5.5
MODBUS RS485 ASCII Bi-Directional Communication Protocol
OMD-525X MODBUS ASCII Protocol
The OMD-525X uses MODBUS ASCII at 19200 Baud, 8 bits of data, 1 stop bit, and Even Parity. The
communication settings are not adjustable on the OMD-525X. The MODBUS ID number can be
changed on the OMD-525X by pressing the MENU button and going into the MODBUS ID Number
sub-menu. This will allow the ID setting to be scrolled up or down between 1 and 99, press ENTER
to save the setting.
If the ESC Key is held on Bootup,
the MODBUS ID number is also displayed on
the bootup screen. Although the OMD-525X uses MODBUS ASCII protocol, the instrument can eas-
ily be read from or written to using Windows Hyperterminal, or Linux puTTy.
MODBUS ASCII structure looks like this:
START FRAME
1 char Starts with colon “:”
ADDRESS FIELD
2 chars logical number
FUNCTION CODE
2 chars As Per Read or Write function
DATA
n chars Data + length will be filled depending on the message type
ERROR CHECK
2 chars Error check (LRC)
END FRAME
2 chars CRLF
The MODBUS ASCIILRC is calculated by adding all data fields together, then negating or inverting
the result for a 1s compliment, then adding 1 for a 2s complement. The starting colon and ending
CR and LF are not included. As an example, command 6 to set OMD-525X unit #1 to 25% manual
ranging would be the following command “:010600000001F8”. In this example we add each field
together, 01+06+00+00+00+01 = 8 or 0000 1000 in binary. The 1s complement is therefore 1111
0111, and adding 1 more gives the 2s complement, which would be 1111 1000 or F8 in Hexadeci-
mal.
The OMD-525X will respond to MODBUS commands 4, 6, and 16. It will also echo back command 3
and 5 for communications testing. In this example the OMD-525X is being polled and echoing back,
“:010500000000FA(CR,LF)” using Modbus Poll software.
Part 5 Annexures
OMD-525X Oxygen Analyzer
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