PM8000 and Modbus TCP/IP

[michael_franz]

TEAM:

We have been experimenting with our fleet of PM8000 meters situated in our lab.  Two out of four would not communicate over Modbus TCP/IP but would report fine with ION protocol.

A colleague of mine, Bryan Robertson and I discussed why Modbus TCP/IP worked on some meters and not others.  After we mapped out all the differences between the devices Bryan noticed that the two meter that were not working on Modbus TCP/IP had their serial port set to gateway mode in ION Setup. 

This morning we disabled this function and now are able to read the device data over Ethernet with the Modbus protocol.  Wanted to let you know and give a shout-out to Bryan for seeing something that the rest of us missed.  . 

There is no reason that enabling the com-port gateway mode should stop Modbus TCP/IP upstream communication.  We trust that this will be addressed in the next firmware revision

Thanks and congratulation to Bryan.

We hope that this save hours of grief for others who have this problem.

ADDITIONAL NOTE:

Apparently the default Modbus address is locked in at 255 when the Gateway mode is enabled.  This may have been covered in training at some time and I forgot.  It would be nice if this were indicated during setup to remind users of this fact.

[Marvin_Foster]

Hey Michael  -  long time,

I believe that this behavior is intentional.  When Gateway is enabled, looks at the Unit Id on the data request and if it is ID=255 it uses the registers in the meter for it's response.  If the ID=something else, it routs that request to the serial port to a presumed downstream devices.

When Gateway is Disabled, the meter ignores the unit id and responds to all register requests from the meter register table.

[michael_franz]

Marvin:

It is great to hear from you.

It is still confusing and buried in documentation.  The rev of the PM8000 manual covering this is missing the address change detail.  It would be nice it ION Setup or the web page of the meter let the user know that the unit ID had changed.  It will become one of those things you just gota know now.

Thanks,

Your friend,

Michael Franz

[patrickw]

This is correct, and for people who find this case in the future, this behaviour is the same for all of the devices with the modbus gateway functionality (ION7550, ION7650, PM8000, ION7400).

Unit ID 255 is one of the reserved addresses in the Modbus spec, typically used to address the gateway device itself while all other unit IDs are passed downstream.