20-COMM-C ControlNet Adapter Replacement Guide

Overview

The Allen-Bradley 20-COMM-C is a legacy ControlNet communication adapter designed to interface PowerFlex 7-Class variable frequency drives (VFDs)—such as the PowerFlex 70, 700, 700S, 700H, and 700L—with redundant ControlNet networks. Integrating via Rockwell Automation’s proprietary Drive Peripheral Interface (DPI), the 20-COMM-C allows real-time, deterministic control, monitoring, and configuring of the drive over a coaxial ControlNet network.

As control systems mature and plant networks standardize on Ethernet-based industrial protocols, the obsolete status of ControlNet hardware presents a significant operational risk. This technical guide outlines the legacy product specifications, compatible replacement strategies (both direct swap-outs and modernization pathways), step-by-step physical replacement procedures, and network configurations required to restore or modernize your drive control loops.

---

Legacy Product Information

The 20-COMM-C is a specialized communication interface card that sits inside the drive's internal communication terminal pocket or in an external DPI expansion kit. It manages control words, reference values, status responses, feedback variables, and drive parameters through input/output (I/O) image mapping.

Technical Specifications

• Catalog Number: 20-COMM-C (Series A, B, and C)
• Protocol: ControlNet (Coaxial media, up to 5 Mbps baud rate)
• Hardware Interface: 20-pin DPI internal ribbon connector (internal) or 5-pin DPI connector (external)
• Media Connections: Dual BNC connections (Channel A and Channel B) for redundant network media topology, and an 8-pin RJ45-style Network Access Port (NAP) for local programming tools
• Addressing Mechanism: Three onboard rotary switches (Hundreds, Tens, Units) to establish network node addresses from 1 to 99
• Datalink Support: Up to 8 configurable Datalinks (4 Input, 4 Output) to transfer 16-bit or 32-bit parameters without explicit messaging overhead
• Lifecycle Status: Obsolete / Discontinued. Official factory support from Rockwell Automation has transitioned to end-of-life, making new-OEM units unavailable.

Common Uses & Application Profile

The 20-COMM-C is widely deployed in legacy distributed control systems (DCS) and programmable logic controller (PLC) configurations (e.g., ControlLogix 5550/5560 systems, PLC-5/C, or SLC 500 with scanner modules). Key industries include municipal water/wastewater, heavy material handling, mining, petroleum refining, and pulp/paper plants where deterministic, scheduled network updates are required over long distances.

---

Recommended Replacements

When a 20-COMM-C fails or is slated for retirement, maintenance engineers have three main paths: drop-in hardware replacement using surplus stock, migrating to EtherNet/IP while retaining the legacy drive, or completely upgrading the drive and network infrastructure.

---

Compatibility Considerations

When selecting a replacement strategy, several technical alignment checks must be executed to prevent commissioning failures.

Wiring and Physical Footprint

• Direct Swap (20-COMM-C to 20-COMM-C): The physical footprint is identical. Slide-in mounting into the drive’s internal option bay. Note that the twin coaxial BNC cables must be reconnected to Channels A and B exactly as before to maintain media redundancy functionality. The 1786-TPS ControlNet tap must not exceed length specifications.
• Network Switch (20-COMM-C to 20-COMM-E): Requires pulling Cat5e or Cat6 Industrial Ethernet cabling to replace the coaxial cables. The 20-COMM-E card fits into the same physical internal DPI slot on PowerFlex 7-Class drives.

Software and Firmware Dependencies

• Network Scheduling (ControlNet Only): ControlNet is a scheduled network. If you swap a 20-COMM-C with an identical module of a differing firmware major revision, the network Active Keeper (typically a PLC ControlNet bridge module, e.g., 1756-CN2R) may reject the connection due to an electronic keying mismatch.
• Electronic Keying: Ensure that the Master Controller’s I/O configuration is set to "Compatible Module" or "Disable Keying" if you cannot guarantee an exact match of the major and minor firmware revisions of the replacement module.
• RSNetWorx for ControlNet: If you must change the node address (via the rotary switches) of the replacement 20-COMM-C, you must rerun RSNetWorx for ControlNet, re-schedule the network, and save the updated configuration to the Keeper(s).

---

Upgrade Benefits

Migrating from the legacy 20-COMM-C ControlNet architecture to modern industrial protocols, specifically EtherNet/IP via the 20-COMM-E, offers operational and performance improvements:

1. Elimination of RSNetWorx Scheduling: ControlNet requires taking the network offline or putting the PLC run-time in scheduling mode to append new devices. EtherNet/IP allows real-time, online changes without network reschedule interruptions.
2. Standardized Diagnostic Diagnostics: Modern Ethernet switches enable managed port mirroring, SNMP diagnostics, and standard web page monitoring built natively into the 20-COMM-E.
3. Reduced Tooling Costs: Eliminates the need for specialized ControlNet interface cards (such as the 1784-U2CN USB-to-ControlNet cable) for local programming.
4. Enhanced Data Throughput: Standard ControlNet networks run at 5 Mbps. Migrating to EtherNet/IP increases physical layer bandwidth to 100 Mbps, facilitating rapid diagnostic data and dynamic explicit parameter changes.

---

Common Migration Challenges

• Nut (Network Update Time) Limitations: ControlNet uses a fixed NUT. Introducing a new node or altering parameter mappings can cause the NUT to exceed allowable limits, causing I/O connection timeouts on critical drives if not configured accurately.
• ControlNet Coaxial Tap Degradation: When unscrewing BNC connectors from the old 20-COMM-C, brittle coaxial cable or loose center pins inside the 1786-TPS tap can break, causing network-wide noise or reflections.
• Datalink Mapping Discrepancy: The 20-COMM-C supports dynamic Datalink mappings (A, B, C, D) configured within the drive's parameters (e.g., DPI Parameter 23 through 30). Failing to manually upload/backup these mapping parameters before removing the failed adapter will prevent the PLC from properly writing to or reading from the drive.

