General Assist Technical Reference
Double Skin Rooftop Package Units
Danfoss MCX Modbus & BMS Integration Guide
Variable list: UNICO_v1.05.00_rev01_FG-000022-ID10.mcxs | Project reference: FG-000022
This article provides a practical reference for service, controls and BMS integration teams working with double skin packaged rooftop units using the Danfoss MCX controller platform. It summarises Modbus communication settings, writable control points, live status, alarms and physical I/O addresses.
Important: The variable list contains factory-level parameters. Do not change writable parameters unless the adjustment has been approved by General technical support or the equipment supplier. Incorrect writes may affect safety logic, compressor staging, fan control, defrost, setpoints or alarm behaviour.
Applies To
Double skin packaged rooftop package units using the Danfoss MCX controller platform.
Reference variable list: UNICO_v1.05.00_rev01_FG-000022-ID10.mcxs
This variable list is for project reference FG-000022 and shows Modbus-related parameters, status variables, alarms and I/O addresses.
Purpose
This guide explains how the variable list is structured, which addresses are commonly required for BMS integration, which values are read-only or read/write, key unit status/setpoint/alarm/I/O points, and what information is required before escalating a Modbus or BMS issue.
Variable List Structure
| Column | Meaning |
|---|---|
| LABEL | Short variable code used by the controller |
| DESCRIPTION | Description of the parameter, status or alarm |
| MIN / MAX | Permitted operating range |
| VALUE/TYPE | Default value or enum type |
| UNIT | Engineering unit, where applicable |
| RW | Read/write status |
| ADU | Modbus address/register reference |
Writable configuration parameters are marked RW. Live status, alarm and I/O feedback points are generally marked Read.
Modbus Communication Settings
Modbus serial settings are located under General > Serial Settings.
| Label | Description | Default / Value | ADU |
|---|---|---|---|
| SEr | Serial address, Modbus | 10 | 7311 |
| bAU | Serial baud rate, Modbus | 19200 | 7312 |
| COM | Serial format, Modbus | 8N1 | 7313 |
External gateway settings are listed under General > Modbus Gateway.
| Label | Description | Default / Value | ADU |
|---|---|---|---|
| XC0 | External communication enable | Auto | 7314 |
| XC1 | External communication baud rate | 19200 | 7315 |
| XC2 | External communication settings | 8N1 | 7316 |
For BMS integration, confirm the controller serial address, baud rate and serial format match the BMS configuration.
Common Writable Control Points
| Label | Description | Range | Unit | Access | ADU |
|---|---|---|---|---|---|
| y01 | System ON/OFF | 0-1 | Enum | RW | 8398 |
| y03 | System heat/cool | 0-1 | Enum | RW | 8400 |
| SC1 | Cooling temperature setpoint | SCL-SCH | °C | RW | 8261 |
| SH1 | Heating temperature setpoint | SHL-SHH | °C | RW | 8264 |
| SPr | Remote setpoint enable | 0-1 | Enum | RW | 7094 |
| Er1 | Maximum power in demand limit | 0-100 | % | RW | 7095 |
Setpoint Limits
| Label | Description | Default | Unit | ADU |
|---|---|---|---|---|
| SCL | Minimum setpoint limit in cooling | 15.0 | °C | 7081 |
| SCH | Maximum setpoint limit in cooling | 30.0 | °C | 7082 |
| SHL | Minimum setpoint limit in heating | 15.0 | °C | 7084 |
| SHH | Maximum setpoint limit in heating | 40.0 | °C | 7085 |
If the BMS cannot write a setpoint, confirm the requested value is within the configured minimum and maximum limits.
Key Read-Only Unit Status Points
The main unit overview is located under Status var > Chiller Overview.
| Label | Description | Unit / Type | Access | ADU |
|---|---|---|---|---|
| A01 | Alarm notification | Enum | Read | 16387 |
| A02 | On/Off status | Enum | Read | 16388 |
| A03 | Compressor status | Enum | Read | 16389 |
| A10 | Remote ON/OFF | Enum | Read | 16398 |
| A11 | Defrost status | Enum | Read | 16399 |
| A12 | Cool/Heat mode | Enum | Read | 16400 |
| A13 | Control probe | °C | Read | 16480 |
| A14 | Actual setpoint | °C | Read | 16481 |
| A15 | Requested power | % | Read | 16484 |
| A16 | Actual power | % | Read | 16485 |
Compressor Monitoring
| Label | Description | ADU |
|---|---|---|
| c01 | Compressor 1 Circuit 1 status | 16417 |
| c02 | Compressor 2 Circuit 1 status | 16418 |
| c03 | Compressor 3 Circuit 1 status | 16419 |
| c07 | Compressor 1 Circuit 2 status | 16423 |
| c08 | Compressor 2 Circuit 2 status | 16424 |
The configuration includes variables for up to six compressors per circuit; not every variable applies to every unit size or configuration.
