combining two pulse streams for counting

Suppose I have two sensors that give off a pulse every time an event occurs; I have a choice of negative-going or positive-going pulse. I also have a counter that is triggered by a falling edge followed by a rising edge. I am looking for a simple way to combine these two streams of pulses to get an accurate combined count. The problem with using an AND or OR gate is that it wouldn't correctly handle the case where one pulse arrives while another is still in progress. So, I was thinking of doing the following: Have one of the streams be normally high and negative-going; have the other be normally low and positive-going. Feed the two signals to the inputs of an XOR gate. That way, a lone pulse on either line would cause the output of the gate to fall and rise; a pair of overlapping pulses should make the output fall and rise twice.

So, what's wrong with this? Usually something this simple is wrong.

"Paul Ciszek" schreef in bericht news:dgnikp$k7f$ snipped-for-privacy@reader1.panix.com...

Alas, it will not be reliable. As apparently both streams are not related, what will happen when two pulses arrive at the same time? Or, maybe even worse, when they arrive almost at the same time? The first thing may be very rare and you will have -theoretically - no output at all. The second will give pulses that are too small or too low to be seen by the counter.

A simple, straight forward solution will be counting both streams seprately and then use a full adder to obtain the result.

Otherwise you will have to build a circuit that samples both streams and give exactly one outputpuls for every inputpuls, separated when both occur at the same time. For not too high frequencies - let's say some hundreds of kHz - a small micro is an ideal component. If you can't use one, you will need some flipflops and glue logic to build a state machine plus a clock to drive it.

petrus bitbyter

Regarding the XOR, simultaneous same duration pulses will fail (3rd example below), as will simultaneous rise or fall with different durations (examples 4 & 5)

Can you control the duration of the pulses from the sensors? I'm envisioning the possibility of "pulses" from sensors that are of random duration. For example, a power failure sensor or low water level sensor could be low (or high) anywhere from seconds to days.

Ed

Example1 ======== -------- Sensor 1: ________| |_________ --- Sensor 2: __________| |__________ - -- XOR : ________| |___| |_________ GOOD!

Example2 ======== -------- Sensor 1: ________| |_________ ---------- Sensor 2: __________| |__________ - --- XOR : ________| |______| |_________ GOOD!

Example3 ======== -------- Sensor 1: ________| |_________ -------- Sensor 2: ________| |__________

XOR : __________________________ BAD!

Example4 ======== -------- Sensor 1: ________| |_________ --- Sensor 2: ________| |__________ ---- XOR : ____________| |_________ BAD!

Example5 ========

-------- Sensor 1: ________| |_________ --- Sensor 2: _____________| |__________ ---- XOR : ________| |_________ BAD!

. . . .

If you get two events at exactly the same time they won't get counted.

or if ome pulse ends as the other begins it produces one pulse on the output.

Bye. Jasen