Quote Originally Posted by tribbles View Post
Simply dropping VREF to 0 would probably drop the output current to 0, which is probably what Q2 is being used for (I'm not familiar with the L297 itself).

If INH1, INH2 and JP1 (pin2) are high, then VREF would be 0.

If the PIC's modulating VREF, then it would allow the holding current to be varied - it was probably felt by the designer to be more configurable than other circuitry (and I would be inclined to agree - you could do some cleverness like make the holding voltage [current] drop off over time the longer you don't use it, and then stabilise at a set value for the minimum hold you need). C23 would smooth the signal.

In your latest circuit, the whole left hand side appears to be doing the work of the PIC - and that's a fairly large circuit. Are you sure you don't want to use a PIC (why not?)
The purpose of IC5,Q2 (IC4A, T1 in circuit 2) is to reduce Vref by 1/3 approx on 1/2 steps to even out torque and get a better 1/2 stepping. This is because, without those additions, the current on half steps (both coils on) is 1.4x current on a full step (one coil on) (sum of the vectors). In circuit 3 the left hand side is a charge pump that detects when the clock pulse train vanishes and switches Vref from Imax to Ihold after a short delay, however its complicated by additional circuitry to force Ihold when the driver powers up.

Whats lacking is the actual code for the PIC, only then can we determine what its doing... should be on the net somewhere...