Can anyone help a newcomer?

[scopit]

Look at Table 4.3 and Table 9.2 of the 16F84A datasheet. When configured as an External Interrupt, RB0 has a Schmitt Trigger input buffer which needs 0.8*Vdd for a logic high and 0.2*Vdd for logic low. Your voltages appear to be marginal.

Thanks Dave, I'll check the voltages again but from your post, the problem may be on the logic low.

[dhouston]

Thanks Dave, I'll check the voltages again but from your post, the problem may be on the logic low.

I agree - after sensing the first high, it never senses a low, so it cannot sense another low-to-high transition.

[dhouston]

Thanks Dave, I'll check the voltages again but from your post, the problem may be on the logic low.

It appears that after sensing the first high, it never senses a low, so it cannot sense another low-to-high transition.

[scopit]

It appears that after sensing the first high, it never senses a low, so it cannot sense another low-to-high transition.

Thanks Dave you were right. I removed the pull up and in the process found a dry joint. This may have been the source of the issue in the first place, but was causing the intermittent behaviour.

Interrupt is working now and I can continue working through the testing.

Thanks everyone for your help.
Danny