Resolving R88D-GT20H Encoder Feedback on CP1E-N40DT-D PLC Inputs

The combination of an Omron R88D-GT20H G-Series AC servo drive and a CP1E-N40DT-D PLC is a common low-cost positioning architecture, but the apparent simplicity of "wire A+, B+, Z+ into the PLC high-speed counter" hides a fundamental protocol limitation. Engineers frequently wire the encoder signal pins directly to CP1E inputs 0.00, 0.01, and 0.04 and find that no pulses are registered, no error is flagged, and the high-speed counter remains at zero. This article explains why the wiring is electrically valid but functionally useless, and provides a complete working replacement architecture using incremental encoder feedback plus INP and ERR drive outputs.

1. Problem Statement

A CP1E-N40DT-D is being used as a positioning controller for an R88D-GT20H drive (typically paired with an R88M-G series servomotor) through the PLS2 (Pulsed Output 2) instruction. The drive moves correctly when commanded, but no encoder feedback pulses are visible to the PLC, even though the wiring is connected:

• Encoder A+ to CP1E input 0.00
• Encoder B+ to CP1E input 0.01
• Encoder Z+ to CP1E input 0.04

The PLC program runs without a fault, but the high-speed counter (HSC) value never changes. There is no error code in the PLC, no alarm on the drive, and the motor continues to follow the PLS2 pulse train as expected. The application requires position verification between the commanded pulse count and the actual motor position.

2. Affected Hardware and Firmware

The fault class is the same across all R88D-GT__H (200 V class) and R88D-GP__H (100 V class) G-Series drives when paired with any CP1E/CP1L/CP1H PLC; it is not firmware-version dependent but is intrinsic to the G-Series encoder interface architecture.

3. Root Cause Analysis

3.1 The G-Series Does Not Expose Absolute Encoder Position as Pulses

The R88D-GT20H motor uses an absolute encoder internally, but the CN2 encoder output connector on the drive does not stream the absolute position as a quadrature pulse train. According to the G-Series user manual, the CN2 pinout is wired to a duplicated channel of the motor encoder and is intended to be used as an incremental feedback signal for an external motion controller. The drive itself holds the absolute multi-turn and single-turn information internally and uses it for homing retention only.

There is no serial bus (such as SSI, BiSS, or EnDat) on the CN2 connector. The G-Series architecture assumes that any "absolute" position is held by the drive and exposed to the controller only through the INP (in-position) discrete output and the deviation-counter alarm behavior. Therefore, even a perfectly wired, perfectly terminated, perfectly configured CP1E HSC will count 0 pulses on a drive that is at rest and correctly positioned, because no incremental pulses are generated while the motor is stationary.

3.2 The Drive Encoder Output Is a Line-Driver Signal

The CN2 encoder output on the R88D-GT20H is a 5 V RS-422 line driver signal. The CP1E-N40DT-D's built-in inputs are 24 V DC sinking/sourcing, with a typical ON threshold of approximately 17 V. Directly wiring a 5 V differential signal into a 24 V input is electrically incompatible: the line driver will be unable to drive the input above its ON threshold, and the high-speed counter will never see a transition.

3.3 The CP1E HSC Inputs Must Be Explicitly Enabled

Even when an HSC-compatible input is physically wired, the CP1E ships with all built-in high-speed counter functions disabled. The 0.00, 0.01, and 0.04 terminals can be assigned to HSC0, HSC1, or HSC3, but only after the configuration is downloaded and the PLC is power-cycled. Until then, those inputs behave as ordinary DC inputs and may register 0/1 transitions at the standard scan rate, not as HSC counts.

4. Encoder Signal Architecture of the G-Series

Understanding the encoder data path is essential for selecting a workable architecture. The R88D-GT20H receives the raw encoder signal from the R88M-G motor over the CN2 cable and:

1. Decodes the absolute single-turn and multi-turn counts for internal servo control.
2. Reproduces an incremental quadrature (A/B) plus index (Z) signal on the CN2 output pins for external use.
3. Generates the INP discrete output when the deviation counter is below the configured threshold (Pn522 on the G-Series).
4. Asserts the ALM (alarm) output when a position, overcurrent, overspeed, or encoder fault occurs.

Therefore, the only feedback signals that are reliably accessible to a CP1E are the discrete INP and ALM/ERR outputs. The quadrature signal is available but is the wrong voltage for direct CP1E input use.

5. Recommended Solution Architecture

The reliable, documented way to verify position on a CP1E-controlled G-Series axis is:

1. Disable the absolute encoder mode in the drive (parameter Pn001 = 0 for incremental, or the equivalent G-Series incremental selection).
2. Add an external home proximity switch wired to a CP1E standard input.
3. Use the INP drive output as the in-position verification flag.
4. Use the drive ALM output as a position-error indicator.
5. Issue ORG (Origin Search) and PLS2 instructions to move the axis.
6. Confirm the INP signal is asserted after every move before issuing the next command.

