Pulsecoder

A pulsecoder is FANUC's name for the absolute serial encoder built into the tail of every servo motor. It reports shaft position to the axis control card over the encoder cable as a continuous serial data stream, giving the controller a fresh absolute reading each servo cycle so the axis position is known the instant the drives come out of e-stop.

It's absolute rather than incremental, which is the reason a small battery hangs off every motor. That battery keeps the pulsecoder's rotation counter running when the cabinet is powered down; lose the battery and you lose the absolute count, which forces re-mastering with the single-axis mastering procedure before the axis can move under program control again.

Faults on this device are among the most common feedback issues in the fleet, and they come in three flavors. Battery-loss alarms (BZAL and RCAL, in the SRVO-062/063 family), internal hardware alarms like SRVO-066 (CSAL / ROM checksum), SRVO-067 (OHAL2 / pulsecoder overheat), SRVO-072 and SRVO-073 (PMAL and CMAL count-integrity), and serial-link alarms like SRVO-068 (DTERR) or SRVO-138 (SDAL). Match the alarm to the family before ordering hardware.

Selection: on modern FANUC arms the pulsecoder ships as part of an integrated motor assembly, and swapping the sensor alone requires disassembly at a repair depot with the alignment fixtures the OEM uses. Field replacement is the entire motor plus pulsecoder as a unit, followed by a re-mastering pass on that axis.

Compatibility limits: pulsecoders are matched to a motor size, a serial protocol revision (αi, βi, iSV variants), and the axis control card in the cabinet. A newer αi-B pulsecoder will not talk to an older axis card; forcing the fit produces immediate serial-data-error alarms. Confirm the cabinet-side revision before ordering a motor with a newer feedback rev.

Troubleshooting is layered. Check the encoder battery voltage first, then the encoder cable's continuity and shield integrity, then the pulsecoder's own diagnostic word on the axis-card screen. A pulsecoder that runs warm to the touch after a short move is heading toward an OHAL2 alarm even if it hasn't tripped yet, and that's a repair-depot signal before the axis quits mid-cycle.

Practical example: an arc-weld cell throwing intermittent SRVO-068 on J5 turned out to have a chafed encoder cable at the J3 elbow. New cable, alarm cleared, mastering held. Chase the cable and battery before condemning the sensor.