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4. Configuration
103
MODBUS Slave Protocol General Parameters
The following pictures show the MODBUS protocol initial screen (Figure 4-15) and its general
parameters:
Figure 4-15. MODBUS RTU Slave Configuration Screen
Configuration
Description
Default
Options
Slave Address
MODBUS slave address
1
1 to 255
Table 4-54. Slave Configurations
Communication Times
The MODBUS slave protocol communication times, found in the “Advanced…” button on the
configuration screen, are divided in: Task Cycle, Send Delay and Minimum Interframe.
Configuration
Description
Default
Possibilities
Task Cycle
(ms)
Time for the instance execution
within the cycle, without considering
its own execution time
50
20 to 100
Send Delay
(ms)
Delay for the transmission response
0
0 to 65535
Minimum
Interframe
(chars)
Minimum silence time between
different frames
3.5
3.5 to 100.0
Table 4-55. Modbus Slave Advanced Configurations
Notes:
Task Cycle:
The user will have to be careful when changing this parameter as it interferes directly in
the response time, data volume for scan and mainly in the CPU resources balance between
communications and other tasks.
Send Delay:
The answer to a MODBUS protocol may cause problems in certain moments, as in the
RS-485 interface or other half-duplex, for example. Sometimes there is a delay between the master
request time and the physical line silence (master delay to put RTS in zero and put the RS-485 in
high impedance state). To solve this problem, the slave can wait the specified time in this field before
sending the new answer. On the opposite case, the first bytes transmitted by the salve, during the
answer could be lost.
Minimum Interframe:
The MODBUS standard defines this time as 3.5 characters, but this
parameter is configurable in order to attend the devices which do not follow the standard.
The diagnostics and commands of the configured MODBUS Slave protocol are stored in variables of
type T_DIAG_MODBUS_RTU_SLAVE_1 (Table 4-56):
Diagnostic Variable
T_DIAG_MODBUS_RTU_SLAVE_1 *.
Size
Description
Diagnostic Bits:
tDiag.*
bRunning
BOOL
The slave is in execution mode
bNotRunning
BOOL
The slave is not in execution (see bit:
bInterruptedByCommand)