>Will a ULN 2803 (8 darlington drivers) protect its inputs from
back-EMF?
>In the case of a PIC driving a 2803 which drives the stepper motor
coils,
>is there any danger of hurting the pic?


Careful!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Yes, the ULN2803 has protection diodes and they all connect to a
single pin. The normal and simple approach is to connect this to V+.
However, this is the WRONG thing to do when driving certain loads
including certain stepper motors. Consdier, when any output tries to
rise above V+ the diode internal will conduct and clamp that pin to a
diode drop above V+. If you have a stepper motor with a centre tapped
winding with the centre tap connected to V+ you efrfectively have a
transformer. When you ground one end of the winding the other end
rises to 2 x V+ - or, it tries to. If you have the other end
connected to V+ by a diode interesting things happen. You will
certainly affect the operation of the stepper adversely and probably
draw more current. There are several solutions.
i - Leave the common pin open - no protection agaunst overshoot.
ii - Connect the protection pin to a voltage of 2 X V+. Fine but
where do you get it in a normal system?
iii - Connect an electrolytic capacitor and a "largish" resistor to
V+. When windings go high the capacitor will be charged to 2 x V+.
Any overshoot will charge it somewhat higher but the resistor to V+
will drop the voltage over time. R must be sized to suit - as highh
as reasonab;e without having the capacitor pump up to far above V+ in
operation.
iv - Resistor only from protection pin to V+ - a reasonable solution.
Size resistor to limit oversjhoot while not being so low as to
severely impact stepper operation.

More than meets the eye at first glance!

All this MAY not apply to your stepper (but most likely will).
Test: Apply a square wave such as you expect to apply in system
across one winding (limit with serties R if you wish). Measure
voltage (a scope is best) of all other leads while doing this. If any
rise higher than the voltage being applied look at the above
comments.

>Will any spikes get back to the PIC?
>Should I stick a 0.1uF or <what value> uF cap on each of the 4 PIC
pins?

Ensure power supplie are adequately decoupled.

Shouldnt really need caps on PIC drive pins.
IF they are used there should be extra resistors from the PIC pins to
the 2803 with the caps on the 2803 pins. Putting caps directly on PIC
pins will result in large loads on the PIC when the cct is first
switched. While a PIC will probably take this without murmur it is
nad design practice and will definitely destroy things in some
circumstances. As far as functionality, caps will not do too much
harm as long as RC time constant is small compared with stepping
time.

I have a design which drives a stepper using a ULN2803 direct driven
by a 16F84. I've had no problems.AFAIR I leave the protection
connection o/c for the reasons mentioned above. I use small resistors
between the driver and the stepper but this is due to the
application - I PWM the stepper drivers and this achieves a pseudo
variable-constant-current :-) drive which allows me to "microstep"
the motor - a whole new and rather arcane area.


regarrds

            Russell McMahon