alexis@panix.uucp (Alexis Rosen) (12/04/90)
I posted a query here a month or so ago, complaining about some very strange
problems with a bunch of 2400bps modems connected to my unix box.
While I'm not sure of this, it now seems likely that the problems are the
result of poor grounding. I think this because the Telebit performs flawlessly,
and the US Robotics nearly so. They both have three-prong (grounded) plugs on
their transformers. The other three modems (a Smart One, a Prometheus, and a
Maxxum) lack grounding, and they frequently die (although the Smart One is
rather less susceptible).
The problem is that the modems and the computer frequently fail to recognize
that DCD and/or DTR control lines have been raised. Thus, the modems may not
answer, and if they do, the getty may not complete the open.
Is there anything I can do to deal with this situation? If not, I suppose I'll
need to get new modems. Which cheap modems have a ground connector? The ones
I know of, which are generally trouble free, don't have these.
Thanks,
---
Alexis Rosen
Owner/Sysadmin, PANIX Public Access Unix, NY
{cmcl2,apple}!panix!alexiskjh@pollux.usc.edu (Kenneth J. Hendrickson) (12/04/90)
In article <1990Dec4.023427.25652@panix.uucp> alexis@panix.uucp (Alexis Rosen) writes: >They both have three-prong (grounded) plugs on >their transformers. >Which cheap modems have a ground connector? I think that this is not relevant. I would be willing to wager that the DC power from the plug-in power supply is floating, the phone lines are somehow isolated, and the modem must be "grounded" to the computer via the RS-232 cable. I wouldn't wory about it. -- favourite oxymorons: student athlete, military justice, mercy killing Ken Hendrickson N8DGN/6 kjh@usc.edu ...!uunet!usc!pollux!kjh
kreed@telesys.cts.com (Kevin W. Reed) (12/04/90)
>>>alexis@panix.uucp (Alexis Rosen) writes: >I posted a query here a month or so ago, complaining about some very strange >problems with a bunch of 2400bps modems connected to my unix box. [stuff deleted] >The problem is that the modems and the computer frequently fail to recognize >that DCD and/or DTR control lines have been raised. Thus, the modems may not >answer, and if they do, the getty may not complete the open. >Is there anything I can do to deal with this situation? If not, I suppose >I'll need to get new modems. Which cheap modems have a ground connector? >The ones I know of, which are generally trouble free, don't have these. I've run into this problem several times. I've seen it caused by two problems: 1) Not enough power to the modem. If you have a bunch of modems plugged into the same strip (outlet) and that circuit is being overdrawn, the ones with the cheap power supplied (or AC/DC converters) may not have enough power to operate properly (This sometimes happens when you use an adapter from another modem type). 2) Sometimes plugging in a phone into a modem causes it to not work properly for answering the phone. The phone rings, but the modem doesn't see it. I doubt it is caused by a ground problem although I'm not an electrician. >Thanks, >--- >Alexis Rosen >Owner/Sysadmin, PANIX Public Access Unix, NY >{cmcl2,apple}!panix!alexis -- Kevin W. Reed --- TeleSys Development Systems -- PO 17821, San Diego, CA 92177 TeleSys-II BBS & telesys.UUCP +1 619-483-3890 - Telebit PEP Line +619 483 0965 UUCP: {nosc,ucsd}!crash!telesys!kreed -------- Internet: kreed@telesys.cts.com
kaufman@Neon.Stanford.EDU (Marc T. Kaufman) (12/05/90)
In article <1990Dec4.023427.25652@panix.uucp> alexis@panix.uucp (Alexis Rosen) writes: >I posted a query here a month or so ago, complaining about some very strange >problems with a bunch of 2400bps modems connected to my unix box. >While I'm not sure of this, it now seems likely that the problems are the >result of poor grounding. I think this because the Telebit performs flawlessly, >and the US Robotics nearly so. They both have three-prong (grounded) plugs on >their transformers. The other three modems (a Smart One, a Prometheus, and a >Maxxum) lack grounding, and they frequently die (although the Smart One is >rather less susceptible). >The problem is that the modems and the computer frequently fail to recognize >that DCD and/or DTR control lines have been raised. Thus, the modems may not >answer, and if they do, the getty may not complete the open. RS-232 pin 7 is the logic ground reference point for signals (e.g. DCD, DTR). In my experience, it is preferrable to ISOLATE the modem from system ground to avoid ground-loop voltage offsets. In any event, the green wire ground is not intended to carry current -- it is a protective circuit in case of failures in the modem power supply. It should be connected to the case and/or chassis, but NOT to logic ground in the modem. A more probable scenario is incorrect wiring of some of your AC outlets -- reversal of the Hot and Neutral leads. I have seen equipment that couples logic ground to the presumed Neutral lead (usually through .1 uf and 10-12K in parallel). If your plug is on another AC phase and wired incorrectly, you will see 60 Hz AC superimposed on your modem signals. Marc Kaufman (kaufman@Neon.stanford.edu)