ProSoft Technology MVI71-MCM - Manual (Page 71)

Reference MVI71-MCM ♦ PLC Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 71 of 111
December 28, 2007
The block number defines the Modbus port to be considered and the slave node
to be accessed. For example, if sending a command to Slave 1, Block ID 1001
should be used (from Port 1). Blocks in the 1000 range are directed to Modbus
Port 1, and blocks in the 2000 range are directed to Modbus Port 2. The slave
address is represented in the block number in the range of 0 to 255. The sum of
these two values determines the block number. The other parameters passed
with the block construct the command. The Internal DB Address parameter
specifies the module's database location to associate with the command. The
Point Count
parameter defines the number of points or registers for the
command (100 registers or 800 coils). The Swap Code is used with Modbus
function 3 requests to change the word or byte order. The Modbus Function
Code
has one of the following values 1, 2, 3, 4, 5, 6, 15 or 16. The Device
Database Address
is the Modbus register or point in the remote slave device to
be associated with the command. When the command receives the block, it will
process it and place it in the command queue. The module will respond to each
event command block with a read block with the following format:
Read Block
Offset Description Length
0 1000 or 2000 1
1 Write Block ID 1
2 0 = Fail, 1 = Success 1
3 to 63 Spare 61
Word two of the block can be used by the ladder logic to determine if the
command was added to the command queue of the module. The command will
only fail if the command queue for the port is full (100 commands for each
queue).
Command Control
Command control blocks place commands in the command list into the
command queue. Each port has a command queue of up to 100 commands. The
module services commands in the queue before the master command list. This
gives high priority to commands in the queue. Commands placed in the queue
through this mechanism must be defined in the master command list. Under
normal command list execution, the module will only execute commands with the
Enable parameter set to one or two. If the value is set to zero, the command is
skipped. Commands may be placed in the command list with an Enable
parameter set to zero. These commands can then be executed using the
command control blocks.
One to six commands can be placed in the command queue with a single
request. The following table describes the format for this block.
Write Block
Offset Description Length
0 5001 to 5006 or 5101 to 5106 1
1 Command index 1
2 Command index 1