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[wars0696]

I have two identical circuit boards using IC MM74HC123 one manufactured by Fairchild Semiconductor the other by Philips. The Fairchild chip when used in the circuit will announce any length of message the Philip circuit won't announce the longer messages. I don't have a circuit diagram for the board. I only difference on the datasheet is differences in the propagation delay, output transition time and power dissipation capacitance. Could anyone please tell me which of these would course the problem and why?

 

[Mikebits]

This is the nature of monostables. Lot to Lot, vendor to vendor. Timing differences between the two. Analog/digital circuits like the 123, the 555 have this inherent quality. These parts should not be used when there is marginal timing. Designs like this usually require vendor specific parts or custom tuning RC elements. Either way it is a pain. The designers made a marginal circuit.

 

[duffy]

You could goof around with Rext to see if you could get them both to work. Stick a pot in there, crank it a little higher, crank it a little lower...

 

[ericgibbs]

I have two identical circuit boards using IC MM74HC123 one manufactured by Fairchild Semiconductor the other by Philips. The Fairchild chip when used in the circuit will announce any length of message the Philip circuit won't announce the longer messages. I don't have a circuit diagram for the board. I only difference on the datasheet is differences in the propagation delay, output transition time and power dissipation capacitance. Could anyone please tell me which of these would course the problem and why?

I have answered your other thread of the same title.!