MAX232, Sharing RxD lines

Guest
I have a system where I have one device talking to two others via RS232. I
have the two receivers RxD lines tied together and to the transmitters TxD.
Is this OK? The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....

Richard
 
I have a system where I have one device talking to two others via RS232. I
have the two receivers RxD lines tied together and to the transmitters TxD.
Is this OK? The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....

Richard
Click to expand...

The classic RS-232 receiver chip, the DS1489, has an input impedance of 4K
ohms. It's pretty much standard to assume a 3K load. If you look at the
MAX232 data sheet, you'll see that the output voltage will bog down about a
volt if you double up (going from a current draw of 3.3 mA to 6.6 mA). Still
got plenty of headroom there.

Short answer -- you've got nothing to worry about as long as you have a
"standard" RS-232 output, or even anything close to it. It's not common
practice, but it's been done before, many times. Go for it.

http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

Good luck.
Chris
 
On Tue, 14 Oct 2003 17:43:05 GMT, <rsoennichsen@sbcglobal.net> wrote:

I have a system where I have one device talking to two others via RS232. I
have the two receivers RxD lines tied together and to the transmitters TxD.
Is this OK?
Click to expand...
Yep, done often.

The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....
Click to expand...
Most RS232 devices have low input resistance.
 
On Tue, 14 Oct 2003 17:43:05 GMT, <rsoennichsen@sbcglobal.net> wrote:

I have a system where I have one device talking to two others via RS232. I
have the two receivers RxD lines tied together and to the transmitters TxD.
Is this OK? The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....
Click to expand...
Yes - one RS-232 transmitter should be able to drive a few receivers.
However, you cannot have multiple transmitters talking on the same
wire.



--
Peter Bennett VE7CEI
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
 
The short answer is that it should be OK but might be a problem if you have
very long cables and low power drivers. If in doubt, load up your
transmitter (include any cable capacitance) and measure the peak to peak
wave form (at the max bit rate) at the input to one of the receivers with an
oscilloscope

I believe that the minimum RS232 logic levels (eg with transmitter loaded)
is +/- 5V and that receivers are specified to operate with input levels as
low as +/- 3V (the difference between +/- 5 and +/- 3 is to provide a noise
margin).


"Peter Bennett" <peterbb@interchange.ubc.ca> wrote in message
news:ttmqovkng5n30t08amcn7mgkqhk22hciar@4ax.com...
On Tue, 14 Oct 2003 17:43:05 GMT, <rsoennichsen@sbcglobal.net> wrote:

I have a system where I have one device talking to two others via RS232.
I
have the two receivers RxD lines tied together and to the transmitters
TxD.
Is this OK? The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....


Yes - one RS-232 transmitter should be able to drive a few receivers.
However, you cannot have multiple transmitters talking on the same
wire.



--
Peter Bennett VE7CEI
GPS and NMEA info and programs:
http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
Click to expand...
 
OK, this is good news. Thanks to all for the comments.

Richard


"CWatters" <colin.watters@pandora.be> wrote in message
news:OJfjb.79654$Av2.4264991@phobos.telenet-ops.be...
The short answer is that it should be OK but might be a problem if you
have
very long cables and low power drivers. If in doubt, load up your
transmitter (include any cable capacitance) and measure the peak to peak
wave form (at the max bit rate) at the input to one of the receivers with
an
oscilloscope

I believe that the minimum RS232 logic levels (eg with transmitter loaded)
is +/- 5V and that receivers are specified to operate with input levels as
low as +/- 3V (the difference between +/- 5 and +/- 3 is to provide a
noise
margin).


"Peter Bennett" <peterbb@interchange.ubc.ca> wrote in message
news:ttmqovkng5n30t08amcn7mgkqhk22hciar@4ax.com...
On Tue, 14 Oct 2003 17:43:05 GMT, <rsoennichsen@sbcglobal.net> wrote:

I have a system where I have one device talking to two others via
RS232.
I
have the two receivers RxD lines tied together and to the transmitters
TxD.
Is this OK? The MAX232 spec shows an input resistance of ~5K ohms
which
seems low.....


Yes - one RS-232 transmitter should be able to drive a few receivers.
However, you cannot have multiple transmitters talking on the same
wire.



--
Peter Bennett VE7CEI
GPS and NMEA info and programs:
http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq
Click to expand...
 
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