Encoder Mode - Measurement Computing PCI-2511 User Manual

PCI-2511 User's Guide Functional Details
Trigger after stable mode behaves more like a traditional debounce function: rejecting glitches and only passing
state transitions after a required period of stability. Trigger after stable mode is used with electro-mechanical
devices like encoders and mechanical switches to reject switch bounce and disturbances due to a vibrating
encoder that is not otherwise moving. The debounce time should be set short enough to accept the desired input
pulse but longer than the period of the undesired disturbance as shown in Figure 11.
Figure 11. Optimal debounce time for trigger after stable mode

Encoder mode

Rotary shaft encoders are frequently used with CNC equipment, metal-working machines, packaging
equipment, elevators, valve control systems, and in a multitude of other applications in which rotary shafts are
involved.
The encoder mode allows the PCI-2511 to use data from optical incremental quadrature encoders. In encoder
mode, the PCI-2511 accepts single-ended inputs. When reading phase A, phase B, and index Z signals, the PCI-
2511 provides positioning, direction, and velocity data.
The PCI-2511 can receive input from up to two encoders.
The PCI-2511 supports quadrature encoders with a 16-bit (counter low) or a 32-bit (counter high) counter,
20 MHz frequency, and X1, X2, and X4 count modes. With only phase A and phase B signals, two channels are
supported; with phase A, phase B, and index Z signals, 1 channel is supported. Each input can be debounced
from 500 ns to 25.5 ms (total of 16 selections) to eliminate extraneous noise or switch induced transients.
Encoder input signals must be within -5 V to +10 V and the switching threshold is TTL (1.3V).
Quadrature encoders generally have three outputs: A, B, and Z. The A and B signals are pulse trains driven by
an optical sensor inside the encoder. As the encoder shaft rotates, a laminated optical shield rotates inside the
encoder. The shield has three concentric circular patterns of alternating opaque and transparent windows
through which an LED shines. There is one LED and one phototransistor for each of the concentric circular
patterns. One phototransistor produces the A signal, another phototransistor produces the B signal and the last
phototransistor produces the Z signal. The concentric pattern for A has 512 window pairs (or 1024, 4096, etc.)
When using a counter for a trigger source, use a pre-trigger with a value of at least 1. Since all counters start at
zero with the initial scan, there is no valid reference in regard to rising or falling edge. Setting a pre-trigger to
1 or more ensures that a valid reference value is present and that the first trigger is legitimate.
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