7
4
MODBUS PROTOCOL
Information.
•
The advanced process recorder operates as a Modbus, Remote Terminal Unit (RTU) slave.
•
Parity checking – used to detect transmission errors in individual characters.
•
Cyclic redundancy checking – used to detect errors in the master messages and slave responses.
•
Non-volatile memory save command.
4.1
Introduction to Modbus Protocol
Modbus communication is based on a master and a slave arrangement. The master sends a message to one slave at a time and
waits for a reply.
The slave cannot accept a new message until the existing message is processed and a reply sent to the master (maximum
response time 90 milliseconds). 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.
To allow the master to differentiate between more than one slave in a system, each slave is given a unique identity address
(between 1 and 99).
A broadcast address (address zero) can be used to access all slave devices with one command. This is limited to write messages
only and there is no slave acknowledgment.
Note. Modbus RTU requires 1 start bit, 8 data bits, 1 parity bit (optional) and 1 or 2 stop bits.
4.1.1
Non-volatile Memory Limitations
Caution. If the number of write cycles to any particular non-volatile memory register exceeds 10
4
cycles, the data
stored may not be retained.
Any changes made to a parameter via the serial link, e.g. Alarm A Trip Point value, are stored in a non-volatile memory register
assigned to that parameter.
The number of write cycles to a particular register can be reduced by disabling non-volatile memory access when making changes
to parameters which do not need to be retained following a power-down. This is achieved using the Non-volatile Save State (NV)
Coil 061 – see Section 7.1.
When the Non-volatile Save State is set to ‘1=Saved’, any parameter changes made via the serial link are written to non-volatile
memory and are retained on power-down. If the Non-volatile Save State is set to ‘0=Not Saved’, parameter changes made via
the serial link are not retained on power down.
The Non-volatile Save State must be adjusted only when necessary and must be reset to the required state each time the
instrument is or the host computer is powered down or the instrument is replaced.
