Specifications

Open CTM (Revision D)

172

MODBUS Messages

11 MODBUS Messages

The CTM-15x modem can be used in MODBUS mode for SCADA applications. This allows the modem to

operate as a simple SCADA device that can understand and format MODBUS messages based on various

modem parameters, such as inputs and outputs, device information and GPS position. The modem can

simultaneously support a MODBUS slave gateway connection.

The modem can be configured to "read" and "write" MODBUS messages using the mode command and

modbusadd commands.

The MODBUS message format is defined by the MODBUS organization.

MODBUS Message Format

MODBUS uses a binary format expressed in HEX. It is a Request/Response protocol with allowance for

Report by Exception Responses from the slave device.

Parameter

Format

MODBUS ID 0xII

1 byte, range 1 to 247

Function code 0xFF

1 byte defined by MODBUS protocol

Data 0xNN……0xNN

Variable size dependant on function code

Error check 0xZZZZ

2 bytes CRC (not used in MODBUS TCP messages)

The following MODBUS messages are supported by the CTM-15x for reading and setting of its

Input/Output.

11.1 Read Digital Outputs

In MODBUS term this is called “read coils” and has function code 0x01.

Request message: 0xII 0x01 0x00 0x00 0x00 0x04 0xZZ 0xZZ

Where:

0xII is the modem’s assigned MODBUS ID

0xZZ 0xZZ is the CRC checksum for this message

Response message: 0xII 0x01 0x01 0xSS 0xZZ 0xZZ