How to Test Encoders With Oscilloscopes
By Ed Wagner
Encoders are common in electronics. They are electro-mechanical devices that perform many functions, which include setting the frequency in radios or altitudes and speed in aircraft instruments. Modern encoders are mostly optical while older ones are mechanical. Depending on the type, they may have serial or parallel data outputs. Serial data transmission is used where devices are widely separated, while parallel data is generally confined to local use.
Connect the oscilloscope probe to one of the output bits of a parallel data encoder. Couple it for DC, 1 volt vertical deflection and set the time base as needed. As the encoder rotates, the data bit should change between 0 and 5 volts. Some very old systems used 0 and 7 volts but they're rare. Each bit may be connected to an integrated circuit that converts the parallel data to serial data -- a repeating series of pulses -- complete with address and control data for use by other devices on the system. Alternatively, parallel data may be sent to buffer amplifiers used for isolation and then sent on to devices on the same or nearby circuit boards.
Connect the oscilloscope probe to the output of a serial data encoder. It may connect to a simple op-amp or a specialized integrated circuit dedicated to performing a more complex task. Like the parallel encoder, the display will show a voltage changing between 0 and 5 volts as the encoder rotates. A simple optical encoder merely indicates motion with pulses, but a quadrature encoder indicates both motion and direction, and an integrated circuit can interpret this data to render speed information too. If this were used to change radio frequencies, for instance, a slow rotation might cause the display to increment by 5 kHz, while a fast rotation may cause a 25 kHz increment.
Watch the pulses coming from the output of the encoder as you rotate the selector. If the pulses do not change, the encoder has probably failed. Look for noise or intermittent operation -- from a mechanical encoder -- that may indicate dust or other contamination. Sometimes erratic operation occurs from an impact or bend in the encoder shaft. Pushing or pulling on the shaft while rotating may induce a failure also. A failed optical encoder cannot be repaired. Replacement is the only option.
I'm a professional electronics technician working on commercial aircraft. My main responsibility is the Honeywell Enhanced Ground Proximity Warning Computer, a 486-based machine that uses GPS information to compare aircraft position, speed and altitude against a simplified terrain map of the planet. This is a flight critical system on modern aircraft, but it's not the sole focus of my work. I've done maintenance on aircraft electronics beginning with the largely electro-mechanical systems on 727s and DC-10s, mostly on autopilots but including navigation and communication radios as well. I enjoy explaining technology to people unacquainted with it, and try to write as simply and directly as possible.