Working With Non-standard Devices

Some non-standard PLCs support the Modbus protocol. However, some of these I/O devices do not support the protocol correctly. When Plant SCADA tries to communicate with these I/O devices, you might get errors on startup or during communication.

To handle these non-standard I/O devices, the Modbus protocol has some special options to change its operation:

1. On startup, Plant SCADA tries to read some digital inputs to verify that the PLC does exist. If Plant SCADA cannot read these bits, it assumes the PLC is bad, and remains off line. Some non-standard I/O devices do not support digital bits, so this command results in an error. With the [MODBUS]InitType parameter, you can change the type of variable the Modbus protocol tries to read on startup, to allow communication to start.

2. The standard MODBUS driver allows reading of 2000 bits in one request. Some non-standard I/O devices do not support such large reads. You can change the maximum read size with the [MODBUS]MaxBits parameter. (The default is 2000.)

   Change the MAX_LENGTH field for the Modbus protocol (in the \Plant SCADA\BIN\PROTDIR.DBF file) to the same value. Re-compile your project after changing this option. Be aware that if you reinstall Plant SCADA, the PROTDIR.DBF file is overwritten and you will need to repeat the change.

3. Some I/O devices that operate over radio modems also require the padding of protocol messages. The padding characters wake the radio modem to allow the rest of the message to be transmitted. Set the [MODBUS]Pad parameter to the number of characters to pad, and the [MODBUS]PadChar parameter to the ASCII code of the character to pad. For example, to pad 20 0xFF characters to the start of the message, set:

   [MODBUS]
   Pad=20
   PadChar=255

4. In the Modbus protocol, LONG data types default to a simplified implementation, with a shortened range of 0 to 655,359,999. To increase the LONG data type to the full range, set the [MODBUS]LongDataType parameter to either 1 or 3.

As a LONG is composed from two registers, the register order used by Plant SCADA must match the order used by the PLC. Refer to the help accompanying the [MODBUS]LongDataType parameter to determine the appropriate value.

Note: The MODBUS driver supports a number of variants of the MODBUS protocol to communicate with some non-standard PLCs. See the topic MODBUS protocol variants MODBUS Protocol Variants to determine which variant you should use when configuring a device.