RS-485 questions. Topology, etc.

[cwrasmussen]

I have a few boards setup with a sort of rs-422 / rs-485 hybrid. I have two "transmit lines" coming from a master board that are connected to the receive lines of the slave boards. Then, there are two separate lines ("receive lines") coming back into the receiver of the master board that comes from the drivers of the slave boards, so that they can respond.

Here's the problem: When transmitting on the "receive lines," I must have only one driver enabled on those lines at a time, but to do that I must only enable the driver when it is time for that slave to transmit (e.g. when it's been polled). For most of the time, nothing is transmitting, so all the drivers are disabled leaving a high-impedance output on the line. This is interpreted as garbage on the line.

I checked several differential-bus receivers for rs-485 and rs-422, and all of their data sheets say that if the voltage is between -0.2V and +0.2V the output from the receiver to TTL is indeterminate (which is where the junk comes from).

QUESTION: How do I set up this network, so that when my slaves aren't transmitting, the receiver on the master doesn't interpret it as a bunch of junk data?


I really appreciate any thoughts you might have on this. Thanks!!

P.S. Feel free to fill me in on any holes in my logic here.

 

[svicent]

To maintain the proper idle-voltage, you must apply bias resistor to force the data line to the idle condition. These bias resistor are pull-up resistor on the data B line, typically to +5V, and pull-down resistors, to ground, on the data A line. You must generate enough DC bias current to maintain at least 200 mV between data A and B lines.

This application note can help you:

h**p://www.bb-europe.com/bb-euro/literature/tech/485appnote.pdf

 

[cwrasmussen]

Man, thanks a million!!! That worked perfectly! I must've been trying to figure this out on my own for about 2 weeks now.

 

[gariantoni]

Would you know how much a RS485 cost? Where do you usually buy this?