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MAX232, Sharing RxD lines

G

Guest

Jan 1, 1970
0
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
 
C

CFoley1064

Jan 1, 1970
0
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


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
 
B

budgie

Jan 1, 1970
0
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?

Yep, done often.
The MAX232 spec shows an input resistance of ~5K ohms which
seems low.....

Most RS232 devices have low input resistance.
 
P

Peter Bennett

Jan 1, 1970
0
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.
 
C

CWatters

Jan 1, 1970
0
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).
 
G

Guest

Jan 1, 1970
0
OK, this is good news. Thanks to all for the comments.

Richard


CWatters said:
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).


RS232.
 
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