Re: Splitting serial cables ?

In article <F7jiQDz4lkLG@xxxxxxxxxxxxxxxxxxxxxxxx>,
briggs@xxxxxxxxxxxxxxxxx writes:
> In article <v%vDf.2975$Iw6.196541@xxxxxxxxxxxxxxxxxxxxx>, "Neil Rieck" <n.rieck@xxxxxxxxxxxx> writes:
>>
>> <briggs@xxxxxxxxxxxxxxxxx> wrote in message
>> news:oCgSttFou3DR@xxxxxxxxxxxxxxxxxxxxxxxxxxx
>>> In article <43DD3DBC.6E32BF1E@xxxxxxxxxxxx>, JF Mezei
>>> <jfmezei.spamnot@xxxxxxxxxxxx> writes:
>> [...snip...]
>>>
>>> Transmit data is on one of those wires. You have two stations both
>>> wired into transmit data. Both of them will be holding that lead
>>> in the space condition (or mark -- I can never remember) when idle.
>>> This is not tri-state logic where the lead floats when idle.
>>>
>>> If you've ever put a breakout box on an RS232 connector, you'll have
>>> noticed that TD and RD are always live.
>>>
>> [...snip...]
>>>
>>> You can have one source sending data to multiple receivers. That's
>>> no problem since the receivers aren't trying to coerce the signal line.
>>>
>>> You can't have multiple sources sending data to a single receiver.
>>> That's a problem since multiple senders are each trying to coerce
>>> the signal line differently.
>>>
>> Umm, if you read my original post you'll notice that I said 2 diodes were
>> required to isolate the two transmit lines from each other. Only one phase
>> of the signal (negative if memory serves) is required to send data from
>> transmitter to receiver. Without the diodes one transmitter would be
>> positive while the other is negative and you would have a problem.
>
> I posted before seeing the reports that diodes were a possible solution
> that had actually been deployed successfully.
>
> What I recall of RS232 in action is that transmit and receive data
> use positive and negative voltages to signal mark and space. Zero voltage
> is never used.
>
> Checking online I see that the signalling range is technically -3 to -25V
> denoting a logic 1 (mark) and +3 to +25V denoting a logic 0 (space).
>
> Actual implementations are more likely to keep the signals down to
> 8 to 14 volts according to the first reference I found.
>
> No matter. A diode can still work in any case. It just needs to be
> capable of dealing with 50 volts end to end without breaking down.
>
> And it needs to leak enough current so that even when both transmitters
> are idle and, consequently, both diodes are blocking current flow
> that enough signal leaks through to keep the receiver seeing the
> mark condition.
>
> One poster mentioned a pull-up resister tied to DSR, presumably to
> address with this leakage requirement.
>
> Mind you, I'm speaking from theory, not from practice. Looking at
> LEDs on a breakout box is about as far as I've ever actually mucked
> about with the relevant hardware.

While many people have written to say this is a sure thing and that the
diodes are the answer, I can assure you that as long as 25 years ago I
had to trouble shoot a terminal<->host problem that turned out to be a
host that output only TTL on it's serial port (+5v mark - 0v space) and
a terminal (Wyse, I think) that followed the standard correctly and
refused to work as it never saw a space. +3 to -3 is defined as not
valid data. I would expect devices today to be much more likely to
stick to standards as it avoids a whole bunch of finger pointing when
things don't work. Be aware!!

bill

--
Bill Gunshannon | de-moc-ra-cy (di mok' ra see) n. Three wolves
bill@xxxxxxxxxxxxxxx | and a sheep voting on what's for dinner.
University of Scranton |
Scranton, Pennsylvania | #include <std.disclaimer.h>
.

Relevant Pages

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