20-COMM-P Profibus DP Adapter Replacement

Overview

The Allen-Bradley 20-COMM-P is a legacy communication adapter designed to integrate PowerFlex 7-Class variable frequency drives (VFDs) and PowerFlex DC drives into Profibus DP networks. Acting as a gateway between the drive's internal Drive Peripheral Interface (DPI) protocol and a Profibus master controller (typically a Siemens SIMATIC S7 PLC), the 20-COMM-P has served as a critical control-plane link in heavy industrial applications for nearly two decades.

Because Rockwell Automation has transitioned the 20-COMM product line to "End of Life" (EOL) status, sourcing immediate, high-quality replacements can be a significant roadblock for maintenance engineers. A failure of this communication card will fault out the drive, halt cyclically updated I/O, and trigger PLC master faults, resulting in severe process downtime.

This guide provides technical specifications, direct-replacement strategies, compatibility tables, and a step-by-step hardware and firmware commissioning breakdown to help you restore operation swiftly and safely.

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Legacy Product Information

The 20-COMM-P is a microprocessor-controlled communication interface that mounts directly in the drive's internal communication bay or, alternatively, in an external DPI expansion conduit box.

Key Technical Specifications

• Protocol Support: Profibus DP-V0 (cyclic data exchange) and DP-V1 (acyclic read/write diagnostics).
• Network Connector: Standard 9-pin Sub-D (female) port, electrically isolated.
• Transmission Rates: Auto-baud detection ranging from 9.6 Kbps up to 12 Mbps.
• Supported Messages: DP-V0 PPO (Parameter-Process Data Object) types 1, 2, 3, and 5.
• Internal Interface: DPI protocol operating at 125 Kbps or 500 Kbps.
• Current Draw: 280 mA at 5V DC (drawn from host drive's control power supply).
• GSD File Requirement: AB_0615.GSD or AB0615.GSD depending on the active firmware revision.

Lifecycle Status: Obsolete

Rockwell Automation has officially discontinued the 20-COMM-P. Modern control networks are transitioning to industrial Ethernet standards like EtherNet/IP or PROFINET. However, migrating an entire plant floor network is highly capital-intensive and time-consuming. Consequently, maintaining, repairing, or sourcing direct-replacement 20-COMM-P modules remains the most cost-effective path to sustain existing physical machinery.

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Recommended Replacements

For maintenance engineers faced with a failing module, three direct paths exist. Your choice will depend on whether you prioritize a continuous "drop-in" swap, a migration to an industrial Ethernet standard, or a complete drive modernization package.

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Compatibility Considerations

Before replacing or upgrading a 20-COMM-P, several compatibility requirements must be carefully analyzed:

1. Drive Compatibility

The 20-COMM-P is naturally compatible with the DPI bus structure of Rockwell’s legacy drive family:

• PowerFlex 70 (Standard and Enhanced)
• PowerFlex 700 (Standard, Vector, and Phase II)
• PowerFlex 700S & 700H
• PowerFlex 700L
• PowerFlex DC Drives

Note for modern drives: If you are attempting to use a 20-COMM-P on a PowerFlex 750-series VFD, you must use a 20-750-20COMM-F1 (or 20-750-20COMM) adapter board to physically bridge the module to the drive’s modern backplane.

2. Physical Layout & Sub-D Connectors

The Sub-D 9-pin connector pinout is conformant to the Profibus standard. Verify proper Shield-to-Ground connections are retained:

• Pin 3: RxD/TxD-P (Data B, positive - Red wire)
• Pin 8: RxD/TxD-N (Data A, negative - Green wire)
• Pin 5: DGND (Data Ground)
• Pin 6: VP (+5V output for active termination power)

3. Software & Firmware Dependencies

• GSD File Revisions: If your original 20-COMM-P utilized an older firmware (e.g., v1.xx) and you install a refurbished card configured with a later firmware (e.g., v3.xx), specific parameter mappings or diagnostics may change slightly. However, in standard DP-V0 cyclic operations, the standard AB0615.GSD profile remains backward-compatible.
• Firmware Flashing: Firmware on the replacement module can be updated over the DPI link using Rockwell’s local software suite (Connected Components Workbench, DriveExplorer, or DriveExecutive) with an aligned interface adapter like a 1203-USB.

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Upgrade Benefits

While swapping with a drop-in 20-COMM-P keeps your physical architecture intact, upgrading your module type or transitioning standard fieldbus networks yields massive operating benefits:

• Sustained Diagnostic Integrity: By using fresh or fully certified refurbished units, you eliminate intermittent connection drops caused by degraded internal capacitors or transceiver thermal wear.
• Simplification of Communications: Converting to a 20-COMM-E module moves your facility towards uniform, high-speed Gigabit star network topologies with no need for the segment length and terminators that are the Achilles heel of standard Profibus chains.
• Future-Proofing via 20-750 Line: Transitioning to the modern 20-750-PBUS module platform preserves your software investment in Siemens PLCs and step-7 code blocks, while allowing physical deployment of modern, readily available PowerFlex 750-Series motor controllers.

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Common Migration Challenges

When removing an old 20-COMM-P and preparing a replacement module, expect these common field issues:

• Inadvertent Address Changes: The 20-COMM-P uses three physical rotary switches under its cover to set its network node address (0 to 125). If these switches are set incorrectly on the replacement module, a duplicate node fault or standard connection failure will occurred. Setting them to 999 defaults node selection to Drive Software parameters instead.
• Active Termination Failures: If the drive being replaced resides at the physical boundary end of a Profibus segment, the DB9 connector's integrated termination switch must be set to "ON." Forgetting to re-engage this physical switch will collapse reliable frame transmission for all adjacent network nodes.
• Incorrect Host DPI Baud Rates: If host DPI communication speed was manually customized to 125 Kbps instead of 500 Kbps, the replacement block may fail to communicate with the drive (showing flashing LED states on PORT status) until parameters are initialized or auto-tuned.

