Hi.

This might be faced by some people and there might be answers, but i have yet to find.

I have connected a ULN2803/4 chip with a AT89c55 controller Port-2. The ULN is supposed to drive 12V relays, that have one of the coil legs connected to 12V and the other leg connected to the ULN.

Since ULN is TTL compatible, writing a logic "1" to the respective port pin, and hence the ULN input, turns ON the relay and a logic "0" turns it off. So far so good.

The problem i am facing is, at startup all relays turn ON, untill the init piece of code executes to put all port pins to Logic "0" state. he problem is that at controller pins are internally pulled-up, hence when the power is turned on, while teh controller is still in reset stage, the port pins are all at logic "1", and the relays turn ON.

This is creating problems for me, as the relays are supposed to drive critical Contactors which are driving a Star-Delta 15KW motor. If all the contactors turn ON at power up (MAIN, STAR, DELTA), then there would be a blast in teh motor, OR the Overload will trip.

Any suggestion how i can keep teh ULN outputs OFF at power up, and ONLY turn the ULN ON when needed ?

Zeeshan