20-COMM-H RS-485 HVAC Adapter Replacement

Overview

The Allen-Bradley 20-COMM-H is a specialized Drive Peripheral Interface (DPI) communication adapter designed to integrate PowerFlex 7-Class adjustable frequency AC drives directly into Building Automation Systems (BAS) and Heating, Ventilating, and Air Conditioning (HVAC) networks. Leveraging the RS-485 physical layer, this adapter natively supports heritage HVAC fieldbus protocols, enabling direct control, monitoring, and parameter configuration of the host drive from centralized automation controllers.

Because Rockwell Automation has discontinued the 20-COMM-H, maintenance engineers and facilities managers must now deal with the challenges of legacy hardware failures on critical air-handling units (AHUs), cooling towers, and pumping stations. This guide provides the deep technical specifications, protocol configurations, wiring details, and step-by-step instructions required to successfully replace or migrate an existing 20-COMM-H module.

Legacy Product Information

The 20-COMM-H serves as an internal gateway between the drive's internal DPI protocol and external RS-485-based networks. It mounts directly inside the drive's enclosure or on a designated communication bracket, receiving its 5V DC power (150 mA maximum) directly from the host drive via a 20-pin ribbon cable.

Technical Specifications

• Protocols Supported: Johnson Controls Metasys N2, Siemens Apogee FLN (P1), and Modbus RTU.
• Physical Layer: RS-485 (2-wire half-duplex, differential signaling).
• Network Connector: 5-pin open-style linear plug.
• Baud Rates: 4800, 9600, 19200, or 38400 bps (dependent on protocol selection).
• DPI Connection Speed: 125 Kbps or 500 Kbps.
• Common Applications: Variable air volume (VAV) boxes, centrifugal pumps, supply/return fans, and chillers utilizing PowerFlex 70, PowerFlex 700, PowerFlex 700H, or PowerFlex 700S drives.

Currently, the 20-COMM-H is classified as End of Life (EOL) / Discontinued. Direct replacements from the original equipment manufacturer (OEM) are no longer manufactured, making the maintenance of these drives reliant on surplus inventory, certified remanufactured parts, or protocol migration gateways.

Recommended Replacements

When a 20-COMM-H adapter fails, several paths can be taken depending on budget, downtime constraints, and long-term facility plans.

Compatibility Considerations

Mechanical & Physical Footprint

The 20-COMM-H is configured for internal mounting within PowerFlex 7-Class drives. If you choose to replace the drive itself with a newer Generation 2 drive (such as the PowerFlex 753 or 755), the legacy 20-COMM-H cannot be mounted directly. Instead, you must use a 20-750-20COMM-F1 physical adapter board to bridge the legacy 20-Series DPI card to the newer 750-Series architecture.

Cabling and Signal Pinout

The external RS-485 connection utilizes a 5-pin linear terminal block. Ensure pin-to-pin mapping is strictly adhered to during replacement:

• Pin 1: Signal Common (or Reference)
• Pin 2: TXD/RXD - (often designated as Data B or '-')
• Pin 3: Shield (Chassis Ground connection point)
• Pin 4: TXD/RXD + (often designated as Data A or '+')
• Pin 5: Not Connected / Terminating Resistor Tie-Point (if external termination is utilized)

Software & Firmware Requirements

• Drive Compatibility: The host PowerFlex drive must run a firmware revision that supports the specific HVAC mapping parameters (typically v2.001 or higher for PowerFlex 700 Vector Control drives).
• Mapping Points: If replacing the adapter with another legacy 20-COMM-H, match the firmware version (v1.xxx vs v2.xxx) of the card whenever possible. Minor changes in adapter firmware can affect the specific Metasys N2 point map or Apogee FLN "P1" register structure, causing communication errors at the BAS controller.

Upgrade Benefits

Migrating from the legacy 20-COMM-H with RS-485 protocols to an Ethernet-based infrastructure (such as BACnet/IP or EtherNet/IP) offers significant operational benefits:

1. High-Speed Data Rates: Legacy RS-485 links peak at 38.4 Kbps or lower. Ethernet architectures operate at 10/100 Mbps, allowing real-time polling of energy usage, motor diagnostics, and run-time parameters.
2. Simplified Topology & Diagnostic Tools: Ethernet-based systems permit modern diagnostic software (like Wireshark or web browser interfaces) to isolate faulty nodes instantly, bypassing the tedious troubleshooting required for daisy-chained RS-485 buses.
3. Modern Security Standard Compliance: Older HVAC protocols like Metasys N2 lack authentication and encryption. Modern industrial gateways and newer Ethernet protocols establish secure boundaries for facility networks.
4. No Single Point of Failure: While a break in a legacy RS-485 daisy-chain can take down every downstream node, structured star Ethernet topology isolates cabling faults to a single drive.

Common Migration Challenges

• Internal Switch Redundancy: Technicians often overlook hardware DIP switches on the 20-COMM-H. These switches (SW1 and SW2) control the physical node address, baud rate, and operational protocol (Modbus, Metasys N2, or Apogee). If swapping modules, these hardware switches must match the exact settings of the removed board, or communication will fail.
• Network Line Termination: RS-485 requires 120-Ohm terminating resistors at both physical ends of the network trunk. The 20-COMM-H includes an internal termination network that can be jumped. Failing to replicate this termination on the replacement card will introduce signal reflections and intermittent packet loss.
• Register Mapping Offsets: While Modbus RTU standards define registers starting at 40001, some third-party controllers address them with a 0-based index. When moving from a 20-COMM-H to an external protocol gateway, register orientation offsets (+1 or -1) may need to be adjusted within the mapping table.

