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An axis's serial pulse coder is returning abnormal data or has detected an internal fault. The control stops trusting that axis's position. It is a measurement alarm, not a power alarm: the machine can be mechanically perfect and still be unable to work, because it does not know where it is.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
Always, because it affects the position measurement.
Reset only after recovering feedback and reference.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
A serial pulse coder is not a simple pulse generator: it is a device with its own electronics that measures position, encodes it into digital frames and sends them to the amplifier with check information. It also runs its own self-diagnostics: it checks that its supply is in range, that the optical signal reading the disc has sufficient amplitude, that the internal counter is consistent and, on absolute types, that the battery backup is still alive. When one of those checks fails, the encoder reports that its data is not trustworthy and the control raises the alarm. That is the difference from a pure disconnection: here communication does arrive, but the encoder itself says something is wrong. The most frequent causes have to do with the environment it lives in: an encoder is mounted on the back of the motor, in surroundings with vibration, heat, oil and coolant. Moisture ingress through degraded sealing is the classic cause and usually gives intermittent faults that get worse over time. Contamination of the optical disc reduces signal amplitude and shows up as progressive degradation. Excessive motor heat shortens the life of its electronics. And vibration eventually loosens couplings or solder joints. So an encoder that starts giving occasional warnings rarely fixes itself: the normal course is for the frequency to increase until the machine is stopped.
In order, from the fastest and cheapest check to the most expensive.
01Note the axis and do not produce with it
While the encoder is giving doubtful data, the dimensions are not trustworthy even though the machine moves. Do not run automatic cycles on that axis and tell the next shift. If the job is urgent, it is better to move it to another machine than to risk a whole batch.
02Reconstruct the context
When it appears: at power-up, in motion, always in the same zone, after hours of running, after a wash-down with a hose. If it appears after the machine has been running and not from cold, suspect temperature or moisture that evaporates and condenses. If it depends on position, suspect the cable.
03Check what is accessible without opening cabinets
With the machine stopped and safe, look around the motor and its encoder: coolant residue where there should be none, degraded seals, chafed cables, connectors not properly seated. Do not unplug encoder connectors and do not open the cabinet: that is for maintenance.
04Do a power cycle and watch
If the alarm comes straight back at power-up, it is a hard fault. If it clears and returns later, it is intermittent, and intermittents are usually moisture, marginal contact or progressive degradation. That fact steers the technician far more than it seems.
05Check whether there are associated battery warnings
On axes with absolute pulse coders, an encoder alarm can come with battery warnings or position loss messages. If so, the correct order is to resolve the battery and the reference first, and only then judge whether the encoder also has a problem of its own.
06Replacement and verification, by maintenance
Changing an encoder involves mechanical and electrical work with the machine locked out, and afterwards that axis's reference has to be re-established by the builder's procedure and zero verified against a known point before producing. The machine is not passed as good until that step is done.
Encoder degradation or progressive moisture ingress. It is confirmed by the occurrence log. It is worth planning the replacement before it stops the machine in the middle of an order, which is how it always ends.
Water or coolant ingress through degraded sealing. It is confirmed by the timing and by moisture in the area. Maintenance dries it out and checks the sealing; if there was ingress, the encoder may already be compromised.
Encoder electronics degraded by temperature, or a motor running hotter than it should. It is confirmed by the time pattern. Besides checking the encoder, you have to look at why that motor reaches that temperature.
Motor and encoder sealing checked inside periodic maintenance, especially on machines that use a lot of coolant. Cleaning without direct jets at motors, connectors and encoder areas. Motor temperature monitored, with the causes of overheating dealt with as soon as they appear. Wiring protected and with bend radii respected in the cable carriers. And a dated log of every warning: it is what lets you replace an encoder during planned downtime instead of in an emergency.
Fanuc detects failure of the axis serial pulse coder, normally related to position feedback. It is an alarm from the Encoder / measurement area, and in the catalog it is listed as “Abnormal serial pulse coder”. On the machine you notice it like this: The axis does not home, Alarm when moving or turning on, Loss of position or unstable feedback.
On the shop floor it almost always comes from one of these causes: Damaged press coder, Encoder cable, Contaminated connector, Amplifier with a read fault. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset only after recovering feedback and reference. Resetting without fixing the cause first only hides the problem: SPC350 trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Note the affected axis, Do not move automatic, Check visible connectors with a safe machine. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With SPC350 active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
Always, because it affects the position measurement. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is critical: there is a real risk of damage to the machine, the tool or a person. Leave the machine stopped, do not keep resetting it to see if it goes away, and call maintenance before producing again. Guidance reference. Always confirm the full text in the exact machine/control manual.
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