User`s guide

Page 84

Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)

5.4.17. Table [20]: Modbus Exception Message Response

Words [4  63] PLC access: Read / Write

This data is filled anytime a communication error is detected due to the remote system sending a 5-byte

Modbus Exception Response message. These are “High-Level” Communication Errors … Application

Level. This table is cleared at the start of a diagnostic scan.

Format:

• Table [20] - Hi-byte: Error’d function code as returned by the remote system.

• Table [20] - Lo-byte: Modbus Exception Type (1  3)

The PLC may reset this table with any value is prefers. To do this, the PLC must insert the new value

into the table and send it to the 1746-C as a write command. The values in this table are not sent out

to the actuators on the network.

A read back of Table [20] will

• If a write command issued: reflect the write command accepted by the 1746-C

• If a read command issued: reflect the current values in the table.

If the PLC is utilizing tables [14 or 15], it should also poll this table. There are many reasons for this.

For example … if there are multiple types of EIM equipment on the network and the PLC requests data

from an actuator that is “not supported by that type of actuator” (but is on the others), the actuator will

return a Modbus Exception for that message and the values will NOT be loaded into tables [14 or 15].

However, the exception codes will be loaded into table [20].

Note: The intention of this table is not to identify (and maintain) this status in “real-time”. It is to only

serve as a general flag to the PLC/operator that an exception has occurred with this actuator

and that the PLC/operator might want to investigate more thoroughly later.