20-COMM-Q ControlNet Fiber Adapter Replacement

Overview

In modern industrial automation, reliable communication between variable frequency drives (VFDs) and central controllers is paramount. For many legacy systems, the Allen-Bradley PowerFlex 7-Class drives (such as the PowerFlex 70, 700, 700S, and 700H) have relied on the 20-COMM-Q ControlNet Fiber communication adapter to transmit high-speed I/O and explicit messages.

The 20-COMM-Q adapter utilizes fiber-optic media to connect these drives to a ControlNet network, providing excellent immunity to electromagnetic interference (EMI) and supporting long-distance physical runs. However, because both ControlNet and the 20-COMM-Q adapter have transitioned to legacy or obsolete life-cycle phases, maintaining these networks requires targeted migration plans or the integration of high-quality refurbished or surplus replacements. This guide details technical replacement pathways, mechanical and electrical considerations, and the exact procedures required to swap or upgrade your 20-COMM-Q module.

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Legacy Product Information

The 20-COMM-Q is a Drive Peripheral Interface (DPI) external or internal communication adapter designed, manufactured, and distributed by Rockwell Automation under the Allen-Bradley brand.

Technical Specifications

• Catalog Number: 20-COMM-Q
• Protocol: ControlNet (Coax-to-Fiber conversion integrated on-card)
• Physical Interface: ST (Straight Tip) fiber optic connectors, dual-fiber (Transmit - TX and Receive - RX)
• Optical Specification: Multi-mode fiber, 62.5/125 micron cable, 1300 nm wavelength
• DPI Data Rates: 125 Kbps or 500 Kbps interface speed with host drive
• ControlNet Data Rate: 5.0 Mbps fixed
• Hardware Address Configuration: Built-in rotary switches for node address selection (01 to 99)
• Power Consumption: 310 mA at 5V DC, drawn directly from the host drive's DPI port or external power supply line

Product Lifecycle and Common Uses

The 20-COMM-Q is officially classified by the manufacturer as Obsolete. Active production has ceased, making direct-from-factory replacements unavailable. Historically, this adapter was widely deployed in environments requiring long-distance conduit runs or exhibiting heavy electrical noise, such as:

• Wastewater treatment plants (isolated pumps and aerators)
• Mining operations (conveyors and crushers)
• Steel and paper mills (large drive lineups subject to massive common-mode noise)
• Power generation facilities

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Recommended Replacements

When a 20-COMM-Q adapter fails, engineers and maintenance technicians face three primary options: sourcing a direct replacement, converting the local node to ControlNet coaxial cable, or modernizing the connection entirely to EtherNet/IP.

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Compatibility Considerations

Replacing a 20-COMM-Q involves aligning electrical, physical, and network parameters to ensure seamless operation.

Footprint & Mechanical Mounting

The 20-COMM-Q can mount directly within the internal communication bay of a PowerFlex 70 or PowerFlex 700 drive. When installed internally, its ST fiber connectors emerge through a specific breakout path. If you are substituting standard coaxial ControlNet modules (such as the 20-COMM-C), ensure you have the appropriate physical clearances for BNC connectors and T-taps, which have a significantly stiffer bend radius than multi-mode optical fibers.

If replacing the card with a 20-COMM-E for EtherNet/IP migration, the physical footprint is identical (standard DPI slot form factor), but the physical cabling changes from dual ST fiber lines to an RJ45 unshielded or shielded twisted-pair (UTP/STP) copper cable.

Software and Firmware Compatibility

• Node Addressing: The 20-COMM-Q uses decimal rotary switches to assign ControlNet node addresses. Any replacement module must be set to the exact same node address as the failed unit before powering up.
• RSNetWorx for ControlNet: If you are performing a 1:1 replacement with an identical firmware revision or a compatible major revision of the 20-COMM-Q, the ControlNet network "Keeper" (typically a 1756-CNB/CN2 module) will recognize the device and allow communication without rescheduling. However, if you transition to a 20-COMM-C, you may need to update the network configuration within RSNetWorx for ControlNet and re-flash the network schedule.
• Drive Firmware: PowerFlex 7-Class drives must have DPI firmware compatibility. Ensure that the drive’s firmware version is compatible with the firmware revision on the replacement card. Generally, 20-COMM-Q firmware v1.xxx or v2.xxx maintains backwards compatibility across most PowerFlex 7-Class drive series.

