20-COMM-S RS-485 HVAC Adapter Replacement

Overview

The Allen-Bradley 20-COMM-S is a legacy Drive Peripheral Interface (DPI) communication adapter designed to interface PowerFlex 7-Class adjustable frequency AC drives with HVAC-specific building automation serial networks. Operating over an RS-485 physical layer, this module enables direct communication with widely deployed automation protocols, including Johnson Controls Metasys N2, Siemens FLN Apogee, and Modbus RTU.

As the 20-COMM-S adapter has reached the end of its official product lifecycle, industrial facilities face mounting pressure to plan for maintenance, replacement, and system migration. When a 20-COMM-S fails in a critical building management system (BMS) or HVAC run-time application (such as variable air volume units, chillers, or cooling towers), direct swaps or structured upgrades are required to prevent broad communication dropouts and subsequent system shutdowns.

This engineering guide provides detailed technical specifications, pinout topologies, direct-replacement options, and modern migration pathways to assist automation engineers in maintaining system uptime.

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Legacy Product Information

Technical Specifications

The 20-COMM-S connects internally to the drive's DPI port, drawing power directly from the drive backplane. It contains an isolated RS-485 transceiver and a 5-pin open-style linear terminal block.

• Catalog Number: 20-COMM-S (Series A and B)
• Protocols Supported:
  • Modbus RTU (up to 38.4 Kbps)
  • Johnson Controls Metasys N2 (9600 bps)
  • Siemens FLN Apogee (4800 or 9600 bps)
• Physical Layer: 2-wire half-duplex RS-485
• Current Draw: 150 mA at 5V DC (supplied by host drive)
• Environmental Ratings: Operating Temperature: -10°C to +50°C (14°F to 122°F)
• Diagnostics: Four tri-color status LEDs (PORT, MOD, NET A, NET B)

Common Applications and Host Drives

The adapter is optimized for legacy DPI-based drives, including:

• PowerFlex 70 (EC and Standard control)
• PowerFlex 700 (Standard and Vector control)
• PowerFlex 700H and PowerFlex 700S
• PowerFlex 753 and PowerFlex 755 (requires a 20-750-20COMM or 20-750-20COMM-F1 adapter carrier)

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Recommended Replacements

As Rockwell Automation has deprecated the 20-COMM series, several paths exist to restore or modernize your control loop. The correct pathway depends on whether you are maintaining the existing RS-485 serial fieldbus or modernizing your entire network architecture to Ethernet-based BACnet/IP.

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Compatibility Considerations

When migrating from a 20-COMM-S, several physical, protocol, and firmware configurations must be cross-referenced to ensure seamless communication.

Wiring and Pinout Topologies

The 20-COMM-S uses a 5-pin terminal block. Traditional RS-485 wiring topologies must be mapped precisely if replacing the unit with a different adapter or a gateway module.

• Pin 1 (Signal - / B): Transceiver Inverting Terminal
• Pin 2 (Signal + / A): Transceiver Non-Inverting Terminal
• Pin 3 (Common / SG): Signal Ground reference
• Pin 4 (Shield): Chassis Ground connection for drain wire
• Pin 5 (Termination): Connect to Pin 1 or Pin 2 if the node sits at the physical end of the RS-485 segment to engage the internal 120-ohm terminating resistor.

Firmware and Parameters

The host drive must run compatible firmware to recognize the module. For example, PowerFlex 70 drives require Main Control Firmware v2.001 or later to fully utilize DPI communication maps.

Configuration of the 20-COMM-S is performed via DriveExplorer, DriveExecutive, or Connected Components Workbench (CCW). Crucial parameters include:

• Parameter 2 [Protocol Select]: Sets the active protocol (0 = Modbus RTU, 1 = Metasys N2, 2 = Apogee FLN).
• Parameter 3 [Comm Rate]: Establishes baud rates matching your physical trunk (e.g., 9600 bps for N2).
• Parameter 4 [Addr Node xx]: Configures network station addresses (1 to 247).

Mounting and Footprints

The 20-COMM-S mounts directly underneath the main cover of a PowerFlex 70 or 700 drive via standard plastic standoffs and a ribbon cable connection. If you are upgrading to a PowerFlex 750-series drive, a slot-based carrier framework is required to host the serial module.

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Upgrade Benefits

Upgrading from the legacy 20-COMM-S to modern protocols (such as BACnet MS/TP via the 20-COMM-B or BACnet/IP) offers significant operational field advantages:

• Higher Bandwidth: Modern networks run at significantly higher transmission rates (up to 76.8 Kbps for BACnet MS/TP and 100 Mbps for BACnet/IP) compared to the legacy 4800/9600 bps serial protocols.
• Rich Diagnostics: Contemporary HVAC networks offer enhanced error reporting, packet logging, and automated failure identification.
• Native Interoperability: Relieving your system of proprietary HVAC communication profiles (Metasys N2 or Apogee FLN) standardizes your entire control infrastructure under the open-source ASHRAE BACnet standard.
• Hardware Longevity: Choosing modern active modules ensures long-term availability of spare units, reducing facility recovery times during outages.

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Common Migration Challenges

• Configuring Node Addressing: Legacy fieldbuses depend entirely on correct node addressing. Dual-device collisions on the network segment occur if the replacement module defaults back to address 1 or 127 upon initial power-up.
• Termination Resistors: RS-485 systems require termination networks at physical endpoints. Removing the old 20-COMM-S can disrupt line impedance if the terminating link is not physically jumpered or configured on the new unit.
• Baud Rate Mismatches: If the host network is set to auto-baud and a replacement adapter is set to a fixed baud rate, communication sync fails.
• Database Mapping in BMS: Modbus registers and N2 points map differently across different generations of communication cards. Changing from a 20-COMM-S to a 3rd party protocol bridge requires manual remapping of BMS points (e.g., Analog Inputs vs. Analog Values).

