MODBUS PROTOCOL
Modbus Remote Terminal Unit (RTU)
Modbus RTU enables a computer or PLC to read and write directly to registers
containing the controller’s parameters. With it you could read all 141 of the
controller’s parameters with five read commands.
Because of the wide array of choices available for setting up a Series 96 controller,
only a subset of the prompts contain parameters in a given situation. This manual
explains the interrelations between prompts. A Modbus read command response of
-32000 indicates that a register is not implemented; -32001, register not active; or
-32002, not read-accessible. A write command will return an exception response of
01 to indicate an illegal function; 02, illegal register; or 03, illegal data. If you try to
write to an inactive prompt the controller will return an illegal data address message
(02).
If you already have a software application that uses Modbus, you can simply skip to
the table in this chapter for the address information your program will need. The rest
of this section on the Modbus provides information for writing a software application
that uses Modbus.
Special Modbus Functions
The following are modbus registers with special functions: Disable Non-volatile
(system) Memory (24)
1
; Alarm Status Output 2 (106); Alarm Status Output 3 (110);
Alarm Status Output 4 (114); Auto-Manual Operation Mode (200); Clear Input Errors
(311); Clear Alarms (331); Silence Alarms (332).
A “0” indicates an inactive state. Send “1” to the register to activate the function. It
will automatically reset to “0” when the function is complete.
Appendix K
MODBUS
Modbus is the protocol used over the serial communications channel if installed.
Using the Modbus interface requires that the controller be equipped with either
EIA/RS-232 or EIA/RS-485.
The following pages give the necessary information for using Modbus in your
application. For wiring information please refer to Appendix J.
ALPHA OMEGA INSTRUMENTS CORP. Instruction Manual
Series 800/850 Thermoelectric Cooler Controller Page 101
1
WARNING: Set this bit if contantly updating variables within any loop via the MODBUS protocol. See next page for details.
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