Modbus Driver - Master/Slave communication issue

Hi, 

From the description, two things stand out: the master/slave direction and the float values always returning 0. Both usually point to Modbus mapping and data format rather than a Kepware service issue.

First, about the Device ID behavior.
If communication only works with <127.0.0.1>,0, it usually means the ALE111 is acting as the Modbus master and Kepware is operating as a slave/server. In that case, the device in Kepware should be configured using the Modbus TCP Server driver, not the Modbus TCP/IP master driver. When Kepware is configured as a master while the DCS is also master, the communication pattern becomes inconsistent and tags may not update correctly.

Second, the float values always showing 0 is very commonly caused by word order or register mapping issues. Yokogawa systems often store floating point values using different byte/word order than the Modbus default.

Things worth checking:

1. Verify the driver type
   If ALE111 is polling Kepware, switch the channel to Modbus TCP Server so Kepware behaves as the slave.

2. Check float byte order
   In Kepware tag settings try different float formats such as:

   • Float (ABCD)

   • Float (CDAB)

   • Float (BADC)
     Yokogawa systems frequently require word-swapped floats.

3. Confirm register alignment
   Float values require two consecutive registers. If the starting register is incorrect (off by one), Kepware will read zeros or invalid values.

4. Verify scan rate vs device update cycle
   If the DCS only updates certain parameters cyclically, set a slower scan rate (for example 500–1000 ms) to avoid stale reads.

5. Check address type
   Ensure the correct Modbus memory type is used (Holding Register vs Input Register). Some DCS parameters are read-only and only exist in Input Registers (3xxxx).

The key point is that the <127.0.0.1>,0 behavior strongly suggests the communication direction is reversed, so confirming whether ALE111 is master and Kepware must run as Modbus TCP Server would be the first thing to verify.

Thanks,