User`s guide

Page 74

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

Words [4 – 7] are only filled with non-zero values if the block of data in words [14 – 63] table are

actually values retrieved from the actuator at the address specified in word [4].

Therefore, if a read of table [15] indicates that

• Word [4] == 0, then the data is not valid (there are not valves with address 0 that can respond

with data).

• Word [4] =/= 0, then the data in the table came from a valve.

If the PLC is utilizing table [15], it is recommended that the PLC also poll table [20]. If an exception

message is returned, then the block of data won’t be inserted into table [15] but the exception message

indicator will be inserted into table [20].

If detected, and the PLC wants to try again, it should initialize table [15] to some known value and either

explicitly clear table [20] or wait until after the next diagnostic scan (which automatically clears table

[20]) before trying again.

Example Scenario: If there are multiple types of EIM equipment on the network and the table [15]

command requests a block of data from an actuator that is “not supported by that type of actuator” (but

is on the others), the particular actuator will return a Modbus Exception for that message and the block

of data will NOT be loaded into tables [15]. However, the exception codes will be loaded into table

[20].

For example, if requesting a block of registers [15 - 30] from unit #5 and it is a

• 320A system … the block of registers is returned.

• TEC2000 system … an exception code is returned.

Notes on table [15] use:

• It is recommended that the PLC only keep this operation active until the data is received and

then deactivate it (setting table [0] – word [14] = 0).

• When the PLC writes data to this table, the 1746-C only overwrites the last 50 words with the

data sent in. The rest of the table is zeroed out.