RS485 Biasing
RS485 Biasing
(OP)
Hi experts,
I have the following RS485 network setup (2400bps):
Slave1 ---1.5Km----Master---300m---Slave2
I know that I must use 120 terminating resistors at
both ends. No problem, because the boards have jumpers
that can select the terminators.
But I must use bias as well (620 ohm resistor in A and B)
My quesyion is: Can I put this bias resistors in the master board (that is more acessible) or must be done also in one end?
Another question: This is a non isolated network with independent power supplies. Should I connect the 3rd wire (ground)? to take care of the common mode voltage? is is not necessary?
Thanks in advance for your help
I have the following RS485 network setup (2400bps):
Slave1 ---1.5Km----Master---300m---Slave2
I know that I must use 120 terminating resistors at
both ends. No problem, because the boards have jumpers
that can select the terminators.
But I must use bias as well (620 ohm resistor in A and B)
My quesyion is: Can I put this bias resistors in the master board (that is more acessible) or must be done also in one end?
Another question: This is a non isolated network with independent power supplies. Should I connect the 3rd wire (ground)? to take care of the common mode voltage? is is not necessary?
Thanks in advance for your help





RE: RS485 Biasing
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: RS485 Biasing
Why the 620 ohm resistors??
Keith Cress
kcress - http://www.flaminsystems.com
RE: RS485 Biasing
@Keith
The 620 ohm resistors are connected bettewen "A" and Vcc (one) and B and GND (the other). Because when the line is in idle mode (no transmit) is at unknow state. The 2 resistors are bias to make sure the line is at "mark" state.
I have read something about this, and the experts say that is better to add this 2 resistors. I just don't know is is better to put them at the end of the line or can be at the middle.
@IRStuff
The mains is the same. Only the power supplies are diferent. (They are industrial switching power supplies).
RE: RS485 Biasing
Your comment about idle makes no sense to me. Unless you actually tristate the transmitter, its idle state is the "1" logic state. The purpose of that is a means of detecting a lost input.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: RS485 Biasing
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: RS485 Biasing
But otherwise you can put the bias resistors where ever it's most convenient - probably the master.
Keith Cress
kcress - http://www.flaminsystems.com
RE: RS485 Biasing
RE: RS485 Biasing
The bias resistors are typically placed at the master, but can theoretically be placed anywhere. According to everything I have read on RS-485, your statement about holding the line in the mark state was correct; the bias resistors are not specifically for detecting a line breakage.
I was recently sent to a data center to diagnose a modbus (RS-485) communications problem between our equipment and the building management system. Debugging the receiver data showed that the first few bytes of the transmission would appear and then nothing else would be received until the line returned to a quiescent state. The problem turned out to be a floating power supply (ground) that did not create reference between the logic ground and the RS-485 'ground'. As soon as the transmission would start, the parasitic capacitance would charge up and push the lines outside of the common mode range. Adding the ground bond from the power supply to chassis, which also connected to the RS-485 ground solved the problem.
RE: RS485 Biasing
RE: RS485 Biasing
Terminating, biasing and add a ground reference solve completly the problems on my network. The switching power supplies are isolated, so I completly lost my reference (normally given by earth wire). Biasing was also important because the network was picking some noise.
I have learn a lot about the so called 2 wire networks, that in fact are 3 wire :)
Thanks for all the help
RE: RS485 Biasing
Yep! Complete BS. Just like the so-called "one wire" sensors.
Keith Cress
kcress - http://www.flaminsystems.com
RE: RS485 Biasing
Note 3 and 5 cover the wiring fairly well.
http://www.robustdc.com/techResources.htm
RE: RS485 Biasing
Time and time again when explaining this I get the blank stare along with...
"I thought 485 is differential..."
Sigh.