RS485 -> RS232 -> V34 -> RS232 -> RS485

Hi,

Apologies if this isn't the correct place to post this. It's not
strictly an electronics design issue. In addition, it may be a stupid
question.

Suppose:

I have an RS485 transmitting device at L1.
I have an RS485 receiving device at L2.

I get from L1 to L2 by the following steps:

At L1 I convert the RS485 data to RS232 data and feed it into a modem.
At L2 I read the data via the modem and convert it from RS232 to RS485
again.

At L1 I mismatch the TX+/- lines (ie. L1 device TX- connects to RS485
converter TX+, and L1 TX+ to RS485 converter TX-).

At L2 I mismatch the TX+/- lines (ie. L2 device TX- connects to RS485
converter TX+, and L2 TX+ to RS485 converter TX-).

Will L2 receive the data transmitted by L1 correctly?

That is, will both mismatches cancel each other out?

Thanks

 

Hmm.. interesting, my hunch (from field experience) is it would work.
But I'm not sure. Maybe the first and last bytes gets corrupted. Anyone
knows for sure?

 

Hi,

I would expect, that it won't work, because the Modem needs to be configured
to dial... and with the wrong polarity of signals most modems wouldn't
understand, what you want.

Marte

 

My arse is itchy, if I scratch it will it stop itching?

 

Ah, but the modems are on a private line and dial/answer regardless of
data being present...

 

Interestingly, if I interface to the RS232 at L2, I have to right bit
shift and then negate the data for it to look similar to what
originates at L1...

So yes, it looks like the mismatches may be cancelling each other out.

I'd best fix that then ... since I need to tap off the RS232 data at
L2.

Locations are (very) remote and outside so thought I'd better check
before sending someone into the wind + rain to open the cabinets and
swap both sets of wires.

 

I guess this depends both upon the size of your arse and the vigour
with which you scratch it...

 

Hello,

This depends on the modem and its settings.
If the modem is set up for example with error correction, it will look at
the asynchronous RS232 charcater framing (start bit, stop bit) which is not
valid any more if you change polarity. In this case you won´t get the
correct data on the other end.
If the modem uses a direct mode without any data framing (=it directly
modulates the data bit by bit) it will work. Unfortulately the other case
is the most usual.


Tom

 

only if you are using synchronous modems.

if using asynchronous modems (eg, common consumer modems) the mis-match
will garble the data. because the start bit won't look like a start bit.
and an idle period will look like a break.

you may get good data some of the time (data dependant) if using async
modems that run the same DCE and line bitrates but modems with any sort
of compression, bit-rate conversion, or error correction will probably
garble your bitstream beyond recognition. (in consumer async modems
this essentially rules out anything on the consumer market capable of
faster than 9600bits per second, and a good number of 1200 and 2400
capable modems,

If you only send odd (or is it even?) bytes it could also work - atleast
in the middle of the packets. ends could still be garbled.

Bye.
Jasen

 

Thanks, the modems are 33.6K Multitech 2834BLK models that have been
set to synchronous mode via the switches on the side and run at
9600bps.

I'll get the jumper settings on each modem sometime Monday, and we'll
see what happens when source and dest wires are reversed.

Tony