Spectrasensors OXY5500, Operator'S Manual, Page: 87 / 140

OXY5500 Operator’s Manual 4
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4 - M
ODBUS
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OMMUNICATION
Introduction
Modbus is a serial communications protocol published by Modicon in 1979 for
use with its programmable logic controllers (PLCs). It has become a de facto
standard communications protocol in industry, and is now the most commonly
available means of connecting industrial electronic devices. Modbus is used
extensively in lieu of other communications protocols because it is openly
published and royalty-free, relatively easy to deploy, and capable of moving
raw bits or words without placing many restrictions on vendors.
This chapter covers the protocols, formats and register data, among other
things, that will be used to communicate with the OXY5500.
Protocol Definition
General Specification
The following general specifications apply to the Modbus protocol:
•
The protocol is Modbus RTU conform.
• The protocol is a master-slave arrangement with the host controller
acting as the master and each individual module as a slave.
• Each module on the bus needs to have a unique device ID (see
register 4095).
• The device has no command buffer, so the host must always wait
until the command is processed.
• Read commands need 10ms processing time
over RS-232 and RS-485 and 300ms over LAN.
• After a write process certain time consuming
tasks are started. After a write process a fixed
timeslot of 150ms over RS-232 and RS-485 and
300ms over LAN after the transmitted response
should be kept.
• RX Input Buffer is 256 bytes.
•
A CRC16 error checking method is implemented. Starting value is
0xFFFF and polynomial type is 0xA001.
•
Some registers are read only. When writing to those, a Modbus error
2 will occur (illegal data address). This will also happen when 4
registers should be written, but the last 2 are read-only, for example.
No register will be changed afterwards.
•
All registers between 1023 and 5708 can be read, there is no read-
protection.