Serial 8 Servo Controller


The 8 servo controller looks like just the piece of hardware I need, but I’m having trouble diagnosing a problem.

When sending inputs via TTL on CON2, the controller acts quite nicely. Connecting via a straight-through serial cable to CON3, however, gets no response from the controller. Yellow and red LEDs are on and don’t change, as though there might be a grounding problem. Solder connections look ok, none of the “optional” components involved with signal conversion seem to be misbehaving, and a quick check with a multimiter shows that signals sent over CON3 are getting voltage to CON2, and vice-versa, but pin 7 of the PIC isn’t getting anything.

It seems to me that I must have introduced a short when I put it together, but I can’t seem to locate one.

Any other thoughts?


Correction: Pin 7 on the PIC does get a signal for inputs via the TTL and DB9 connectors. I don’t have a way of checking the integrity of that signal, though.



Can you provide better detail about the LEDs? Yellow and red on should not be one of the possibilities. You should get either all on or just yellow on, and that should change once you start sending data.

- Jan

Hi again,

I didn’t think that yellow and red was a possible combination, but alas… As I’d mentioned, everything works properly via TTL input; servos behave normally, and lights behave just as the manual describes. With the controller connected by the DB9 connector, though, the only combination of LEDs lit is yellow and red, regardless of the serial port’s state (also the case if nothing is connected and the controller is given power without grounding the signal pin at CON2.)

Odds are I pooched something in the assembly phase, but got lucky enough to have a halfway-working controller. I’m content to work around it for now if that seems to be the case.


We haven’t heard of the problem you’re having, so there isn’t a specific recommendation we can make to fix the problem. The only quick solution I can think of is if you have a strange serial cable. The servo controller is made for a straight-through cable (you can also plug it directly into the computer if you’re not sure about your cable).

Otherwise, you could start tracing out the serial-related connections with a meter to try to determine what’s wrong.

- Jan