Tommy Servo Control Board

armis2000

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Geek question:

I was trying to get my servo board to work properly, but for some reason it was not activating the circuit past the 74HC174 flipflop. The Data input was toggling. The CLK input pulsing (altho my logic probe shows pulsing, but at logic high for most of the duty cycle).

Seeing that the data line was not strobed in any way (logic high when the blinders are supposed to be there, and low when not), I figured the 74HC174 was bad - socketed/new chip - same result. So, I tried simply jumpering the input of the FF to the output (pin 3 to pin 2, with the chip removed). The servo driver works now as expected.

Is there any reason why U4 is there at all? Maybe it was a throwback to being able to add servos to the lamp matrix? (CLK could be a row, and Data a column?)

Could someone validate the clock signal for me? If its bad, all I can think is the PIA has a bad line output going to the 74154.
 
Hard to tell without putting a scope on it but I would look at the CLR line (pin 1 of the FF) as well. It should be active high. If that's stuck low the FF will never enable.
 
Yep - CLR is static high level - Only thing I can think is the CLK needs to strobe with a longer low level; I was hoping someone could check what that signal looks like on their Tommy.

But since jumpering the In to the Out pins seems to work, I'm probably OK leaving it, unless someone can inform me why that would be a bad idea.

Cheers!
 
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