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Step-by-Step Replacement Procedure

Follow these detailed steps to perform a safe and highly reliable unit substitution:

Step 1: Pre-Replacement Verification

1. Place the parent machine and surrounding process loops into a safe, controlled state.
2. Isolate the drive from its main AC feeder line using appropriate lock-out, tag-out (LOTO) protocols.
3. Wait at least 5 minutes for the drive's internal DC bus capacitors to fully discharge. Always verify 0V DC with an appropriately rated digital multimeter on the drive's power terminal blocks before proceeding.

Step 2: Extracting the Faulty Unit

1. Unplug the 9-pin Sub-D Profibus network connector from the 20-COMM-P's front port. Inspect the cable and pins for structural sign of oxidation or damage.
2. If mounted internally, remove the drive cover.
3. Locate the mechanical mounting screws securing the communication module to the drive’s internal bracket.
4. Carefully disconnect the internal DPI ribbon cable (if utilized in your specific drive layout frame size) or gently pull the module out of its integrated backplane slot.

          [PowerFlex Drive Bay]
                   |
     (Gently pull module straight out)
                   |
         [20-COMM-P Adapter]  <--- Disconnect Internal DPI Cable
                   |
         [9-Pin Sub-D Profibus] <--- Unplug DB9 Connector

Step 3: Preparing the Replacement Module

1. Locate the three rotary switches on the replacement 20-COMM-P PCB.
2. Align the dial settings (hundreds, tens, ones) to match the node address from the original hardware. Example: For node 42, set "Hundreds" to 0, "Tens" to 4, and "Ones" to 2.
3. Verify that the grounding clip or grounding cable is aligned properly to ensure clean dissipation of industrial high-frequency electromagnetic noise.

Step 4: Installation & Reassembly

1. Align the replacement module with the guide rails in the drive chassis.
2. Push the module firmly into the connection slot, verifying that the DPI bridge header pins mate cleanly without bending. If a ribbon connector is used, secure it until the side latches lock.
3. Tighten the physical retaining screws to ground the module housing properly.
4. Re-plug the DB9 fieldbus cable. If this node is at an end-of-run location, confirm the physical termination switch on the DB9 outer shell is flipped to "ON."
5. Reinstall the plastic safety covers on the VFD.

Step 5: Power-Up and Calibration

1. Restore control power to the drive.
2. Observe the module’s diagnostic LEDs:
   • PORT LED: Should state Solid Green, indicating proper DPI interface communication.
   • MOD LED: Should state Solid Green, indicating the functional processor internal cycle.
   • NET LED: Should flash or stay green depending on whether the Master PLC is cyclically polling the node.
3. If the NET LED flashes red, check parameter 03 [DP Addr Source] or inspect PLC master configuration mismatching.
4. Perform an operational motor dry-run test to check dynamic speed command update rates.

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Frequently Asked Questions

Q1: What is the significance of the 20-COMM-P LED states?

• PORT LED: Solid Green = DPI linked. Flashing Red = DPI connection failed.
• MOD LED: Solid Green = Normal logic runtime. Red = Boot/Internal firmware crash.
• NET LED: Solid Green = Online and active cyclic exchange with CPU. Flashing Green = Module linked but idle (PLC in hold or program mode). Red = Complete timeout of communication sequence.

Q2: Do I need to alter the Siemens PLC program when replacing a 20-COMM-P with the same part number?

No. An identical, fresh or refurbished 20-COMM-P module set to the exact same physical node address using the hardware dial wheels functions as a direct drop-in. No changes to TIA Portal, Step-7, or the underlying system firmware configuration parameters are required.

Q3: What should I check if my new module has a solid green "PORT" LED but a flashing green "NET" LED?

A flashing green NET LED indicates that while the module is functionally ready and online, it is not currently exchanging cyclic I/O data with the Profibus master. This is typically due to the Master PLC being placed in "Stop" or "Program" mode, or the Node address configured on the hardware dials not matching the exact Profibus configuration database in the PLC.

Q4: Can I use the 20-COMM-P on a newer PowerFlex 753 drive?

Yes, but you will require a hardware carrier card known as a 20-750-20COMM-F1. This card inserts into Slot 4 (or other optional communication ports) of the 753 frame and adapts the modern drive interface to standard, legacy 20-COMM modules.

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Related Products & Families

• PowerFlex 700 / 70 / 700S series AC drives
• 20-COMM-E (EtherNet/IP Web-enabled Communication Adapter)
• 20-COMM-C (ControlNet Coaxial/Fiber Communication Adapter)
• 20-COMM-D (DeviceNet Network Interface Adapter)
• 20-750-PBUS (Native Profibus DP Module option for modern PowerFlex 750 platforms)

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Need Help?

Finding high-quality, obsolete parts to keep your legacy systems online can be a major challenge. Palm Parts Solution can supply high-quality replacement modules, including new, refurbished, or surplus 20-COMM-P units. Each module is thoroughly inspected, cleaned, and testing-certified by senior engineers, and ships with an industrial-grade warranty. Contact our parts division today for prompt shipping across North America.