RS485 Modbus RTU - Active / Passive Master Possible?

harryting

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Need a sanity check here. It is possible to have redundant Modbus RTU master, but only one master is actively polling at a time?

The background is that I want to run RS485 to a remote location through unstable terrain, so in the case of broken wire, I will still have the data through a backup radio that will be powered by a battery pack.
 
Yes, although broken could mean shorted, and broken could mean you lose the terminating resistor. so consider using a 485 repeater or some other means to cater for this failure mode.
 
Ian makes a GREAT point.

If both your masters are connected via daisy-chained RS485 at your master location, you want a signal isolator between your master station and the rugged wilderness. Maybe even one with a PLC-controllable power supply so you can disconnect any spurious signals from the hardwired line.

There's no standard way to make a Modbus RTU master "listen" or "standby". Either you will need full control of the master, or a device that compares their status and chooses which is enabled.
 
If I were trying it, I'd use a signal quality relay with gold plated contacts to switch between the hard-wired isolator and the RS-485 radio modem. That way, the master is only communicating over one of the two over-the-distance links.



RS-485-radio-modem-back-up-to-wiring.jpg


The question will be whether the #2 RS-485 radio modem on the field side will 'tri-state' if the master has nothing on its RS-485 inputs. If it tristates, then the hardwired connection is likely to work OK, although one might have to experiment with parallel connection of the radio modem and hard wiring on the field side or a serial 'daisy chain' connection.


My diagram shows a parallel connection, but I'd probably start with a daisy chain 'serial' connection on the field side.


Good luck trying to find a radio modem vendor who can tell you what happens on the #2 end when the #1 has no signal.
 
Thanks, Ian, Ken, and danw.

It's great point on the signal isolator or repeater and I'll have to do some bench testing to see how all it will work out. The existing setup is a completely battery powered remote location with Banner Radio that acts like a 485 wire replacement and we are trying to get low voltage power supply wires there along with signal but keep the existing radio as a backup in case of line break. The master side is a compactlogix with Prosoft Modbus card. It will be a while before we try this out but I will try to come back and give an update.
 

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