Modbus Rtu Master; Using The Modbus Rtu Master Instructions - Siemens SIMATIC S7 System Manual

● Modbus RTU slave protocol: Modbus communication uses a CRC (cyclic redundancy
check) to ensure the integrity of the communications messages. The Modbus slave
protocol uses a table of pre-calculated values to decrease the time required to process a
message. The initialization of this CRC table requires about 11.3 milliseconds. The
MBUS_INIT instruction performs this initialization, which normally happens during the first
scan after entering RUN mode. You are responsible for resetting the watchdog timer if
the time required by the MBUS_INIT instruction and any other user initialization exceeds
the 500 millisecond scan watchdog time. Writing to the outputs of the module resets the
output module watchdog timer.
MBUS_SLAVE extends the scan time when it services a request. Calculating the Modbus
CRC extends the scan time by about 40 microseconds for every byte in the request and
in the response. A maximum request/response (read or write of 120 words) extends the
scan time by approximately 4.8 milliseconds.
9.3.2

Modbus RTU master

9.3.2.1

Using the Modbus RTU master instructions

STEP 7-Micro/WIN SMART and the S7-200 SMART CPU support two Modbus RTU
Masters. For a single Modbus RTU Master, use the instructions MBUS_CTRL
(Page 481)and MBUS_MSG (Page 483). For a second Modbus RTU Master, use the
instructions MB_CTRL2 (Page 481) and MB_MSG2 (Page 483).
If you use two Modbus masters in your project, make sure to use different port numbers for
MBUS_CTRL and MB_CTRL2.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Libraries
9.3 Modbus RTU library
479