Wiring 11 — Quadrature Encoder to PLC High-Speed Counter Input

Quadrature Encoder to PLC High-Speed Counter Input

The Quadrature Principle

An incremental quadrature encoder produces two digital pulse trains — A and B — that are 90° out of phase with each other. Because of this phase offset, the PLC can determine both position (by counting edges) and direction of travel: when A leads B the axis is moving forward; when B leads A the axis is moving in reverse. A third signal, Z, produces a single pulse per revolution at a repeatable mechanical position. This index or marker pulse is used to establish an absolute reference after a power cycle or alarm recovery — without it the PLC only ever knows relative displacement from wherever it started.

Why Line-Driver / Differential Wiring

Encoders are available in two output circuit types. Open-collector encoders use three signal wires (A, B, Z), a supply, a common, and a shield — six conductors total. They are cheap and adequate for cable runs up to roughly 5–10 metres.

In an industrial cabinet sharing cable trays with VFDs, contactors, and power wiring, open-collector signals are unacceptably noisy over longer runs. Line-driver (RS-422 differential) encoders solve this by sending each phase as a complementary pair: A and A' are always opposite in polarity. The HSC card's receiver subtracts A' from A; any noise that appears equally on both wires — called common-mode noise — cancels out mathematically. This is the same noise-rejection principle used by analog 4–20 mA loops and RS-485 networks. Line-driver wiring is the industrial default whenever cable runs exceed a few metres or EMI sources are present.

5V TTL vs 24V HTL Encoders

Line-driver encoders come in two voltage families. 5V TTL encoders are the traditional choice: they support higher pulse rates (up to several MHz), are cheaper, and suit servo drives with built-in 5V encoder supplies. 24V HTL encoders swing the signal rails to the full 24V bus, making them more immune to resistive voltage drop over long cable runs and compatible with standard PLC I/O supply rails. This lesson uses a 24V encoder — the more common choice in general-purpose industrial automation.

Shield Grounding — One End Only

The encoder cable shield must be bonded to protective earth at exactly one point. In this topology, the shield bonds at the PLC HSC card's SHIELD terminal, which is internally connected to the cabinet ground bar. The encoder end of the shield is left floating. Bonding both ends creates a low-impedance loop between two ground points; any difference in earth potential between the machine frame and the control cabinet drives a circulating current through that loop, inducing noise directly on the differential signal — the exact problem you are trying to prevent. This single-point shield rule is identical to the rule you applied in lessons 06 and 07 for analog signals.

HSC Card vs General Digital Input

General-purpose DI cards can read encoder A and B signals at low speeds, but they route pulses through the PLC backplane and firmware scan cycle. Above a few kHz the firmware cannot keep up and pulses are lost. HSC cards contain dedicated quadrature decode logic in hardware (typically an FPGA counter): they latch every edge in real time, maintain a 32-bit position register, and detect direction without consuming any CPU scan time. Use an HSC card whenever an encoder operates above 1–2 kHz or when direction information is required with guaranteed accuracy.