Modbus Protocol; Introduction - ABB C250 User Manual

4 MODBUS PROTOCOL

Notes.
• The COMMANDER 250/V250 operates as a MODBUS, Remote
Terminal Unit (RTU) slave.
• Parity checking – detects transmission errors in individual characters.
• Cyclic redundancy checking – detects errors in the master messages
and slave responses.
• Negative numbers are represented in 'two's complement' format, e.g.:
1000
–1000

4.1 Introduction

MODBUS communication uses the master/slave principle to send messages to one
or more slaves. Each slave is given a unique identity address (between 1 and 99).
A broadcast address (address zero) can be used to write to all slave devices
simultaneously, using one command. In this instance there is no slave
acknowledgment.
Slaves cannot accept new messages until the current message has been processed
and a reply sent to the master (maximum response time 250ms). The slave monitors
the elapsed time between receipt of characters. If the elapsed time without a new
1
character is 3 / character times, the slave assumes the next character received is the
2
start of a new message.
Note. MODBUS RTU requires 1 start bit, 8 data bits, 1 parity bit (optional)
and 1 or 2 stop bits.
= 0x03E8
= 0xFC18
7