What Does the 351 Alarm Actually Mean?
First, "SPC" is the abbreviation of Serial Pulse Coder, and its job is to feed the motor's actual position back to the CNC system and the servo. The meaning of the 351 alarm is actually very plain: the CNC system and the serial pulse coder of some axis "can't talk to each other" anymore — either the communication is broken, or the data received is wrong, and the system can't recognize it normally.
According to what's said in the FANUC 0i-A maintenance manual (B-63505EN/02), this type of alarm may be related to these few things:
- The feedback cable connection is not normal
- The coder's supply voltage is insufficient
- The related servo control circuit has a problem
The key point is this: 351 only tells you "there's a communication problem," it does not tell you which part is bad. The feedback cable, the power supply, the communication interface — any link is possible. Especially when it appears right after the motor was replaced, or right after someone touched things in the electrical cabinet, you should think toward these places even more.
What to Check First? Common Causes of the 351 Alarm
1. Pulse Coder Feedback Cable (Check This First)
Our experience is that most 351 cases end up failing right here: the plug isn't pushed all the way in, the connector has poor contact, or the cable has a broken core inside.
The point most easily overlooked: the wire looking intact doesn't mean it's conducting inside. The outer jacket isn't broken, the plug isn't loose, and yet the core inside may already be snapped, or making contact good and bad by turns. So if the alarm popped up after installing or removing a motor, or after changing a cable, first confirm both ends' connectors one by one, and don't rush to a conclusion.
2. The Coder Itself Isn't Responding
After the machine runs for a long time, the inside of the pulse coder may also go wrong, so the communication data can't be sent out. But this is by elimination: only when the cable and connections are all fine does it come to the coder and its power supply.
3. The Coder Supply Voltage Is Not Normal
The coder needs a power supply that meets spec before it can do its job. If the voltage isn't enough, or the power connection has a problem, then even if the coder itself isn't bad, the communication will still fail. So before deciding to replace the coder, it's best to follow the corresponding FANUC repair materials and have an experienced person measure the supply situation.
4. Servo Module or CNC Side Control Circuit
If the feedback cable, power supply, and coder are all confirmed normal and the alarm is still there, then you have to look further back: the related servo control module, and in some systems the Base PCB also has to be put on the suspect list. This step belongs to the case of "only suspect it after everything before has been ruled out." Don't fix your eyes on it right from the start.
Diagnostic Screen No.203: Important Clues Are Hidden There
For the applicable FANUC 0i-A and Power Mate systems, you can use diagnostic number 203 to view the related status of serial pulse coder communication errors. Note, different model FANUC systems may have different diagnostic items, so first confirm the specific system model of the machine in your hands, then look at it against the corresponding maintenance manual.
Inside 203 there will be these flag bits DTE, CRC, STB, which can help you judge which kind of communication abnormality it is:
- DTE: the coder isn't answering normally, or communication failed. Focus on checking the pulse coder, the feedback cable, and the power supply.
- CRC: the serial communication data is wrong. Check the cable, the coder, and the related communication circuit.
- STB: the stop bit has a problem, or the communication data is abnormal. Likewise you need to check the feedback line, the coder, and the control circuit.
These flag bits won't directly tell you "which part is bad," but they can help you narrow the troubleshooting range. For example: if it shows the coder isn't answering, first fix on the feedback cable connector and the power supply; if it's a data error, then keep checking the communication line and the coder state.
One small habit is worth building: don't just copy a "351" off the screen and call it done. Write down the complete alarm information, the axis with the problem, and the state of the 203 diagnostic screen together. The value of this information for judging the fault is far greater than a single alarm number alone.
When Checking the Feedback Line and Coder, Which Details Are Easy to Miss?
If the 351 appeared after replacing the
servo motor, the coder, or the feedback cable, it's suggested to first check those connections that were touched during the repair.
Specifically, you can look at these few points one by one:
- Is the feedback plug pushed completely all the way in
- Is the connector damaged
- Are both ends of the cable connected correctly
- Has the cable been pinched or pulled
These faults look like nothing much, but they're enough to keep the CNC system from receiving normal coder data.
Once the connections are all fine, then go check the coder's power supply. Measuring the voltage has to follow the spec of the FANUC system you're using. Don't just take another model's test values and apply them.
If the power supply and feedback line are both normal and the coder still can't establish communication, only then do you need to do further testing on the pulse coder itself. Only when the diagnostic result confirms the coder is bad do you consider replacing it.
One more thing to remind: the 351 alarm doesn't mean the
servo amplifier has to be replaced. Only when the coder, cable, and power supply are all ruled out do you need to check the related circuits on the servo module or the CNC control board.
What Else to Watch Out For After Replacing the Pulse Coder?
* Verify the Model Before Buying
Before confirming the coder is really bad and buying a new one, first verify the servo motor's actual model and the coder configuration. Different series of FANUC motors and coders may have different designs. Just by the 351 alarm or the motor's appearance, you can't judge whether they're interchangeable.
* After Replacing, Confirm Communication Is Restored
After replacing, follow the repair procedure for the corresponding FANUC system to confirm communication has returned to normal. If this repair touched the absolute position data, or affected the reference point state, you also have to confirm according to the specific configuration whether the reference point needs to be re-established.
* Still Alarms After Replacing, Don't Replace Randomly
If the coder is replaced and the alarm is still the same, go back and check the feedback line, power supply, and related control circuit. Don't rely on trying one coder after another to test your luck. That way costs money and time, and may not even solve the problem.
SongWei CNC: FANUC Pulse Coder and CNC Parts Testing and Repair
SongWei CNC provides new and second-hand FANUC servo motors, pulse coders, servo amplifiers and CNC system boards, and can also provide
testing and repair support for these parts.
If you're being troubled by FANUC 351 alarm / SPC ALARM: n AXIS COMMUNICATION, when contacting us please provide these pieces of information together:
- CNC system model
- The axis that alarms
- The complete alarm information
- The complete model of the servo motor or pulse coder
- The diagnostic state of that system
If the alarm only appeared after changing the motor, changing the coder, or changing the feedback cable, please also describe that repair. With this information, we can better judge whether the problem is closer to the communication line, the coder power supply, the pulse coder itself, or the servo control circuit, and narrow the fault range before you buy spare parts.
Summary
The FANUC 351 alarm is a serial pulse coder communication alarm, generally displayed as SPC ALARM: n AXIS COMMUNICATION. Possible causes include: abnormal feedback cable connection, pulse coder fault, insufficient supply voltage, and abnormal related servo or CNC control circuits.
Don't just go replacing the coder or servo amplifier based on the 351 alarm alone. First find out the real cause of the communication break, then decide which to repair and which to replace. Usually this saves a good amount of wasted spare part money, and also a few days less downtime.
