Use of Modem and Null Modem Cables

1
Use of Modem and Null Modem Cable "Modem and
null modem cables are used for data
transmission between two electronics such as
a computer to another computer, central
processing unit to monitor and computer to
programmable logic controllers. They are used
for parallel transmission of data between
computers. These cables are used to
transmit N-bits of data simultaneously "
1
2

• Thus, they enable relatively faster
  transmission of bytes and facilitate quick
  processing. A parallel connection must use this
  cable for fast and reliable connection. Modem and
  null modem cables have N- channels which are
  the number of wires making the cables.
  Each of these channels is used to transmit
  a bit so that all the channels help in
  transmission at the same type. Also, the
  frequency of bit transmission might vary in
  different channels. Modem cables are
  prone to interference at high
  frequency because of cross
  connection. Interference will degrade the quality
  of the signal being transmitted.
• Modem and null modem cables are of various types
• Modem cable
• DB9 null modem cable
• DB25 null modem cable
• Bidirectional null modem cable
• These cables are readily available at the store.
  They can either have the same connector on both
  side or DB9 on one side and DB25 on the other
  side. The cord is sold with either a male
  connector on one side or a female connector
  on the other. The choice depends on where you are
  going to plug the connector. Usually, DB9
  and DB25 have a standard size for use in all
  electronic equipment.

3
NULL MODEM CABLES This cable is used in direct
communication between devices by use of RS232
cable. This cord has a cross-link
connection between the transmitting and
receiving line. DB9 and DB25 are two null
modem cords. They can be cross-linked so as
to be used as an adapter for a device
with DB9 port and the other device with DB25
port. DB9 means that the connector has
nine-pin configuration and DB25 means the
connector has 25-pin configuration. DB 25
transmits nine types of signals. These
signals are frame ground, transmitted data,
received data, request to send, clear to send,
signal ground, data set ready, data carrier
detects and data terminal ready.
4
DB9 cords do not transmit frame ground
signal but all other signals. Bidirectional
null modem cable has DB9 on one end and DB 25 on
the other end so as to ease adaptation to
devices. These cables are applicable for
file transfer between personal computers or
in remote operation of electronic devices such
as debugging. Also, they are used when managing
switches, micro-controlled systems, and
rack-mounted server units. RS 232 CABLE This
cable is found in the industrial machine,
scientific instruments and networking equipment.
It has 25-pin layout that transmits ten types of
signals. These signals are data terminal ready,
data carrier detects, data set ready, ring
indicator, request to send, clear to send,
transmitted data, received data, common ground,
and protective ground. This cable is used
for connecting data terminal equipment to
data circuit terminating equipment. RS 232
has lost its popularity in the modern
computers, thus they must be linked by use of
external USB to RS 232 adapter or by
use of internal expansion card to RS 232
peripheral. RS 232 cable can be a DB9 or a DB25.