---

Step-by-Step Replacement Procedure

Follow these technical procedures to perform a safe, direct replacement of a failed 20-COMM-C adapter.

Step 1: Safety & Isolation

1. LOTO: De-energize the PowerFlex drive in accordance with Lockout-Tagout protocols.
2. Voltage Check: Verify that zero voltage is present across the incoming three-phase AC terminations (L1, L2, L3) and the internal DC bus terminals (+DC, -DC). Wait at least 5 minutes to allow the drive's internal capacitors to fully discharge. Check the DC bus with a reliable multimeter.

Step 2: Media and Option Removal

1. Record which coaxial cable is attached to Channel A and Channel B of the 20-COMM-C. Gently unscrew the BNC connections. Do not pull or twist the coaxial cable directly.
2. If present, disconnect the Network Access Port (NAP) cable.
3. Open the drive's front cover assembly or enclosure door.
4. Unscrew the module's securing screws (if utilizing an external DPI bracket) or unclip the internal card.
5. Unplug the internal 20-pin DPI ribbon cable from the top header of the 20-COMM-C module.

Step 3: Hardware Addressing Configuration

On the replacement 20-COMM-C module:

1. Locate the three rotary address switches labeled Hundreds, Tens, and Units.
2. Using a small screwdriver, rotate the switches to match the node address of the decommissioned module (range 01 to 99).
3. Ensure the address matches the configuration programmed in the PLC's ControlNet scanner profile.

Example: For Node Address 45
[Hundreds] -> Set to 0
[Tens] -> Set to 4
[Units] -> Set to 5

Step 4: Installation and Connection

1. Align the replacement module with the internal card mounting rails inside the drive or the external DPI pocket.
2. Securely connect the 20-pin DPI ribbon cable to the module's header. Ensure the polarity-keyed connector snaps locked.
3. Slide the 20-COMM-C into position and tighten any grounding or retention screws.
4. Re-attach the ControlNet BNC connectors to Channel A and Channel B respectively. Verify mechanical tightness.

Step 5: Power-On and Initialization

1. Close all drive covers.
2. Restore AC line power to the PowerFlex drive.
3. Observe the four LED diagnostic indicators on the face of the 20-COMM-C:
   • PORT LED: Solid Green indicates correct DPI communication with the host drive.
   • MOD LED: Solid Green indicates the module itself is operating normally.
   • NET A / NET B LEDs: Alternating red/green indicates network search. Solid Green signifies that the ControlNet connection is active, scheduled, and communicating with the PLC Master.
4. Verify that the drive's LCD Human Interface Module (HIM) does not report a "F122 DPI Port X Error".

Step 6: Network Re-Scheduling (If Required)

If you replaced the module with an alternative firmware version causing a connection fault:

1. Connect your PC to the ControlNet network using a 1784-U2CN interface module connected to the 20-COMM-C NAP port.
2. Open RSNetWorx for ControlNet.
3. Perform a network "Go-Online" scan.
4. If a module mismatch or unscheduled node error exists, click the "Enable Edit" button, allow the software to optimize the schedule, and download the new configuration to the keeper.

---

Frequently Asked Questions

Q1: Can I replace a 20-COMM-C with a 20-COMM-E directly?

A: Yes, physically the 20-COMM-E fits into the same internal option slot. However, this replaces the ControlNet media with EtherNet/IP cabling. You must modify the I/O tree configuration in your PLC program (Studio 5000) and change the physical connection from coaxial coax to standard Ethernet network drops.

Q2: What does a flashing red NET A or NET B LED mean on the replacement card?

A: A flashing red network LED indicates a connection timeout or that the node is not scheduled by a path master. Ensure the rotary switches match the original node address, the PLC is in Run mode, and the network schedule has been successfully saved to the Keeper.

Q3: My drive is saying "DPI Port 5 Fault" after swapping out the card. How do I fix this?

A: This fault indicates the drive has lost communication with the option card. Check the physical seating of the internal 20-pin ribbon cable. If the adapter was swapped while the drive logic power was active, cycle power to the drive to re-initialize DPI board enumeration.

Q4: Does the 20-COMM-C require a separate 24VDC power supply?

A: No. The 20-COMM-C draws its logic-level operational power directly from the host PowerFlex drive through the 20-pin ribbon cable interface.

Q5: Is it possible to configure Datalinks on the replacement module without RSNetWorx?

A: True Datalinks are configured via the drive's parameter list using the HIM cradle or through Connected Components Workbench (CCW) software. However, the ControlNet scanner configuration in Studio 5000 must have matching input/output assembly sizes, which corresponds to the schedule initialized in RSNetWorx for ControlNet.

---

Related Products & Families

• PowerFlex 70 / 700 / 700S / 700H Drives: The core variable frequency drives that directly host the 20-COMM-C.
• 20-COMM-E: The EtherNet/IP equivalent DPI communication card for PowerFlex 7-Class drives.
• 1786-TPS & 1786-TPYS: ControlNet Taps, coax connectors, and terminating resistors (75 Ohm BNC) essential for physical network health.
• 1756-CN2R / 1756-CNB: ControlLogix ControlNet bridge modules that serve as the network scheduling keepers.

---

Need Help?

Whether you require a direct replacement 20-COMM-C to restore production quickly or are planning a systematic upgrade to EtherNet/IP communication profiles, engineering support is available. At Palm Parts Solution, we specialize in providing high-quality new, refurbished, and surplus industrial automation components. Every module we supply is thoroughly tested by technical personnel and backed by a comprehensive warranty to ensure reliable operation when returned to active service.