Controlling the compressor via Modbus

The compressor can be monitored and controlled via Modbus.

In case the compressor shall be controlled via Modbus, the parameter "Modbus control" must be set to "Modbus directly". If the gateway for the BITZER digital network (BDN, access via myBITZER) is connected between the system controller and the IQ module, "Modbus control" must be set to "Modbus via BDN Gateway". By default, "Modbus control" is set to "Deactivated", so that the compressor can be controlled via digital/analogue signals only.

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:

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

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 (CN2:3 relay C powered)
  • In case parameter "Modbus control" is not "Deactivated"
    • Operation is enabled (Bit 3 of the "Serial Control Word" set to 1)

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)
  • 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)
  • Start command given (either via digital input start or Modbus)
  • In case "setpoint control characteristic" = "0 .. Max": setpoint > 1%

13

Critical

0 = No critical alarm(s)
1 = A critical alarm is active

14 .. 15

Not used