| Run Hours | Description | ADU |
|---|---|---|
| c0a | Compressor 1 Circuit 1 hours | 16801 |
| c0b | Compressor 2 Circuit 1 hours | 16803 |
| c0c | Compressor 3 Circuit 1 hours | 16805 |
Rooftop Status Variables
| Label | Description | Unit / Type | ADU |
|---|---|---|---|
| RS00 | Supply damper on/off | Enum | 17001 |
| RS01 | Supply fan on/off | Enum | 17002 |
| RS02 | Supply fan speed | % | 17003 |
| RS03 | External damper on/off | Enum | 17004 |
| RS04 | External damper position | % | 17005 |
| RS05 | Return fan on/off | Enum | 17006 |
| RS06 | Return fan speed | % | 17007 |
| R023 | Free cooling demand | % | 17024 |
| R024 | Free heating demand | % | 17025 |
| R026 | Air quality demand | % | 17027 |
Supply Fan Parameters
| Label | Description | Default | Unit | ADU |
|---|---|---|---|---|
| SF0 | Regulation type | FIX | Enum | 7317 |
| SF1 | Minimum speed | 10.0 | % | 7318 |
| SF2 | Maximum speed | 75.0 | % | 7319 |
| SF3 | Time at maximum speed at startup | 0 | s | 7320 |
| SF6 | Enable supply damper | TIME | Enum | 7323 |
| SF7 | Fan delay after damper | 60 | s | 7324 |
| SF8 | Fan delay at OFF | 60 | sec | 7325 |
SF2 Maximum speed is set to 75% by default.
External Damper / Economiser Parameters
| Label | Description | Default | Unit | ADU |
|---|---|---|---|---|
| dE0 | Enable FreeCoolHeating | NO | Enum | 7346 |
| dE1 | Minimum opening | 20.0 | % | 7347 |
| dE2 | Maximum opening | 100.0 | % | 7528 |
| sC0 | Summer free-cooling enable | YES | Enum | 7350 |
| wH0 | Winter free-heating enable | YES | Enum | 7355 |
| wC0 | Winter free-cooling enable | YES | Enum | 7365 |
These points may only be relevant where the unit is fitted with the applicable damper, economiser or air quality options.
Alarm Monitoring
Alarms are shown in the ALARMS section. Alarm points are mapped as individual bits within ADU registers, so the BMS may need to read the register and decode the relevant bit.
| Alarm Type | Examples | ADU |
|---|---|---|
| General / phase / high pressure / low pressure | A01, A02, AH1, AL1 | 20481-20482 |
| Compressor protection | AC1, C01, C02, C03 | 20482-20483 |
| Fan protection | A60, A61, AF1, AF2 | 20483-20484 |
| Probe and sensor | P05, P14, P21, P23, P24 | 20485-20486 |
| Network and communication | N01, N02, N03-N10 | 20488-20489 |
| Rooftop-specific | SFA, RFA, SFL, FIR, SFW | 20491-20492 |
Common Rooftop-Specific Alarms
| Label | Description | Reset | ADU / Bit |
|---|---|---|---|
| SFA | Supply fan alarm | Manual | 20491 .06 |
| RFA | Return fan alarm | Manual | 20491 .07 |
| SFS | Supply fan safety switch | Auto | 20492 .12 |
| FIR | Fire/smoke alarm | Manual | 20492 .13 |
| SFW | Supply flow | 3 | 20492 .14 |
| SFL | Supply filter | Auto | 20492 .15 |
| FR1 | Frost alarm Circuit 1 | Auto | 20492 .00 |
| FR2 | Frost alarm Circuit 2 | Auto | 20492 .01 |
I/O Configuration
Analogue Inputs
| Input | Description | Signal | Unit | ADU |
|---|---|---|---|---|
| AI1 | Return temperature | NTC-10K | °C | 21804 |
| AI2 | Outdoor temperature | NTC-10K | °C | 21768 |
| AI3 | Suction pressure Circuit 1 | 4-20 mA | barg | 21775 |
| AI5 | Discharge pressure Circuit 1 | 4-20 mA | barg | 21766 |
| AI12 | Variable setpoint | 0-10 V | - | 21769 |
| AI16 | Supply temperature | NTC-10K | °C | 21803 |
Digital Inputs
| Input | Description | Contact | ADU |
|---|---|---|---|
| DI1 | Remote ON/OFF | N.C. | 21309 |
| DI3 | High pressure switch Circuit 1 | N.C. | 21299 |
| DI7 | Protection Compressor 1 Circuit 1 | N.C. | 21253 |
| DI8 | Protection Compressor 2 Circuit 1 | N.C. | 21254 |
| DI11 | Protection Compressor 3 Circuit 1 | N.C. | 21255 |
