Modbus Mode versus Character Mode

The serial ports can operate in either Modbus mode or character mode.

When operating in Modbus mode, the serial port can be accessed by:

  • local HMI for management and diagnostics

  • a programming PC, for maintenance and diagnostics

  • a modem connected to a central location for SCADA and data collection via RS232

When operating in character mode, you can use a serial port to:

  • Send ASCII characters to a printer or a modem.

  • Display data on the screen of a terminal and receive data from its keyboard.

Configuring Modbus versus Character Mode

By default, the serial ports operate in Modbus mode, but can be switched to character mode using the L9_XMIT or XXMIT instruction in the application program. When the instruction is completed, the serial port remains in character mode. To return to Modbus mode, cycle power to the processor or download the original configuration to the processor.

In Modbus mode, the serial port communicates using the Modbus protocol. In Modbus mode, you can use the XXMIT EF block in Control Expert to:

  • Designate each serial port to serve as either a Modbus master or slave.

  • Specify the signaling protocol that will be used by the RS282/RS485 configurable serial port.

NOTE: When using XMIT, L9_XMIT, or XXMIT EF blocks on the two serial ports of the 171 CBU 78090 CPU, create an EF block instance for each port. Do not dynamically change the port number. Set the port number as an initial value for each instance because the block stores one single port configuration.

Port Parameters

The RS282/RS485 configurable serial port and the RS485 fixed serial port present the following properties and range of settings when operating in Modbus mode:

Parameter

Settings Include...

signaling protocol

  • fixed serial port

    • RS485 (fixed)

  • configurable serial port

    • RS232

    • RS485 (default)

operations mode

  • slave (default)

  • master (using the XXMIT instruction)

baud rate

  • 300

  • 600

  • 1200

  • 2400

  • 4800

  • 9600 (default)

  • 19200

parity

  • even (default)

  • odd

  • none

mode/data bits

  • 8-bit RTU (default)

  • 7-bit ASCII

stop bit

  • 1 (default)

  • 2

Modbus address

  • 1...247

  • 248 indicating point-to-point addressing

Default = 1