Servos on toddler board won't work

Benoit

I got the Toddler robot kit last December, but couldn't find the time to play with it until now.

I'm at the very first steps, where I'm to center the servos using a small program on the Toddler board. I got the program in, then fired it up, and ... nothing . Darn, that's disappointing. I went back to the PC, and I'm sure I had no mistakes in the program. What could I mess up ? It's only got 3 code lines in a loop.

I added a few debug statements, and sure enough, I could see the program moving along.

So I used my parallax scope to see if anything got out, and I couldn't see any activity on the servo connector pins ( X6=pin 12 and X7=pin 13) , but there's definitively a pulse coming out of the port, because when I put the probe directly on the PIC's pins, or on the socket besides the breadboard I can the pulse in the scope.

I traced it to the 2 small SMTs right besides connector X6. There's a signal on one side, and nothing on the other? Are these resistors? I couldn't find them on the toddler schematics ( it's a rev C board ). If they are resistors, I would assume that they're blown. That's a brand new board, very first time I used it. I swear I didn't do anything to it

What would happen if I simply bypassed them? I've used servos directly on BS2 pins before.

-Ben

Franklin

I'd check them with a meter first. Power off and directly across the device should show a resistance (if they are indeed resistors, I didn't check)

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- Stephen

Benoit

I did. Nothing. They aren't diodes either, I tried reading the continuity both ways, positive one side, then the next, and still nothing.

I was thinking of simply putting a glob of soldier on top of each, so the signal would go through, but I'm worried that these things are there to protect the BS2.
What makes me think they aren't though, is that I can get a signal from the experimenter ports pins right next to the breadboard, so these must be going straight to the processor.

-Ben

Franklin

Switch in position 2? Jumpers to Vss? If so I'd send it back for replacement because according to the schematic there isn't anything between the pic and the servo ports but traces.

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- Stephen

Benoit

Switch in position 2 [noparse]:)[/noparse] There's 5 volts on the center pin, I checked. Replacing the board is going to be expensive. Parallax charges about 60$ to ship things to Canada, and I expect it would cost me about 20$ to send it back.

I think I'm going to take the chance and jump the suckers...

-Ben

Benoit

Well,
After more tracing of the circuit, I managed to figure out why it didn't work... It was a combination of a cold solder on the board, and a mistake I made

The 2 surface mounts close to X6 are 100ohms resistors. There was a side with a cold solder that didn't conduct. A quick heat on each with the solder iron fixed that. But that wasn't enough to get the servos going, and here's where I messed up: I had confirmed that there was power on the R pin of connectors X6 and X7, but I hadn't checked the negative pin. I had figured that this would have been going straight to the common ground. Well, it happens that it's not! the B pin of X6 goes to Vs1, and X7 goes to Vs2. It seems that the ground pins for the servos are separated from the rest of the board.

It happens to me once in a while, I skip instructions... On page 6 of the manual ( advanced robotics with the Toddler) step B clearly shows that you have to jumper Vs1 and Vs2.

-Ben