Serial Direct
Cable Connection
Introduction
The Serial Direct Connection is the one which
utilizes the COM ports of your computers. Every
computer has at least 2 COM ports, COM1 and COM2.
The "COM" stands for "Communications". It's pinouts
are a lot simpler when compared to the parallel
port, but the speed is also a lot slower :)
To give you an idea of how fast (or slow) a serial
port is, at its best you will get around 12 to 14 KB
per second. That's pretty slow when you're used to a
network connection, but let me show you how serial
data is transferred so you can also understand why
it's a lot slower:
The above picture gives you an idea on how serial
data is transferred. Each coloured block that is
numbered is sent from PC 1 to PC 2. PC 2 will
receive the data in the same order it was sent, in
other words it will receive data block 1 first and
then 2, all the way to block 7. This is a pretty
good representation of data flow in a serial cable.
Serial ports transmit data sequentially over one
pair of wires (the rest of the wires are used to
controll the transfer).
Another way you can think of it is like a one lane
road where the road is wide enough to only fit one
car at a time (one data block at a time in our
example above), so you would imagine that the road
cannont process several cars at one time.
The Serial port
Most new computers have two COM ports with 9 pins
each, these are DB-9 male connectors. Older
computers would have one DB-9 male connector and one
DB-25 male connector. The 25 pin male connector is
pretty much the same as the 9 pin, it's just bigger.
Let's have a look at a serial port to see what we
are talking about:
Different pinouts are used for the DB-9 and DB-25
connectors and we will have a look at them in a
moment. Let's just have another quick look at the
COM ports of a new computer:
Notice the COM ports, they are both DB-9 connectors,
there is no more DB-25 ! The connector above the two
blue COM ports is an LPT or Parallel port.
The serial port of a computer
is able to run at different speeds, thus allowing us
to connect different devices which communicate at
different speeds with the computer. The following
table shows the speeds at which most computers'
serial ports are able to run and how many
KB/sec
they translate to:
Now we will have a look at the pin outs of both DB-9
and DB-25 connectors:
The Cable
All that's left now is the
pinouts required to allow us to use the serial cable
for direct connection. There is a special term for
this type of a cable, it's call a "null
modem" cable, which
basically means you need to have TX and RX crossed
over. Because you can have different configurations,
e.g DB-9 to DB-9,
DB-9 to DB-25,
and DB-25 to DB-25,
I have created different tables to show you the
pinouts for each one:
1)
DB-9 to DB-9.
You use this configuration when you need a cable
with a DB-9 connector on each end:
2)
DB-9 to DB-25.
You use this configuration
when you need a cable with one DB-9 and one DB-25
connector on either end:
3)
DB-25 to DB-25.
You use this configuration
when you need a cable with a DB-25 connector on each
end:
Well, that pretty much covers everything about
serial direct connection via a null modem cable.
If you're using third party
software to connect your computers, you probably
won't stumble into big problems, but if you're using
Windows software be sure you have
unique names
for each of your computers
because Windows will treat the direct connection as
a "network" connection. This means you will be able
to see the other computer via Network Neighborhood. |
