ProSoft Technology MVI71-MCM - Manual (Page 92)

MVI71-MCM ♦ PLC Platform Reference
Modbus Communication Module
Page 92 of 111 ProSoft Technology, Inc.
December 28, 2007
Write
Block
Offset Internal
Database
Register Content Description
52 5060 Minimum
Command
Delay
This parameter specifies the number of milliseconds to wait
between issuing each command. This delay value is not
applied to retries.
53 5061 Command
Error
Pointer
This parameter sets the address in the internal Modbus
database where the command error will be placed. If the
value is set to -1, the data will not be transferred to the
database. The valid range of values for this parameter is -1
to 4999.
54 5062 Response
Timeout
This parameter represents the message response timeout
period in 1-millisecond increments. This is the time that a
port configured as a master will wait before re-transmitting a
command if no response is received from the addressed
slave. The value is set depending upon the communication
network used and the expected response time of the
slowest device on the network.
55 5063 Retry
Count
This parameter specifies the number of times a command
will be retried if it fails. If the master port does not receive a
response after the last retry, the slave devices
communication will be suspended on the port for Error
Delay Counter scans.
56 5064 Error
Delay
Counter
This parameter specifies the number of polls to skip on the
slave before trying to re-establish communications. After the
slave fails to respond, the master will skip commands to be
sent to the slave the number of times entered in this
parameter.
60 5069 Guard
Band
Timeout. A value of 0 uses the default baud rate or you can
set a timeout value in milliseconds (0 to 65535)
Guard Band Parameter
With multiple nodes in the network (multidrop), the module must be able to define
the time period used to properly recognize the received data sequence as a valid
Modbus message. This time period is defined through the guardband timer
parameter. Failure to set the Guard Band functionality may cause successive
communication errors in the Modbus network.
This functionality is only used for Modbus RTU mode, when the module is
configured as a slave device in a multidrop network (RS-422 or RS-485 wiring).
In RTU mode, when the module receives sequence of data from another node, it
will recognize the end of the message when it detects a 3.5 character gap. The
next byte received after a 3.5 character gap will be recognized as the start of a
new Modbus message.
The 3.5 character gap depends on the port communication settings, especially
the baud rate. The higher the baud rate, the lower the corresponding time period
equivalent to the 3.5 character delay.
To use the default Guard Band Timeout parameter, set the Guard Band Time as
follows:
Guard Band Time = 0