Siemens S7-200 SMART Common questions about serial communication

1. Does the S7-200 SMART support Modbus ASCII communication mode?

STEP 7- The Micro/WIN SMART software does not provide the Modbus ASCII communication mode instruction library. If the S7-200 SMART CPU is used for Modbus ASCII communication, it requires the user to use the free port communication mode for programming.

2. Can the RS485 port (port 0) and the SB CM01 signal board (port 1) integrated with the S7-200 SMART CPU function as the Modbus RTU master station or slave station at the same time?

S7-200 SMART V2.2 supports two communication ports as the primary MODBUS RTU station at the same time, but not as the secondary MODBUS RTU station.

3. How does the S7-200 SMART CPU as Modbus RTU master access the hold register whose Modbus address range is greater than 49999?

Generally, the Modbus protocol's hold register range is between 40001 and 49999, which is sufficient for most applications, but some Modbus slave hold register area addresses will be outside this range. The Modbus RTU master protocol library supports holding register address ranges (40001 to 49999) or (400001 to 465536). If the Modbus slave address range is 400001-465536, simply assign the appropriate value to the Addr parameter when invoking the MBUS_MSG subroutine, such as 416768.

4. When the S7-200 SMART CPU, as the Modbus RTU master station, invokes the MBUS_MSG command for many times, why does the command show 6# error code?

1. Only one MBUS_MSG command can be activated at a time. If multiple MBUS_MSG commands are activated, ** MBUS_MSG commands will be executed. All subsequent MBUS_MSG commands will abort execution and 6# error codes will appear. Multiple MBUS_MSG commands need to be executed in polling mode.

2. The proposed address area allocated in the Modbus library storage area overlaps with the V storage area already used in programming.

5. The S7-200 SMART CPU functions as the Modbus RTU master station. When the slave station is faulty or the communication line is disconnected, the master station attempts to send multiple request packets, which results in a long communication time. How to reduce the number of retransmissions of the master station to improve communication efficiency?

When the MBUS_MSG command is activated, it will send the Modbus request message frame. If the master station does not receive the response from the slave station within the time interval defined by the Timeout parameter, the Modbus RTU master protocol library will send two more Modbus request message frames by default. The Done completion bit of the MBUS_MSG command is set to 1 only after the last request frame is sent and no reply is received after the interval defined by the Timeout parameter. Changing the value of the mModbusRetries variable from 2 to 0 can cancel the retry times of Modbus request frames. The ** address of the mModbusRetries variable can be obtained by querying the symbol table of the Modbus RTU master protocol library, as shown in Figure 1.