A G320 can lock the servo shaft yet produce no commanded motion when its control-side +5 V supply is missing at pin 12. In the documented case, the step signal appeared at the breakout board, but a hidden short inside the shielded +5 V conductor prevented power from reaching the drive.
Observed Symptoms and Known Hardware
| Item | Evidence | Diagnostic meaning |
|---|---|---|
| Servo motor | 24 V | The motor locked when Err/Res was connected to +5 V, confirming that the drive could energize it. |
| Encoder | Tamagawa, 500 CPR, channels A, B, and C; C is the index | Encoder supply is 5 V, but the evidence does not establish whether feedback signals were verified at the drive. |
| Step input | A logic probe showed pulses | This confirmed activity only at the test point, not correct voltage and current at every G320 terminal. |
| Direction input | Pulled high or low | A static direction state alone does not prove that the drive receives valid step commands. |
| Pin 12 | Expected +5 V from an external breakout-board supply | The supply was present at the breakout board but absent at the G320. |
Root Cause
The +5 V conductor had a short hidden inside its shield. Voltage had initially been checked at the breakout board rather than at G320 pin 12, so the upstream measurement concealed the open power path to the drive. After the conductor fault was corrected and +5 V reached the G320, the system operated normally.
Diagnostic Procedure
- Measure the external +5 V supply directly at G320 pin 12, using the drive-side reference point. Do not accept a breakout-board measurement as proof that voltage reaches the drive.
- If +5 V is present upstream but absent at pin 12, isolate and continuity-test the complete conductor, its shielded section, terminations, and connectors.
- After restoring pin 12 power, command motion and verify actual shaft movement. Recheck step and direction signals at the G320 terminals if the motor still does not move.
Buffer Drive Capability
The breakout board used a 74HC245. One recommendation was to replace it with a 74ACT245 or 74AC245 because the 74HC device was described as marginal at 8 mA for a 0.4 V logic-low level; an earlier comment characterized its drive current as 4 mA. These statements do not establish the exact G320 input-current requirement, so treat buffer replacement as a signal-integrity action, not the confirmed root-cause repair. In this case, the 74HC245 worked after pin 12 power was restored, while an attempted LS-family substitute did not.
FAQ
Why does my G320 lock the motor but not respond to step pulses?
Measure +5 V directly at G320 pin 12. The documented failure locked the motor and showed upstream step activity, but a hidden conductor short prevented +5 V from reaching pin 12.
Is checking +5 V at the breakout board enough?
No. Verify the voltage at G320 pin 12 because a damaged wire, termination, or connector can leave the breakout board powered while the drive receives no +5 V.
Should I replace a 74HC245 used with G320 step and direction signals?
Consider a 74ACT245 or 74AC245 if drive capability or logic-low margin is suspect; the 74HC245 was described as marginal at 8 mA and 0.4 V. However, verify pin 12 power first because the 74HC245 operated successfully after the wiring fault was repaired.