Making Things Talk

52 MAKING THINGS TALK

First, there’s the protocol that the microcontroller speaks,

called TTL serial:

•Physical layer

The Arduino module receives data on digital I/O pin 0,

and sends it out on pin 1.

•Electrical layer

It uses pulses of 5 volts or 0 volts to represent bits.

•Logical layer

A 5-volt signal represents the value 1, and a 0-volt signal

represents the value 0.

•Data layer

Data is sent at 9600 bits per second. Each byte contains

8 bits, preceded by a start bit and followed by a stop bit

(which you never have to bother with).

•Application layer

At the application layer, you sent one byte from the PC to

the Arduino and processed it, and the Arduino sent back

one byte to the PC.

But wait, that’s not all that’s involved. The 5-volt and 0-volt

pulses didn’t go directly to the PC. First they went to a

serial-to-USB chip on the board that communicates using

TTL serial on one side, and USB on the other.

Second, there's USB, the Universal Serial Bus protocol.

It differs from TTL serial in many ways:

•Physical layer

USB sends data on two wires, called Data+ and Data–.

Every USB connector also has a 5-volt power supply line

and a ground line.

Making the Connection: The Lower Layers

You’re already familiar with one example of serial communication, between a microcon-

troller and a personal computer. In Chapter 1, you connected an Arduino module to a

personal computer through the computer’ ...