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Step-by-Step Replacement Procedure

Follow these steps to safely replace a damaged 20-COMM-S with an identical replacement module.

1. Safety and Power Down

• Disconnect Power: Implement standard OSHA Lockout/Tagout (LOTO) protocols at the incoming power source.
• Capacitor Discharge: Wait at least 5 minutes to allow internal DC bus capacitors on the PowerFlex drive to discharge fully. Verify that the drive's internal "DC Bus Under Voltage" indicator lights are off.

2. Physical Extraction

1. Remove the front cover panel of the PowerFlex drive.
2. Locate the 20-COMM-S module mounted inside the chassis.
3. Gently pull the 5-pin RS-485 network plug out of the module. Secure the physical wire harness to prevent it from dropping.
4. Carefully pull the internal DPI ribbon cable out of the socket on the top edge of the 20-COMM-S card.
5. Depress the securing clips on the plastic standoffs and pull the legacy adapter straight out.

[PowerFlex Host Drive]
       │
       └───► [Internal Ribbon Cable] ──────► [20-COMM-S Adapter Port]
                                                    │
                                                    └──► [5-Pin Terminal Block]
                                                           ├── Pin 1: Signal -
                                                           ├── Pin 2: Signal +
                                                           ├── Pin 3: Signal Common
                                                           └── Pins 4/5: Shield/Termination

3. Setting Up the Replacement Module

Before inserting the replacement module, inspect the onboard DIP switches if configured for hardware-based addressing:

• Set the SW1 and SW2 address switch blocks on the replacement card to match the exact switch profile of the old card (or prepare to configure the parameters digitally using CCW).

4. Installation and Wiring

1. Mount the replacement module onto the plastic standoffs inside the drive chassis.
2. Direct-connect the internal DPI ribbon cable to the adapter's port. Ensure the polarizing tabs are aligned correctly.
3. Insert the 5-pin RS-485 connector back into the module's terminal block. Ensure wire leads are tight with no exposed copper strands.

5. Commissioning and Configuration

1. Re-attach the drive's protective covers.
2. Restore incoming utility power to the host variable frequency drive.
3. Access the drive parameters using a Human Interface Module (HIM) or a connected personal computer running Connected Components Workbench.
4. Navigate to the port hosting the communication module and verify that Parameter 2 [Protocol Select] and Parameter 4 [Addr Node xx] match your system layout.
5. Cycle drive power if parameter adjustments are executed.
6. Observe the diagnostic LEDs on the module faceplate:
   • PORT LED: Solid green indicates a normal, established internal DPI connection with the host drive.
   • MOD LED: Solid green indicates the communication module is in a healthy, operating state.
   • NET A / NET B LEDs: Flashing or solid green indicates active protocol data exchange with the BMS.

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Frequently Asked Questions

Q1: Can I use standard Category 5e/6 cables to rewire the RS-485 segment on a 20-COMM-S?

While Cat 5e/6 cabling is physically capable of transmitting serial data, RS-485 fieldbuses are designed around shielded twisted-pair (STP) cable possessing an impedance characteristic of 120 ohms. Standard Ethernet wire can cause signal reflection and data dropouts over long distances. High-quality 120-ohm RS-485 cable (such as Belden 9841 or equivalent) is recommended.

Q2: What should I do if the MOD LED turns solid red on power-up?

A solid red MOD LED signals an unrecoverable hardware fault. Power down the drive, re-seat the internal DPI ribbon cable, and verify that there are no short circuits across the 5-pin RS-485 plug. If the error persists after power cycling, the 20-COMM-S module must be replaced.

Q3: My BMS protocol is Modbus RTU, but the replacement configuration defaults to Metasys N2. How do I change this?

You can select the correct physical communication protocol by adjusting Parameter 2 [Protocol Select]. Setting this value to 0 assigns the card to run Modbus RTU. Remember to cycle drive control power to save the setting and re-initialize the transceiver.

Q4: Is it possible to bridge a modern Ethernet-enabled drive to a legacy N2 line using are older 20-COMM cards?

No. To use a modern drive on an old N2 or Apogee FLN line, you must use a dedicated, external industrial protocol converter (such as a ProSoft gateway) configured to convert Modbus/TCP or EtherNet/IP down to the serial RS-485 protocol level.

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Related Products & Families

• Allen-Bradley PowerFlex 70 Series AC Drives
• Allen-Bradley PowerFlex 700 Series AC Drives
• 20-COMM-B (BACnet MS/TP Communications Adapter)
• 20-COMM-E (EtherNet/IP DPI Communications Module)
• 20-750-20COMM (DPI Carrier Frame for PowerFlex 750 Series)

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Need Help?

Replacing obsolete industrial networking hardware requires precision, compatibility checks, and verified hardware. Part of a critical building automation or industrial HVAC operation cannot afford extended downtime.

Palm Parts Solution can supply high-quality new, refurbished, or surplus 20-COMM-S and 20-COMM series communication modules to keep your facility running smoothly. Every unit we supply undergoes rigorous technical testing and comes backed by our comprehensive warranty. Contact our support team for help selection hardware, finding direct replacements, or evaluating migration options.