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
MAX232, Sharing RxD lines
Oct 14, 2003
4 Replies
Yep, done often.
Most RS232 devices have low input resistance.
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
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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).
ITxD.
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OK, this is good news. Thanks to all for the comments.
Richard
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