ProSoft Technology MVI46-MCM - Manual (Page 67)

MVI46-MCM ♦ SLC Platform Reference
Modbus Communication Module User Manual
ProSoft Technology, Inc. Page 67 of 108
March 29, 2011
Slaves are defined to the system when the module initializes the master
command list. Each slave defined will be set to a state of one in this initial step. If
the master port fails to communicate with a slave device (retry count expired on a
command), the master will set the state of the slave to a value of 2 in the status
table. This suspends communication with the slave device for a user specified
scan count (ErrorDelayCntr value in the MCMPort object for each port). Each
time a command in the list is scanned that has the address of a suspended
slave, the delay counter value will be decremented. When the value reaches
zero, the slave state will be set to one. This will enable polling of the slave.
Block ID Description
3002 Request for first 128 slave status values for Modbus Port 1
3003 Request for last 128 slave status values for Modbus Port 1
3102 Request for first 128 slave status values for Modbus Port 2
3103 Request for last 128 slave status values for Modbus Port 2
Block Request from Processor to Module
Word Offset Description Length
0 3002 to 3003 or 3102 to 3103 1
1 to 247 Spare 246
The module will recognize the request by receiving the special write block code
and respond with a read block with the following format.
Block Response from Module to Processor
Word Offset Description Length
0 Reserved 1
1 Write Block ID 1
2 to 129 Slave Poll Status Data 128
130 to 248 Spare 119
249 3002 to 3003 or 3102 to 3103 1
Ladder logic can be written to override the value in the slave status table to
disable slaves (state value of 3) by sending a special block of data from the
processor to the slave. Port 1 slaves are disabled using block 3000, and Port 2
slaves are disabled using block 3100. Each block contains the slave node
addresses to disable.