Problem: Can't receive RS485 by more than one receiver

[eehadi]

Hi. I have a problem using RS485.
I can transmit data using RS485 (ISL83485 IC) and receive it while only one receiver is connected, but when I connect both receivers, I can't receive data correctly in the farther device (located about 2 meter far from sender).
Am I wrong about connecting more than one device to sender or the transmitter IC can't support enough current. I connected R+, R- of two receivers directly to D+, D- of transmitter. is any resistor needed?

 

[betwixt]

Most RS458 transmitters can drive 32 receivers so there shouldn't be any problem. Yes, you should have terminating resistors, at least at the most remote end and you should be using a balanced cable, preferably a twisted pair of wires. You should still also have a ground connection between the two ends, although the signal is differential if you omit the ground, one or both signals can go outside the permitted limits for the receiver.

Brian.

 

[eehadi]

There are termination resistors in boards and cables are balanced but ground of the farther device is isolated, I hope connecting grounds solves the problem.
Is ground wire necessary? Is OK that I connect ground of all circuit directly to each other?

 

[betwixt]

Check you only have ONE terminator at the most distant receiver, in some circumstances you may need another at the sending end but there should never be more than two in a network.

Yes, a ground is required, the data is generated by making one wire carry a higher votage than the other and at the receiver it detects which wire has the highest voltage to recover the data. Because it is differential, it has very good tolerance of differences in ground voltage between each station but if the difference is great enough that one or both wires carries voltage above the allowed level with respect to that stations ground, it will not work. So you do need a ground but it doesn't have to be as good as required by some other signalling methods.

Brian.