Migrating CQM1 Encoder Cam Logic to Modicon M221 HSC

Overview: What the CQM1 Program Is Actually Doing

A mid-1990s Omron CQM1 running an encoder-driven cam programmer must be retrofitted to a Schneider Electric Modicon M221. Before any M221 code is written, the legacy logic has to be decoded exactly, because the whole machine sequence hangs on four elements:

• The DM6642 PLC Setup word (value 100 as read on the programming tool), which enables and configures the built-in high-speed counter.
• Encoder inputs on the first input words: IR 000, IR 001, IR 002 are referenced by the application.
• A first-scan preset pair: MOV 230 → DM0100, MOV 0 → DM0101, then INI(61) to load the counter present value (PV) and enable counter evaluation.
• BCMP(68) comparing the counter source word against 16 range pairs starting at DM0000, writing the result bit map into HR00.

The result is a software cam switch: one instruction converts the encoder position into up to 16 zone bits. Each bit is a cam segment output.

Decoding the Block-by-Block Logic

The BCMP range table

BCMP(68) consumes 32 consecutive words as 16 range pairs. The mapping is strictly positional:

Result logic per pair: source < lower limit → bit = 0; lower ≤ source ≤ upper → bit = 1; source > upper → bit = 0. In this machine only DM0000…DM0013 carry cam values, i.e. seven active segments driving HR00.00…HR00.06.

Resolving the "230" Ambiguity

Two readings of the number 230 are circulating, and they lead to completely different M221 designs. Do not pick one silently — test it.

Verify on the live machine before the swap:

1. Open a data monitor on the BCMP source operand and jog the machine by hand. If the value follows the shaft, the source is the counter PV word, not a constant.
2. Cross-reference DM6642 in the program listing. If nothing writes it, the value came from the PLC Setup and is read at power-up only.
3. Confirm the CPU model suffix (the CQM1-CPU__ part number). High-speed counter capability and the PV storage word depend on the CPU variant; check the operation manual for that exact model rather than assuming.
4. Record whether the counter is configured for incremental (up/down) or up/down phase-difference (quadrature) operation, and whether the Z input performs a reset.

DM6642 and the input assignment

DM6642 is part of the CQM1 PLC Setup area. Setup words are read at power-up, so a change requires a power cycle to take effect — and a value read as 100 on the tool may be displayed in hexadecimal or decimal depending on the monitor format. Note the display base before you convert it. The referenced inputs IR 000, IR 001, IR 002 occupy the dedicated high-speed input positions; on a quadrature setup these carry phase A, phase B and the Z/marker pulse. Confirm the physical wiring at the terminal block and the encoder output type (push-pull, open collector, line driver, 12 V or 24 V) — this determines the M221 input hardware you need.

Mapping the Function to the M221

The M221 provides high-speed counter function blocks configured in EcoStruxure Machine Expert – Basic. Verify the following against the Modicon M221 Logic Controller Programming Guide and the hardware guide for your exact reference, because HSC channel count, maximum frequency and dedicated input positions vary by controller reference.

Reproducing BCMP on the M221

The M221 has no single block-compare instruction with identical semantics. Implement the seven active segments as an explicit comparison, using the counter current value in place of the CQM1 PV word:

(* Position source: HSC current value copied to %MW200 each scan *)
%MW200 := %HSC0.V;

(* Segment 0: DM0 / DM1 -> HR0.0 *)
%M100 := (%MW200 >= %MW0) AND (%MW200 <= %MW1);
(* Segment 1: DM2 / DM3 -> HR0.1 *)
%M101 := (%MW200 >= %MW2) AND (%MW200 <= %MW3);
(* ... repeat through Segment 6: %MW12 / %MW13 -> %M106 *)

If a downstream routine expects the packed word, rebuild it bit by bit into %MW210 using the bit-access notation %MW210:X0 … %MW210:X6, mirroring the original HR00 layout. Keep the same bit order so that any documented output mapping remains valid.

Commissioning and Verification

1. Capture the baseline. Upload the CQM1 program and dump DM0000…DM0031, DM0100, DM0101 and DM6642. Photograph the encoder terminal block and record the encoder pulses-per-revolution from its nameplate.
2. Log live behaviour. With the machine running at normal speed, record the counter source word and HR00 together. Note the exact position value at each cam transition — this is your acceptance criterion, independent of any interpretation of the code.
3. Static test on the M221. With the drive locked out, force %MW200 through the full range and confirm each segment bit turns on and off at the recorded limits, inclusive of both endpoints (the CQM1 test is ≤ and ≥, not strict inequality).
4. Direction and edge test. Turn the encoder by hand forward and reverse. The count must increment forward and decrement in reverse. If it counts backwards, swap A and B; if it counts only in one direction, the channel is configured for single-phase pulse counting instead of quadrature.
5. Marker test. Rotate through the Z pulse and verify the position value re-syncs to the intended preset every revolution with no cumulative drift over 20+ revolutions.
6. Speed test. Run at maximum machine speed and compare the pulse rate (PPR × rev/s) against the HSC maximum input frequency for your M221 reference. Miscounting at speed with correct counting at low speed points to frequency limits, input filtering, or encoder cabling/shielding.
7. Overlap check. Confirm that segments intended to be mutually exclusive never assert simultaneously, and that no gap exists between adjacent segments where all bits fall to 0.

One further option worth pricing before committing to the retrofit: if only two or three machines run Omron and the rest of the plant is Schneider, standardising on the M221 is justified on spare parts and skills alone. If instead the goal is minimum engineering risk, a later-generation Omron CPU keeps the counter semantics closer to the original and eliminates the transcription errors that come with retyping cam tables by hand. Weigh the two against how much of the remaining program logic must also be rewritten.

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