MODBUS line resistance

[geir_eas_no_bfo]

Hi,

have a PAS800 connected to a RMH (LV481004)

The RMH has no function for easy termination of line resistor (120ohm).

The question is if it is realy nessecary? Could it be included in the RMH already?

It works without fine without, but generally it should be a line resistor in all topology`s star/ serial.

BR

GIA

[Romain_Polette · Accepted solution]

Hello @geir_eas_no_bfo , 

You can consider the following general rules for RS485 2 wires bus:

1. 120 R resistor is needed on each extremity of the bus to avoid signal reflection
2. 120 R can be avoided if your bus is less than 50 m
3. 650 R resistor is needed at one point to polarize the bus

Panel Server integrate both polarization and termination features.

If you have only an RMH connected to the Panel Server, its fine to remove termination on both side of the bus (deactivate feature on PAS).

Regards, 

[Guillaume_Evrard]

Hello @Romain_Polette ,

I would keep the termination on the PAS side ?

Kind Regards