8
4
MODBUS PROTOCOL
Information.
•
The COMMANDER 500 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.
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 125ms). The slave monitors the elapsed time between
receipt of characters. If the elapsed time without a new character is
3
1
/
2
character times, the slave
assumes the next character received is the 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.