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Upgrade Benefits

While sourcing a direct refurbished 20-COMM-Q represents the quickest fix to minimize immediate downtime, transitioning to an EtherNet/IP architecture via the 20-COMM-E offers distinct, long-term operational advantages:

1. Elimination of RSNetWorx Scheduling: ControlNet requires specialized software (RSNetWorx) to compile and schedule network bandwidth every time a configuration changes. EtherNet/IP relies on standard cyclic I/O connections (Requested Packet Intervals, or RPI), which are configured directly inside Studio 5000 / RSLogix 5000 without requiring network-wide downtime.
2. Simplified Diagnostic Capabilities: The 20-COMM-E contains an embedded web server, allowing technicians to ping the card, view drive parameters, diagnostic logs, and connection statistics via a standard web browser over the plant intranet.
3. Hardware Availability: Modern industrial ethernet switches and CAT5e/CAT6 media are highly ubiquitous, standardizing parts inventory across the plant and significantly lowering maintenance costs.
4. Higher Bandwidth: ControlNet runs at a fixed speed of 5 Mbps. EtherNet/IP communication modules support 10/100 Mbps auto-negotiation, lowering network latency and supporting more robust monitoring databases.

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Common Migration Challenges

Migrating away from or replacing a fiber-optic ControlNet module presents specific hurdles that must be managed:

• Fiber-Optic Connector Integrity: ST connectors are highly sensitive to particulate contamination. Standard plant dust, oil mist, or moisture on the fiber tips will degrade the signal margin. Always clean the fiber tips with specialized lint-free optical wipes and fiber-cleaning pens before insertion.
• Bend Radius Constraints: Multi-mode fiber can experience micro-bends or macrobends if pulled too tightly around cabinet runs. Maintain a minimum bend radius of at least 30 mm (or as specified by the cable manufacturer) during re-routing.
• Keeper Configuration Conflicts: If replacing the card with a unit that has a different firmware major release, the ControlNet active keeper might reject the connection, showing a "Nut (Network Update Time) mismatch" or a general configuration fault on the PLC scanner module.
• Legacy I/O Mapping: If upgrading to EtherNet/IP, the physical configuration of the controller's I/O tree in the Logix designer software must compile the drive on an Ethernet port instead of a ControlNet local/remote scanner module. This step requires taking the PLC CPU out of Run mode to download the new hardware configuration.

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Step-by-Step Replacement Procedure

Follow these detailed steps to perform a safe, structured 1:1 replacement of the 20-COMM-Q ControlNet Fiber card.

                  TYPICAL DRIVE CONTROLLER BAY
       +-----------------------------------------------+
       |             [ PowerFlex Drive ]               |
       |                                               |
       |    +-------------------------------------+    |
       |    |            20-COMM-Q                |    |
       |    |                                     |    |
       |    |   +-------+   [ Rotaries ]          |    |
       |    |   | ST-TX |   (0)(1) - (9)(9)       |    |
       |    |   +-------+                         |    |
       |    |   +-------+   [ LED Array ]         |    |
       |    |   | ST-RX |   [PORT] [MOD] [NET]    |    |
       |    |   +-------+                         |    |
       |    +---|--|------------------------------+    |
       |        |  |                                   |
       +--------|--|-----------------------------------+
                |  |  Fiber Optic Cable Run
                v  v  (62.5/125um ST-ST)

1. Safety and Isolation

• Implement standard Lockout/Tagout (LOTO) protocols on the host PowerFlex drive.
• Isolate all high-voltage sources (480V/600V AC) powering the drive. Verify that the DC bus capacitor voltage has fully dissipated by checking the voltage test points or waiting for the drive's onboard LED indicator to fully extinguish (typically 5 to 10 minutes depending on drive frame size).

2. Disconnect Existing Media

• Locate the 20-COMM-Q communication card inside the drive cabinet or external DPI enclosure.
• Carefully detach the two ST fiber optic connectors by pressing them inward slightly and twisting counter-clockwise to release the bayonet spring mechanism.
• Immediately install protective dust caps onto both the fiber cable ends and the transceiver optical ports of the adapter to prevent contamination.

3. Physical Removal of Card

• If the card is mounted directly in the internal DPI slot, loosen the retaining screws or release the plastic mounting clips holding the card assembly in place.
• Gently slide the 20-COMM-Q module straight out of the DPI connector slot on the main drive control board. Avoid twisting the board, which could bend or break the pins of the internal DPI connection header.

