Chapter 4 - Final Checkout
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© 2017
GM1212 Geist Watchdog Wi Instruction Manual
Modes of Transmission
The Modbus protocol uses ASCII and RTU modes of transmission. The Watchdog Wi
supports only the RTU mode of transmission, with 8 data bits, no parity and one stop bit.
Every Modbus packet consists of four fields:
Slave Address Field
Function Field
Data Field
Error Check Field (Checksum)
Slave Address Field
The slave address field is one byte in length and identifies the slave device involved in the
transaction. A valid address range is between 1 and 254. The slave address is set from the
Com Port1/Modbus/Bacnet Configuration web page.
Function Field
The function field is one byte in length and tells the Watchdog Wi which function to
perform. The supported functions are 03 (Read 4xxxx output registers), 04 (Read 3xxxx
input registers), 06 (Preset single register), 16 (Preset multiple registers).
Data Field
The data field of the request is a variable length depending on the function. The data fields
for the Watchdog Wi are 16-bit registers, transmitted high order byte first (big-endian)
Error Check (Checksum) Field
The checksum field lets the receiving device determine if the packet has transmission
errors. The Watchdog Wi’s RTU mode uses a 16-bit cyclic redundancy check (CRC-16).
Exception Responses
If a Modbus master sends an invalid command to the Watchdog Wi or attempts to read
an invalid register, an exception response is generated. The response packet will have the
high order bit of the function code set to one. The data field of the exception response
contains the exception error code.
Exception Codes
Code
Name
Description
01
Illegal Function
The function code is not supported
02
Illegal Data Address Attempt to access an invalid address
03
Illegal Data Value
Attempt to set a variable to an invalid value