This approach trades the goal of "read raw encoder count" for the more practical goal of "confirm the axis reached the commanded position," which is what the application actually requires.

6. Wiring and Termination

6.1 Drive to PLC Control Wiring

6.2 Encoder Output Wiring (Optional, If HSC Verification Is Required)

If a closed-loop count is required (for example, for a registration application), a line-receiver board must be inserted between the drive's CN2 output and the CP1E HSC input. A typical implementation uses a TI AM26LS32 or equivalent RS-422-to-TTL receiver, followed by a 24 V level-shifter to drive the CP1E input. The Z (index) channel is wired to 0.04 as a count reset.

For most positioning applications, this hardware addition is unnecessary. The combination of PLS2 plus INP assertion provides equivalent functional verification at a fraction of the wiring cost.

7. CX-Programmer Configuration Steps

The following procedure configures the CP1E-N40DT-D to use outputs 100.00 and 100.01 as the PLS2 pulse output channel, and input 0.02 as the in-position verification flag.

7.1 Configure the Pulse Output Channel

1. Open the project in CX-Programmer and double-click Settings in the project tree.
2. Select the Built-in Input/Output tab.
3. On the Pulse Output 0 row, set Use = Used, Output Mode = CW/CCW, and confirm the assigned outputs (100.00 and 100.01).
4. Click Transfer to PLC, confirm the dialog, and wait for the transfer to complete.
5. Cycle power to the CP1E. The new pulse output assignments do not take effect until a cold start.

7.2 Configure the In-Position Input as an Interrupt

1. In the same Settings dialog, click the Built-in Input tab.
2. Map input 0.02 (the INP input) to the desired interrupt task, for example Interrupt Task 140.
3. Write the in-position handling routine into the interrupt task: set a completion flag, latch the current PLS2 target register, and clear the busy flag.
4. Transfer and power-cycle the PLC again.

7.3 Sample PLS2 Ladder

| W0.00  --[ ]--+--[PLS2(887) D0 D10 D20]--|
| (Move Cmd)   |   S: D0   = Pulse Frequency (Hz)
|              |   N: D10  = Pulse Count
|              |   Accel: D20 = Acceleration rate
|              |   Decel: D22 = Deceleration rate
|              |
| 0.02 (INP) --[ ]--( W1.00 )--|  In-Position OK
| 0.03 (ALM) --[/]--( W1.01 )--|  Alarm clear

After the PLS2 instruction is executed, the program monitors W1.00 for completion. The next move is gated on W1.00 = ON and W1.01 = ON (no active alarm).

8. Drive Parameter Settings

The G-Series drive must be configured for the CP1E command interface. The relevant parameters are:

9. Verification Procedure

After wiring and configuration, perform the following verification sequence:

1. Power the drive and PLC. Confirm the drive front panel shows bb (base block) or run with no alarm code.
2. From CX-Programmer, force W0.00 ON. The drive should rotate at the commanded frequency for the commanded pulse count.
3. Monitor W1.00. It should transition ON within the positioning window defined by Pn522.
4. Move the load manually and verify that W1.01 (alarm) does not assert unless the deviation counter exceeds Pn520.
5. Issue a series of varying PLS2 commands and confirm that W1.00 consistently asserts within the expected time window.
6. If W1.01 asserts, read the drive's alarm code from the front panel and consult the alarm list in the G-Series user manual.

10. Alternative: Motion Controller Architecture

If true position feedback is required rather than in-position verification, the application is using the wrong controller category. The CP1E is a programmable logic controller with pulse-output capability; it is not a motion controller with servo-loop closure. For closed-loop position control, use one of the following:

For G-Series replacement in new applications, the 1S-Series is the modern equivalent and provides native absolute encoder data through EtherCAT, eliminating the position-feedback problem at its source. For a 1S-Series tuning and commissioning walk-through, the Omron 1S-Series Multi-axis Setup and Tuning Getting Started Guide (I827) provides step-by-step procedures. A broader view of the Omron servo and inverter product family is available on the Servos and Inverters category page.

11. Troubleshooting Matrix

12. Safety and Commissioning Notes

• Always confirm the ALM (alarm) output is wired into a PLC input that participates in the safety stop logic. The G-Series alarm output is open-collector and must be configured as normally-closed or normally-open according to the application.
• Set Pn520 (deviation counter overflow) to a value that is well above the normal following error at maximum acceleration. A value of 8192 pulses is a typical starting point for a 200 W axis.
• After any drive parameter change, cycle the drive control power (SERVO OFF then SERVO ON) for the new value to take effect.
• Do not hot-plug the CN2 encoder cable. The R88M-G encoder is sensitive to ESD and supply rail disturbances during insertion.
• When migrating from G-Series to 1S-Series, replace the CN1 wiring scheme entirely. The 1S-Series uses EtherCAT on CN1 and does not accept pulse-train commands.