Hardware Reference
In-Depth Information
Layers of Agreement
Before you can get things to talk to each other, you have to lay some ground rules for
the communication between them. These agreements can be broken down into five
layers, each of which builds on the previous ones:
•
Physical
How are the physical inputs and outputs of each device
connected to the others? How many connections between
the two devices do you need to get messages across?
This is a simplified version of a common model for thinking
about networking, called the
Open Systems Interconnect
(OSI)
model. Networking issues are never really this neatly
separated, but if you keep these elements distinct in your
mind, troubleshooting any connection will be much easier.
Thinking in layers like this gives you somewhere to start
looking for the problem, and a way to eliminate parts of the
system that are
not
the problem.
•
Electrical
What voltage levels will you send to represent the bits of
your data?
•
Logical
Does an increase in voltage level represent a 0 or a 1?
When high voltage represents 1 and low voltage represents
0, it's called
true logic
. When it's reversed—high voltage
represents 0 and low voltage represents 1—it's called
inverted logic
. You'll see examples of both in the pages that
follow.
No matter how complex the network gets, never forget
that the communication between electronic devices is all
about pulses of energy.
Serial communication
involves
changing the voltage of an electrical connection between
the sender and receiver at a specific rate. Each interval
of time represents one bit of information. The sender
changes the voltage to send a value of 0 or 1 for the bit
in question, and the receiver reads whether the voltage
is high or low. There are two methods (see Figure 2-3)
that the sender and receiver can use to agree on the rate
at which bits are sent. In
asynchronous serial commu-
nication
, the rate is agreed upon mutually and clocked
independently by sender and receiver. In
synchronous
serial communication
, it's controlled by the sender, who
pulses a separate connection high and low at a steady
rate. Synchronous serial communication is used mostly
for communication between integrated circuits (such as
the communication between a computer processor and its
memory chips). This chapter concentrates only on asyn-
chronous serial communication, because that's the form
of serial communication underlying the networks in the
rest of the topic.
X
•
Data
What's the timing of the bits? Are the bits read in groups
of 8, 9, or 10? More? Are there bits at the beginning or end
of each group to punctuate the groups?
•
Application
How are the groups of bits arranged into messages? What
is the order in which messages have to be exchanged for
something to get done?