Step-by-Step Replacement Procedure

Follow this procedure to replace a faulty 20-COMM-H adapter with a functional unit.

1. Safety and Isolation

DANGER: HIGH VOLTAGE. AC drives contain large bus capacitors that store lethal voltages after input power is disconnected.

1. Isolate all electrical power feeding the host PowerFlex drive. Lock out and tag out (LOTO) the main disconnect switch.
2. Verify the drive's internal DC bus voltage has completely discharged. Wait at least 5 minutes (or the duration indicated on the drive's warning label) and verify with a calibrated digital multimeter at the +DC and -DC test terminals that the voltage is below 50V DC before proceeding.
3. Attach an electrostatic discharge (ESD) grounding wrist strap to your person and connect the clip to an unpainted chassis ground point inside the drive.

2. Disconnection & Physical Removal

1. Gently pull the 5-pin open-style linear RS-485 connector out from the bottom of the 20-COMM-H module. Do not pull on the individual wires.
2. Unscrew the retaining screws securing the 20-COMM-H module to the mounting bracket.
3. Carefully disconnect the internal 20-pin DPI ribbon cable from the drive mating port and pull the adapter clear of the housing.

3. Hardware Switch Configuration

Before mounting the replacement board, copy the DIP switch configurations of SW1 and SW2 from the failed module to the replacement.

• SW1 (Node Address / Baud Rate): Set the binary-encoded switches to identical address settings (1 to 247).
• SW2 (Protocol Selection):
  • For Metasys N2, set switches 1–3 to the designated binary configuration.
  • For Apogee FLN (P1), adjust switches to match the legacy selection.
  • For Modbus RTU, set switches to the required baud rate and parity settings.

4. Installation and Wiring

1. Align the replacement 20-COMM-H onto the drive mounting bracket.
2. Firmly plug the internal 20-pin DPI ribbon cable into the card’s DPI connector slot.
3. Secure the module to the drive chassis using the mounting screws.
4. Re-insert the 5-pin linear RS-485 plug into the network port. Ensure the shield wire is grounded correctly and differential polarity matches ($A+$ to Pin 4, $B-$ to Pin 2).

5. Commissioning and LED Verification

1. Remove LOTO locks and apply control voltage to the drive.
2. Monitor the onboard status LEDs on the face of the 20-COMM-H:
   • PORT LED (Drive Status): Should be solid Green, indicating healthy communication between the adapter and the host drive.
   • MOD LED (Module Status): Solid Green indicates normal internal operation.
   • NET A / NET B LEDs (Network Status): Flashing Green indicates active network packet transmission and reception. If either light is solid Red, check for address conflicts, incorrect baud rates, or swapped polarity on the RS-485 line.

Frequently Asked Questions

Q1: Can I replace a 20-COMM-H with a 20-COMM-E (EtherNet/IP)?

Yes, but only if you also update your BAS or PLC network architecture. The 20-COMM-E communicates over EtherNet/IP, whereas the 20-COMM-H communicates over serial RS-485 (N2, FLN, Modbus RTU). You will need to install an external protocol gateway (such as a ProSoft or HMS Anybus module) or configure an EtherNet/IP driver in your automation system to bridge this transition.

Q2: Is there a built-in termination resistor on the 20-COMM-H?

Yes. The 20-COMM-H features internal termination jumpers. If the replacement unit is positioned at either physical end of the RS-485 daisy-chain, you must enable this termination to prevent data corruption. If it is in the middle of a run, keep termination disabled.

Q3: My replacement card has 'PORT' flashing yellow. What does this mean?

A flashing yellow PORT LED indicates that the adapter is not communicating with the host drive's DPI port. Verify that the 20-pin ribbon cable is seated squarely on both the adapter and the drive control board. This can also indicate that the drive’s internal DPI port has been disabled or requires a parameter reset via the Human Interface Module (HIM).

Q4: Are the parameter configurations stored on the drive or the 20-COMM-H?

The network parameters (node address, protocol configuration, baud rate) are read directly from the physical DIP switches on the 20-COMM-H during startup. However, the drive's communication parameters (such as those modified using a HIM) are saved to non-volatile memory on the host drive’s main control board. Swapping the 20-COMM-H card will not wipe out your drive’s primary parameter set.

Related Products & Families

When modernizing or repairing your PowerFlex drive communication networks, you may find the following related products and components useful:

• 20-COMM-E: 22-Pin DPI to EtherNet/IP Adapter Module.
• 20-750-20COMM-F1: DPI Option Card Adapter for PowerFlex 750 Series Drives.
• 20-HIM-A6: Enhanced LCD Human Interface Module (HIM) for programming DPI-compatible devices.
• 22-COMM-T: DSI to LonWorks adapter (commonly used in older PowerFlex 40 and 400 HVAC drives).

Need Help?

Whether you require a drop-in refurb, a premium surplus unit, or specialized advice on transitioning your system, Palm Parts Solution can assist. We maintain an extensive inventory of discontinued legacy automation hardware, including the Allen-Bradley 20-COMM-H communication adapter. All units are thoroughly tested by our technicians and come fully backed by our warranty. Contact Palm Parts Solution today to request a quote or speak with an application engineer about your HVAC network requirements.