CNC probe mixer

[G. Hoffman]

OK, so I've got two probes for my CNC machine (one for setting Z axis depth zero, and one for probing objects).  Annoyingly, they operate at opposite logic states!!!  Rather than having to reset the input every time I change probes, I want to make some kind of mixer for the two probes.  I was thinking I could just do it with a 4049 hex inverter, which would be great since I have all the parts here and wouldn't have to buy any parts, but I pretty quickly realized you can't just mix logic signals like you do analog signals.  So now I'm thinking I want to use a quad AND gate.  I'm thinking I can use one gate for each input as a buffer, and by switching the second input between logic states I'll also get the ability to switch polarity.  The third gate will be just an AND gate to mix the two probes appropriately, and the fourth gate will be another buffer for the output (also switchable polarity). 

Like so:




Makes sense, or no?


Gabriel

[g_u_e_s_t]

close, but an AND gate will not act as an iverter.  if one of its inputs is 0, the output will always be 0, regardless of the other input.  you need an XOR gate to act as an optional inverter.  alternatively, you could use your NOT gates, and do diode mixing of the signals to make the AND you want.  basically, each output goes through a diode, so they dont fight each other, and then to a common resistor.  if the diodes are facing towards the output (so current can only flow into the output of the gates), and the resistor goes to +Vcc, then both diodes will need to be high for the input of the next stage to see a high signal, making an AND (or making a NAND if youre using NOT gates, but follow that with another NOT and you get an AND).

[G. Hoffman]

Damn.  I was able to find a XOR quad gate that fits the board I designed (74AS86 — AS for the 1nS rise time!  It matters in this case), but damn they are about 5 times more expensive.   

Thanks for the double check, though.   That would have been damned frustrating.

[G. Hoffman]

It worked, first power up.  Yeah!


Gabriel

[g_u_e_s_t]

good to hear it worked.