modbus over rs485

[jonkers]

Hello all. 

Hoping somebody could maybe give me some ideas on things to try. 

I have a Prosoft MVI56E-MCM Master unit.  There is 3 nodes on it to begin with that were able to be read.  An Atlas Copco mkV gateway was added and 3 more nodes for the compressors.  As soon as the gateway is connected all comms where lost. 

I started to look into it not knowing too much about Modbus or RS485.  I found that there was a lot of errors even with the gateway disconnected.  I found that it was wired in a star, not as a bus.  I have since fixed this and now all the errors have gone.

But as soon as you connect the gateway it drops out.  I was messing around with the terminating resistor and noticed just by putting in the terminating resistor at the end with the gateway not connected it still drops out.  I am not sure what else to try so thought I would ask if anybody else had any ideas.

Thanks

[AndersJ]

There are multiple ways of defining the RS-485 signals,
such as A,B,+,- and possibly others as well.
In addition these are often mislabled, and the standard misunderstood.

Perhaps you need to reverse your connection to the gateway.

[jonkers]

I checked that.  I thought it may be wrong in the db9 connector at the gateway. 

I used a serial to USB adaptor and can Modbus scanner.  I removed the Prosoft master and from the laptop, I was able to poll all the devices and read the registers.

[ajb]

If adding the terminating resistor causes the link to drop, assuming the terminating resistor is connected correctly, that suggests that the bus is loaded to the point where it's only marginal functional anyway.  Are there perhaps extra terminating resistors connected?  Perhaps when it was wired as a start someone had put a termination resistor at the end of each arm?

You might try measuring the resistance between the A/B terminals with no driving device connected (possibly with all of the devices off if that can be arranged), if it reads ~120R then it's terminated properly at one end, if it reads ~60R then it's properly terminated at both ends.  If it reads lower with everything powered off then it may be biased somewhere or you might have an extra termination.  You can also measure the DC voltage on the bus when it's driven to a known state, and note what happens when the termination resistor is connected.

[jonkers]

Thanks for the reply.  I was thinking something is wrong when it dropped by adding the resistor.  I can unplug all the connectors when I get in tonight and have a look. 

I know at the moment there is only 1 resistor at the plc card.  I will double check what size it is.  I know it also drops when going directly to the gateway.  Unfortunately, we can not touch the gateway so I can not double check it's settings.