| DI15 | Supply filter | N.C. | 21335 |
| DI17 | Phases/voltage status | N.C. | 21311 |
| DI18 | Remote cooling/heating | N.C. | 21314 |
| DI19 | Protection fans | N.C. | 21287 |
| DI20 | Demand limit | N.O. | 21315 |
| DI21 | Supply fan alarm | N.C. | 21326 |
| DI22 | Unit second setpoint | N.O. | 21310 |
Analogue Outputs
| Output | Description | Signal | ADU |
|---|---|---|---|
| AO1 | Sink/source side regulation value 1 | 0-10 V | 22018 |
| AO3 | Supply fan | 0-10 V | 22026 |
Digital Outputs
| Output | Description | Contact | ADU |
|---|---|---|---|
| DO2 | Compressor 1 Circuit 1 | N.O. | 21506 |
| DO3 | Compressor 2 Circuit 1 | N.O. | 21507 |
| DO4 | Supply damper | N.O. | 21571 |
| DO6 | Compressor 3 Circuit 1 | N.O. | 21508 |
| DO10 | Four-way valve Circuit 1 | N.C. | 21536 |
| DO12 | Supply fan | N.O. | 21568 |
| DO13 | Sink/source side regulation enable 1 | N.O. | 21532 |
| DO14 | General alarm | N.O. | 21534 |
| DO20 | Operating status | N.O. | 21567 |
BMS Integration Recommendations
- Confirm the fitted communication option from the model code and wiring diagram.
- Confirm whether the site is using Modbus RTU or another protocol gateway.
- Confirm Modbus address, baud rate and parity settings.
- Agree which points are monitoring-only and which points are writable.
- Avoid writing to factory parameters unless approved.
- Use status variables for monitoring rather than physical I/O where possible.
- For alarms, confirm whether the BMS can decode individual bits within alarm registers.
- Validate all values on site against the controller display before handover.
Recommended Minimum BMS Point List
| Purpose | Recommended Variables |
|---|---|
| Unit enable/status | y01, A02, A10 |
| Mode | y03, A12 |
| Setpoint | SC1, SH1, A14 |
| Control temperature | A13 |
| Demand/output | A15, A16 |
| Alarm status | A01 plus alarm bit registers |
| Compressor status | A03, c01, c02, c03 |
| Fan status | RS01, RS02 |
| Damper status | RS00, RS03, RS04 |
| Filter/fan alarms | SFA, SFS, SFL |
| Communication settings | SEr, bAU, COM |
Troubleshooting
| Issue | Checks |
|---|---|
| BMS cannot communicate | Confirm Modbus address, baud rate, parity, wiring polarity, shield and termination. |
| Values are incorrect | Confirm register type, scaling, signed/unsigned format and whether the BMS is offsetting register numbers. |
| BMS cannot write ON/OFF | Confirm write access, controller mode, remote enable configuration and whether local controls are overriding BMS. |
| Setpoint write rejected | Confirm the setpoint is within SCL/SCH or SHL/SHH limits. |
| Alarm not matching display | Confirm the BMS is decoding the correct ADU and bit position. |
| Fan command present but fan not running | Check RS01/RS02, supply fan alarm, supply flow alarm and physical fan enable/output. |
| Unit enabled but compressors not starting | Check requested power, compressor status, active alarms, safety chain and anti-short-cycle timers. |
Escalation Information to Collect
- Full model number and serial number
- Variable list file name and revision
- Controller software version, if available
- BMS protocol and hardware details
- Modbus address, baud rate and parity
- Register map used by the BMS contractor
- Screenshot/export of BMS point configuration
- Photos of communication wiring and controller terminals
- Active alarm list from the controller display
- Values read from key points A02, A12, A13, A14, A15, A16 and A01
Confirm the actual controller configuration and software revision on site before applying register information.