Thanks for the replys. I am using the monostable configuration that is shown in the datasheet (unsure of manufacture but I think its a universal configuration), it uses two external caps between pin 5 and 6 and a resistor between pin
7 and 8. A falling edge is supposed to trigger a high output from the
555 timer but doesn't when the input to the 555 is taken across the pull up resistor of the compartor.