4. Configuration of the Replacement Card

• Lay the failed card side-by-side with the replacement unit on an ESD-safe anti-static conductive mat.
• Locate the binary or decimal rotary address switches on the replacement 20-COMM-Q.
• Using a small precision flathead screwdriver, dial in the matching MAC ID (network address) to match the setting of the failed card (for example, if the original card was set to Node 12, set the switch TENS dial to 1 and the ONES dial to 2). Ensure the node address matches the exact address configured in your PLC program's ControlNet scanner device map.

       Rotary Switches Example (Node 12):
          [TENS]       [ONES]
           (1)          (2)
        0  -  9      0  -  9

5. Installation of the Replacement Card

• Carefully align the internal DPI connector pins of the new 20-COMM-Q with the slot header in the PowerFlex drive.
• Push firmly to slide the card into the slot until it is fully seated. Secure any mounting screws or snap-lock clips.
• Remove the dust caps from the fiber cables. Clean the cable ends with an optical-fiber cleaning pen.
• Insert the TX and RX fiber cables into their respective ST transceiver ports on the 20-COMM-Q. Rotate the connector lock clockwise until it clicks into the locked detent. Note: Ensure TX from the source/repeater enters the RX port of the 20-COMM-Q, and vice versa.

6. Power Verification & Tuning

• Restore main AC power to the PowerFlex drive.
• Observe the LED diagnostic array on the faceplate of the 20-COMM-Q:
  • PORT LED: Should display solid green, indicating a healthy DPI connection to the host drive control board.
  • MOD (Module) LED: Should display solid green, indicating successful internal module initialization.
  • NET (Network) LED: Must transition from flashing red/green to solid green once the active host ControlNet Keeper establishes cyclic data connection with the card.
• If the NET LED remains flashing red or solid red, verify fiber cable continuity, polarities (ensure TX and RX are not swapped), and double-check that the rotary node address is correct.

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Frequently Asked Questions

Q1: Can I hot-swap the 20-COMM-Q card while the drive is powered on?

No. Hot-swapping the 20-COMM-Q can cause electrical arcing across the DPI connector pins, potentially damaging the communication adapter, the drive's main control board, or causing an instantaneous drive hardware fault. Always fully de-energize the drive before replacing communication hardware.

Q2: What is the maximum physical distance supported by the fiber optics on the 20-COMM-Q?

Using standard 62.5/125 micron multi-mode fiber optic cabling (1300 nm wavelength), the maximum distance between active ControlNet nodes or repeaters using standard ST connectors is generally 2,000 meters (approximately 6,500 feet), assuming low attenuation across physical splices.

Q3: My replacement card has a solid red MOD LED. What does this mean?

A solid red MOD LED indicates an unrecoverable hardware failure within the module itself, or that the firmware of the unit has been corrupted during a flash update. If cycling power does not clear this error, the module must be replaced.

Q4: Do I need to perform a network reschedule in RSNetWorx after replacing a 20-COMM-Q with an identical model?

Typically, no. If the replacement module is set to the same ControlNet node address and runs compatible firmware, the network master (Keeper) will recognize it as the scheduled node and restore communication immediately. If physical module types are changed (e.g., swapping a 20-COMM-Q for a 20-COMM-C), a network reschedule is mandatory.

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Related Products & Families

• PowerFlex 7-Class Drives: PowerFlex 70, PowerFlex 700, PowerFlex 700S, PowerFlex 700H, and PowerFlex 700L AC Drives.
• ControlNet Communication Accessories: 20-COMM-C (Coax ControlNet Card), 1786-RPFM (Fiber-to-Coax Repeater module), 1786-TPS (T-Tap connectors), and 1756-CNB/CN2 series ControlLogix scanners.
• EtherNet/IP Alternatives: 20-COMM-E (Single-port EtherNet/IP adapter), 20-COMM-ER (Dual-port EtherNet/IP adapter with Device Level Ring support).

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Need Help?

Replacing legacy communication components can be challenging when managing outdated industrial networks. Palm Parts Solution has inventory and technical expertise to assist you. We can quickly supply new-in-box, high-quality refurbished, or reliable surplus 20-COMM-Q adapters—all backed by our comprehensive warranty. Contact our technical support and parts team today to find the exact parts and migration routes you need to keep your facility running smoothly.