Trying to understand a Motor Shield

Using Arduino Motors, Mechanics, Power and CNC
1

I bought a Make It Robotics kit since I'm pretty green in this space:

Kit: http://comingsoon.radioshack.com/make-it-robotics-starter-kit/2770168.html#.VZh42xNViko
Code: GitHub - RadioShackCorp/2770168-MakeItRobotics-Starter-Kit: Support files, user's guide, library files, and example sketches for RadioShack Make: it Robotics Starter Kit (SKU 2770168)

I'm trying to understand how the motor shield works so I can use it in other ways. I see that they send power to the motors using:

#define DC_SEND_HEADER   0x56
#define DC_RECV_HEADER   0x76
#define DC_CMD_DIRA      0x73
#define DC_CMD_DIRB      0x74
...
#define DC_CMD_PWMA      0x80
#define DC_CMD_PWMB      0x81
...
#define FW               0xff
#define BW               0x00

Header File

void MakeItRobotics::line_following_turn_left(int speed)
{
  int duty = 0;
  int half = 0;
  if(speed == 0)
  {
    duty = 0;
    half = 0;
  }
  else if(speed >= 255)
  {
    duty = 1;
    half = 128;
  }
  else
  {
    duty = 257 - speed;
    half = 257 - speed*3/4;
  }
  dc_write(DC_CMD_DIRA, BW);
  dc_write(DC_CMD_DIRB, FW);
  dc_write(DC_CMD_PWMA, half);
  dc_write(DC_CMD_PWMB, duty);
...

}
...
void MakeItRobotics::dc_write(int type, int value)
{
  Serial.write(DC_SEND_HEADER);
  Serial.write(type);
  Serial.write(value);
  delay(20);
}

CPP File

And this makes almost no sense to me--it's all very new. Please point me in the correct direction.

First, what is the purpose of DC_SEND_HEADER?

Second, when I do something like Serial.write(0x56), what am I accomplishing?

My guess is that the shield is taking its instructions from the serial port, rather than getting a message from a pin, and these are triggers or specialized messages for the embedded microcontroller. When I say that, I sound like I know what I'm talking about, but I don't...

Thanks for any help!

2

I think so - your analysis seems reasonable.