ION 7650 dual modbus master gateway

[Anonymous user]

          Is it possible to set an ION 7650 to act as a dual gateway for COM1 and COM2 using modbus master protocol? At the moment, there is 8 x PM5300 meters connected to COM1 using RS485 and 10 x PM5300 meters connected to COM2 using RS485, however I am only able to see one of those ports at a time, not both, depending on the serial connection I select as the gateway.

[sesa56307_bridg]

The ION7650 can certainly act as a Modbus Master on both COM1 and COM2 (retrieving data from downstream devices on the two serial loops, and making that data available in the ION framework for processing/logging).  But I believe the Modbus Gateway functionality (where a separate Modbus Master communicates using Ethernet through the ION7650 to downstream serial devices, in parallel with any Modbus Mastering by the meter) will only work on one serial port at a time, based on the "Modbus Gateway Connection" setup register.  Logically, the meter has no way to tell whether a Modbus Gateway request is for one port or the other, since the requests are just based on unit ID (which could be duplicated on the two serial loops).  So this is decided at configuration time.

If you need gateway-type functionality on both serial ports at the same time, you could consider using the EtherGate protocol instead, which permits a direct transfer of serial data to/from a downstream serial loop.  There are some caveats with EtherGate though (the Ethernet protocol is slightly different from Modbus TCP, the meter cannot Modbus Master that loop at the same time, and multiple master connections are not supported).  Please see the user guide for further information if this sounds interesting for your application.

[Hal_Etheridge]

Generally you want to avoid using ModbusRTU protocol over Ethergate.  Due to how the ModbusRTU protocol is designed certain error conditions are not detectable by the modbus master and can result in malformed, corrupted or invalid data when used over Ethergate.  It will result in an increased error rate.  How much these issues will be present is dependent how a number of factors including the network latency and variability, which device types are present and the reliability of the devices themselves (among several others).  These issues may be acceptable tradeoffs for some simple installations.

A better solution would be to either:

- Combine the serial loops together (avoiding duplicate unit ids of course).  This will reduce performance due to the increase in the number of devices on the loop but if the loops were not saturated previously there may not be any visible performance decrease.

- Use a Modbus TCP to RTU gateway (like the new ComX devices or the old EGX series or even another 7650).

The second option will most likely give better overall performance but at increased cost.  The first is (possibly) cheaper if there are not that many devices and/or duplicate unit ids on the existing loops which will require reconfiguration of those devices.

Which option is better for you is dependent on your individual system configuration.