Replace an Output Driver on G4 CNC

Output driver IC's on the 5AX motion card are used to actuate various relays in the spindle cabinet. Failure of M03, M04, M08, M12, M13, or M25 could be caused by a bad output driver. The Control Ready (CRDY) signal (which enables servos to turn on) is also actuated by a driver IC.
If troubleshooting all switches, cables, connectors, etc, does not fix the problem, one of the driver IC's may be bad.

After troubleshooting all switches, cables, and connectors associated with the M‑function that doesn't engage, you can discover if a driver IC may be bad by checking voltages on either terminal strip TB1, or HDR101A on printed wiring board, in the spindle cabinet. If one of the optional outputs M17, M19, M21, or M27 is the issue, the measurements are made at HDR102A on PWB.

To make these measurments, the spindle cabinet must be open, the CNC controlled powered up, and the servos must be on.

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Measure Voltages

On machines equipped with TB1, TB1 Terminal Stripyou will be measuring voltages on the long terminal strip that runs down the left side of the electrical panel. Terminal 1 is the top terminal on the strip.

On machines equipped with PWB, PWB (Printed Wiring Boardthe control is connected to HDR101, but you will be measuring voltages at HDR101A.HDR101 and HDR101A which provides access to all the of the pins. Terminal 1 is the right-most pin on that header.

MDI mode is used to issue the m‑function.

*For M12 & M13, write program with eight M12's or M13's (one M12 or M13 is a 1/2 second pulse, so eight will keep function on for four seconds).

• Before the m‑function is issued, for all measurements, the meter must read about 12 volts DC between the two terminals.
• If the voltage does not measure about 12 volts there may be a bad connection between test point and 5AX motion card. Click for schematics
• After the m‑function is issued the meter should read 1 - 2 volts DC.
• If the voltage does not drop to about 1 - 2 volts when m‑function is issued, replace the relevant driver IC. (Scroll down to Replace Driver IC, below.)

Again, this assumes that you've verified that all switches, cables, connectors and relays associated with the issue check good. Click for schematics

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Replace Driver IC

If the voltage does NOT drop to about 1.5VDC when M-function is issued, the 5AX card isn’t pulling the pin down, possibly because of a bad output driver chip, or there’s a bad connection in the CN401 ribbon cable.

To check the ribbon cable, turn off the CNC, then unplug ribbon cable
CN401 Ribbon Cable between 5AX and Connect Card between CN401 on 5AX card and connect card.
NOTE: Current 5AX cards, shipped after May, 2025 are smaller, so the ribbon cable
CN401 Ribbon Cable between 5AX and Connect Cardis easier to access.

Examine the connectors to insure the ribbon is securely in place. unplug and replug both ends, power up the CNC and see if you can turn on the M‑function. If not, recheck the voltage at TB1 or HDR101A as above, with and without the M‑function applied.

If still not about 1.5VDC with M‑function applied, you will need to replace the appropriate driver IC on the 5AX board.

If your CNC is equipped with first generation 5AX card,
First Generation 5AX Cardthe driver IC for M03, M04 and M25 is U413.
Driver IC U413 The driver IC for M08 is U412.
Driver IC U412 A spare driver IC is provided: U414.
Spare Driver IC U414

• To replace either U412 or U413, set control power off and unplug the power cord for the control, remove the screws that hold the triangular brackets
  5AX Mounting Brackets to the chassis, and tilt the card back to get access. Remove either U412 or U413
  5AX Driver IC's U412, U413 and U414 (spare) and set it aside, then remove U414 and carefully plug it in to either U412’s or U413’s socket.

If your CNC is equipped with second generation 5AX card,
Second Generation 5AX Cardthe driver IC for M03, M04 and M25 is U307.
Driver IC U307 The driver IC for M08 is U306.
Driver IC U306 A spare driver IC is provided: U308.
Spare Driver IC U308

• To replace either U306 or U307, set control power off and unplug the power cord for the control. Remove either U306 or U307
  5AX Gen2 Driver IC's U306, U307 and U308 (spare) and set it aside, then remove U308 and carefully plug it in to U306’s or U307’s socket.

When removing the IC's from their sockets, be careful not to bend the leads- work them out gradually by slipping a small screwdriver or similar tool under one end, then the other. When plugging them into their sockets make sure that none of the leads get bent.

Replacement driver IC UL2803A is available at DigiKey.com).