Scott Edwards Mini SSC

Forum 2005-2010 (read only) Hardware Interfacing
1

I have a Scott Edwards Mini SSC (serial servo controller) in an older project of mine. I would like to replace the current controller with an Arduino. I have not been able to get communication established to the SSC. Scope tells me signal is there and I have checked the SSC with another computer. It wants to hear a wake up byte of "255", then a servo number (0-7) followed by a position byte (0-254).

The following is the program I am using:

//First attempt to use Scott Edwards SSC controller
// It uses serial input to control up to 8 servos.
// Wow, I have spent a LOT of time on this. Likely 5hrs so
// far and no joy yet.
// I wonder if my problems might be the level of the signal?

char sync = 255;
char pos = 127;
char servo = 4;

void setup() // run once, when the sketch starts
{
Serial.begin(9600); // set up Serial library at 9600 bps
}

void loop() // run over and over again
{

Serial.print(sync, BYTE);
Serial.print(servo, BYTE);
Serial.print(pos, BYTE);
delay(1000);
pos = 254;
Serial.print(sync, BYTE);
Serial.print(servo, BYTE);
Serial.print(pos, BYTE);

pos = 127;
delay(1000);

}

Any pointers would be greatly appreciated.

Bill

2

That certainly looks correct, so I would definitely wonder if you are trying to drive PC RS232 levels on the SSC with the 5V TTL levels on the Arduino 0 and 1 pins.

3

I was not aware of (actually, had forgotten) the difference between the Arduino logic levels and the PC logic levels on the RS-232 output.

That may well be the problem. Any simple way to test it? Or not so simple?

Many, many thanks

Bill

4

Yes, there is a simple way to test it. RS232 logic is AC. It swings both positive and negative. It also has a larger voltage range, usually around 14V +-. 5V TTL logic swings between 0 and 5V. So using a scope you can view the communication between the current devices to see what the signal looks like. An RS232 signal will have a large swing plus and minus while a TTL level signal will not switch polarity and will have a lower voltage swing.

And, if you do need rs232 signalling then look into a MAX2322 chip (and get 5 capacitors of 1uf) or someone recently posted a little circuit board from Wulfden: http://wulfden.org/TheShoppe/pa/index.shtml