ProSoft Technology MVI69-MCM - Manual (Page 122)

Reference MVI69-MCM ♦ CompactLogix or MicroLogix Platform
User Manual Modbus Communication Module
Page 122 of 167 ProSoft Technology, Inc.
March 22, 2011
5 When the internal countdown register decrements to zero (0), the Slave
Status register for that slave will be set to a value of one (1), which will allow
a command to be sent in an attempt to re-establish communication with the
slave.
The Slave Disable and Slave Enable Special Control Blocks can be used to
access the Slave Status table and control slave polling through ladder logic by
forcing values into the Slave Status table in a manner similar to how the module's
internal firmware operates automatically.
Using the Disable control will force a value of three (3) into the internal module
register that holds the Slave Status information for that port and slave address. In
this situation, when a Slave Status register is set to three (3), the slave at the
corresponding address will not be polled, even if there are commands in the
Command List that are enabled and addressed to that slave. Since the E
RROR
D ELAY C OUNTER
is not involved in this operation, the status register will remain
set to 3, disabling the slave, until it is re-enabled with the Enable control.
Using the Enable control will force a value of one (1) into the table. When a Slave
Status register is set to one (1), the slave at the corresponding address will be
polled as specified in the Command List.
Here are the Special Control Block codes for performing these functions.
Block ID Description
3000 Disable list of slaves on Port 1
3001 Enable list of slaves on Port1
3100 Disable list of slaves on Port 2
3101 Enable list of slaves on Port 2
Port 1 slaves can be disabled using block code 3000 and can be enabled using
block code 3001. Port 2 slaves can be disabled using block code 3100 and can
be enabled using block code 3101. Each output (Write) block can contain a list
of up to 59 slave addresses to disable or enable.
Write Block - Disable Slaves
Here is structure of the Disable Slaves output image block.
Offset Description Length (words)
0 3000 or 3100 1
1 Number of slaves to disable with this block 1
2 to 60 List of addresses of the slaves to disable 59
61 to ( n ) Spare (present only if Block Transfer Size > 60) ( n - 60 )
( n ) = 120 or 240 (if configured)
The module will respond with an input image block with the same identification
code and indicate the number of slaves disabled by this function.