Information Technology Reference
In-Depth Information
Figure 15.1 shows how two computers connect to each other using RS-232 converters and
modems. The RS-232 converter is normally an integral part of the computer, while the mo-
dem can either be external or internal to the computer. If it is externally connected then it is
normally connected by a cable with a 25-pin male D-type connector on either end.
Modems are either synchronous or asynchronous. A synchronous modem recovers the
clock at the receiver. There is no need for start and stop bits in a synchronous modem. Asyn-
chronous modems are, by far, the most popular types. Synchronous modems have a typical
speed of 56 Kbps whereas for asynchronous modems it is 33 Kbps. A measure of the speed of
the modem is the baud rate or bps (bits per second).
There are two types of circuits available from the public telephone network: either direct
dial or a permanent connection. The direct dial type is a dial-up network where the link is
established in the same manner as normal voice calls with a standard telephone or some kind
of an automatic dial/answer machine. They can use either touch-tones or pulses to make the
connection. With private line circuits, the subscriber has a permanent dedicated communica-
tion link.
Computer
RS-232
Modem
Telephone
connection
Computer
RS-232
Modem
Figure 15.1
Data transfer using modems
15.2 RS-232 communications
The communication between the modem and the computer is via RS-232. RS-232 uses asyn-
chronous communication which has a start-stop data format. Each character is transmitted
one at a time with a delay between characters. This delay is called the inactive time and is set
at a logic level high as shown in Figure 15.2. The transmitter sends a start bit to inform the
receiver that a character is to be sent in the following bit transmission. This start bit is always
a '0'. The following data bits are sent as a 7-bit ASCII character, followed by a parity bit and
finally either 1, 1.5 or 2 stop bits. The rate of transmission is set by the timing of a single bit.
Both the transmitter and receiver need to be set to the same bit-time interval. An internal
clock on both of them sets this interval. They only have to be roughly synchronised and ap-
proximately at the same rate as data is transmitted in relatively short bursts.
15.2.1 Bit rate and the baud rate
One of the main parameters for specifying RS-232 communications is the rate at which data
is transmitted and received. It is important that the transmitter and receiver operate at roughly
the same speed.
