Controlling the compressor via Modbus

Control mode "Evaporating temperature" and "Room temperature"

The IQ MODULE CM-RC-02 independently regulates the respective temperature, it starts and stops the compressor and adjusts its capacity.
In these control modes, the Modbus control timeout function, the serial control word and the serial setpoint have no influence.

Control mode "External: Modbus directly" and "External: Modbus via BDN Gateway"

With the "External: ..." control modes, the start and stop of the compressor and its capacity control can be specified by external signals.

If the compressor is to be controlled via Modbus, the control mode "External: Modbus direct" must be selected.
If the gateway for the BITZER Digital Network (BDN, access via myBITZER) is connected between the system controller and the IQ MODULE CM-RC-02, "External: Modbus via BDN gateway" must be selected. See also

CT-311: BITZER Gateway Installation Guide

When Modbus control is enabled, the setpoints from the different interfaces (analogue setpoint and serial setpoint) are merged and the resulting "Setpoint" can be read via the Modbus interface. The setpoint is limited to 100%, even if the sum of the setpoints may be above 100%.

Modbus control timeout function

If the Modbus communication is interrupted, the IQ module can be configured for different reactions to this interruption by the parameters "Modbus control timeout function" and "Modbus control timeout".

Configuration possibilities for the timeout function:

None --> Do nothing (if compressor is running, operation will be continued)
Stop --> Stop the compressor
Fault --> Stop the compressor and trip with the fault alarm "11-00 Serial Control Timeout"

Any received Modbus telegram resets the timeout function.

Serial control word

The serial control word is set via the Modbus holding register 110:

Parameter	Unit	Data type	Scaling	Address (I/H)	Register count
Serial control word	None	uint16		110(H)	1

Bit definition of the serial control word:

Bit	Function	Description
0 .. 2	Reserved	Should be set to 1 (for compatibility to the CSV)
3	Enable operation	0 = Operation not enabled 1 = Compressor released for operation This might mainly be used for removing the external release via Modbus in case of the "Operating mode" = "System control mode". Otherwise the bit must be set to 1.
4 .. 5	Reserved	Should be set to 1 (for compatibility to the CSV)
6	Start	Start command 0 = Stop 1 = Start active
7	Reset	Alarm reset command is active when bit is set from 0 to 1. Positive edge triggered.
8 .. 9	Reserved
10	Data valid	Must be set to 1 to instruct the controller to accept the serial control word. 0 = The serial control word as well as the serial setpoint are ignored. This means that if the start command was active just before the data valid bit is set to "0" the command remains active until the data valid bit is set to "1" and the start bit is set to "0". 1 = The serial control word and serial setpoint will be accepted. The resulting control word (I102) will always have this bit set.
11 .. 15	Reserved

Examples of serial control word setups:

Command	Hexadecimal value	Decimal value	Binary
			Reserved	Data valid	Reserved	Reset	Start	Reserved	Enable operat.	Reserved
No command / Stop	43F	1087	00000	1	00	0	0	11	1	111
Start	47F	1151	00000	1	00	0	1	11	1	111
Reset	4BF	1215	00000	1	00	1	0	11	1	111

The neutral value of the control word is 43F hex (= 1087 dec = 10000111111 binary).
To start the compressor, the control word must be 47F hex (= 1151 dec = 10001111111 binary).

Serial setpoint

The serial setpoint is set via the Modbus holding register 111:

Parameter	Unit	Data type	Scaling	Address (I/H)	Register count
Serial setpoint	%	int16	100	111(H)	1

Status word

The actual status of the IQ module can be seen via the Modbus input register 103.

Parameter	Unit	Data type	Scaling	Address (I/H)	Register count
Status word	None	uint16	1	103(I)	1

Status word bit definitions

Bit	Function	Description
0 .. 1	Not used
2	Operation enabled	0 = The compressor is not ready for operation 1 = The compressor is ready to operate without considering the recommended stillstand and start delays. A warning alarm (30-22, 30-24 or 30-26) might appear when starting the compressor. No fault (Bit 3 = 0) Safety chain input activated (CN10: Safety In powered) In case parameter "Modbus control" is not "Deactivated" Operation is enabled (Bit 3 of the "Serial Control Word" set to 1) In case parameter "Operating mode" = "System control mode" Parameter "System operating mode" = On External release given (CN3: start/stop activated)
3	Fault	0 = No fault alarm(s) 1 = A fault alarm is active. The compressor is stopped.
4 .. 6	Not used
7	Warning	0 = No warning alarm(s) 1 = A warning alarm is active
8	On reference	0 = The compressor is ramping or not running. 1 = The compressor is operating at setpoint.
9	Not used
10	Limiter active	0 = No limiter function is active 1 = A limiter is actively regulating the setpoint to keep the compressor in operation (reducing the risk of a fault stop)
11	Running	0 = Compressor is not running 1 = Compressor is running Direct on line, star delta, part winding: motor start completed (K1/K2 control) VARIPACK connected via Modbus: VARIPACK reports back that the compressor is running Frequency inverter, soft starter: running is assumed when the start input is closed.
12	Start active	0 = Compressor start is not active 1 = Compressor start is active. This includes the activation of the start unloader, pre-run of the condenser and additional fan as well as start procedure of the motor starter function (star/delta and part winding contactor activation). Operation enabled (Bit 2 = 1) No fault (Bit 3 = 0) "Operating mode" = "Compressor operation mode" Start command given (either via digital input start or Modbus) In case "setpoint control characteristic" = "0 .. Max": setpoint > 1% "Operating mode" = "System control mode" The cooling capacity control requests the compressor to start. The recommended start to start interval and stop to start time counted down and the max. starts per hour have not been exceeded (see parameter "Remaining time for short cycling protection").
13	Critical	0 = No critical alarm(s) 1 = A critical alarm is active
14	Ready for operation (available from firmware versions 2.0.12.0)	0 = Not ready for operation 1 = Ready for operation Operation enabled (Bit 2 = 1) The remaining time for protection against short cycling has expired.
15	Not used