Documentation

Differential Axis

WARNING – ESD

Components are static sensitive. Observe ESD precautions when working on the board – see ESD Protection.

How each differential axis output on MASSO G3 is protected, and how to test and repair the TVS diodes and 0-ohm protection resistors.

ESD sensitive

1. How the axis output works

Part	Role
Differential driver IC	Generates the differential Step and Direction signals.
TVS diodes (Step±, Dir±)	One across Step+/Step− and one across Dir+/Dir−, protecting against static and accidental external voltages. They don't affect normal signals. The SM712 is asymmetric: it starts to conduct outside a working range of about −7 V to +12 V. A large or sustained overvoltage can make a diode fail short to MASSO GND – this protects the driver IC but disables that output line.
0-ohm resistors	One on each of Step+, Step−, Dir+ and Dir−; acts as a fuse in extreme faults.

INFORMATION

Each TVS package holds two diodes, so one line can fail while the other still works. A failed diode may look normal or burnt – test it with a multimeter in diode test mode.

Differential axis output circuit

2. Components and board locations

	Component	Part
	X & Y differential driver	Texas Instruments DS26LS31CM
	Z differential driver	Texas Instruments DS26LS31CM
	A & B differential driver	Texas Instruments DS26LS31CM
	Step+/Step− TVS diode	Bourns CDSOT23-SM712
	Dir+/Dir− TVS diode	Bourns CDSOT23-SM712
	Step+ 0-ohm resistor	WALSIN WR04X000 PTL
	Step− 0-ohm resistor	WALSIN WR04X000 PTL
	Dir+ 0-ohm resistor	WALSIN WR04X000 PTL
	Dir− 0-ohm resistor	WALSIN WR04X000 PTL

Axis output locations on the board

	X axis
	Y axis
	Z axis
	A axis
	B axis
	14-pin chips (differential drivers)

3. Test the TVS diodes

Bourns CDSOT23-SM712. Use a multimeter in Diode Test mode. Turn MASSO off and leave it off for all testing, and disconnect all axis wiring.

Test	Probes	Good reading on S+, S−, D+, D−
1	Black on MASSO GND, red on each output in turn	Open circuit (OL, 1 or −1 depending on meter). 0 = short.
2	Red on MASSO GND, black on each output in turn	About 0.67 V. Compare with a known good axis.

REMOVING A FAULTY TVS DIODE

The TVS diode isn't needed for normal operation – if it's the only fault, removing it restores the axis. You can replace it or leave it off, but without it the axis has no ESD protection. Clean the pads and make sure no solder bridges remain.