probably fry). ^ ^Other ground wire options include: ^ Since you are both in the same building (I infer from your post), ^ you can both hook the RS-232 grounds to the building ground to ^ complete the circuit. If possible, you would use a water pipe, ^ then a building frame ground (assuming it is really grounded) -- ^ many times window frames, door frames will be connected -- you ^ need to check this first. ^ ^ Last choice would be the ground in a 3 prong outlet -- this is ^ dangerous if the building was not wired correctly. Sometimes the ^ ground wire is wired to the neutral wire or even worse the hot ^ wire. If so, this becomes dangerous. Also, I don't like this ^ option since it puts a wire near 120 VAC. ^ ^ Good Luck, ^ Rob Ginn ^ eraps1@nadc.arpa -- ------------------------------------------------------------ All opinions are my own, as are the spellings any deviation from the norm is just an effect of late nights and early mornings, with little sleep between.
hah@inteloa.intel.com (Hans A. Hansen) (10/10/89)
In article <7225@thor.acc.stolaf.edu> henning@thor.acc.stolaf.edu (Mark D. Henning) writes:
$
$Here is my promised posting of the solution to my 2 wire serial
$problem. I got two answers (one of them by several people).
$
$The first answer would be very expensive and unprofitable.
Stuff deleted.
$
$The other solution I have posted below. It is very good, however, I
$am unsure whether or not it works, because I finally convinced
$telecommunications to let me rewire the jacks (the 4 wire had been
$run, but only 2 connected.)
$
$Anyway, Thanks again to all who sent me the ideas.
$
$^Mark,
$^You actually have three wires available.
WARNING *****
This solution while sounding good will give you nothing but grief!
This is a very noisy, both in terms of signal quality and RF emissions,
answer to your problem.
You said that you have 4 wires available... use them. You will need a
good ground return for both TX and RX signals. Twisted pair wires also
offer common mode noise rejection and RF emissions reduction.
If you are still not convinced then I suggest that you invest in RS-232
transceivers... you'll need them. To really solve this with this type of
circuit you will need opto-isolators.
Hans
More stuff deleted.