RS485 - Effect of termination resistor??

[microcon555]

What is the effect of termination resistor while using single slave unit?

Reading various posts in this topic i found that RS485 will work without terminating resistor, But if I add a terminating resistor in a single slave network, it stopped working. What is the reason for this and how to solve this.

I am using max485 for slave and usb-rs485 converter from ftdi for checking.

Thanks
MicroCon

 

[IanP]

What is the effect of termination resistor while using single slave unit?

Reading various posts in this topic i found that RS485 will work without terminating resistor, But if I add a terminating resistor in a single slave network, it stopped working. What is the reason for this and how to solve this.

I am using max485 for slave and usb-rs485 converter from ftdi for checking.

Thanks
MicroCon

The role of the termination resistor is to match the impedance of the transmission cable and reduce reflections. With this resistor in place, sometimes, the levels can be close to, or below the threshold level of the receiver. To fix levels issue you have to fit bias resistors, one between Vcc and (+), and one between GND and (-)..see attached picture..
There are also other things to keep in mind. 1st, on a short cables this resistor is not used. 2nd, the converters (usb-485) sometimes are designed in a doggy way and do not simulate true rs-485 transceiver..
:wink:
IanP

 

[microcon555]

Thanks for your interest..

The max485 which have fail safe build in. Is it necessary to bias it externally?

Does this biasing will have any effect on the network when it is used in large number of slaves, say 10 slaves?

Thanks again
MicroCon

 

[IanP]

Is it necessary to bias it externally?
Does this biasing will have any effect on the network when it is used in large number of slaves, say 10 slaves?

Biasing will only have positive effect on the network, regardless of how many devices are connected; it defines the correct dc-levels.
I'd suggest you connect the bias resistors together with the termination resistor at one place ...
:wink:
IanP

 

[microcon555]

Thanks for your information. I'll test it and update with results

MicroCon