User's Manual

RS-232 RADIO MODEMS

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234839, Revision 2.2

RS-232 Radio Modems

ommunication systems cannot directly transport digital data without distortion affecting the signal. A

device is required to ‘transform’ the digital signal into one that can be transported without distortion. A

radio modem is a device used to convert serial digital data into a form that can be transmitted by Radio

Frequency (RF). This is exactly like the landline modem except that it transmits its data down your phone

line instead. The receiving modem ‘transforms’ the signal back into serial data. This is summarised in Figure

2-1.

ta is

transfer

red in

blocks

‘chunks

’. The communicating modems must be synchronised so that they both recognise the beginning and end of

the data blocks. Data transmitted in the RF path is typically sent in large blocks or packets, where many data

bits represent a packet. The data on the Terminal or DTE/DCE end by comparison is sent asynchronous

where start and stop bits are included in every block of 8 data bits. This decreases the amount of useful data

inside each block

If data is transferred in one direction at one time only, it is known as Simplex Communication. If data is

transferred in both directions at the same time, it is known as Duplex Communication and is divided into

two systems, Half and Full. In Half Duplex Communication, data can only flow in one direction at a given

time and in Full Duplex Communication data flows in both directions simultaneously. This is illustrated in

Figure 2-2.

RS-

Sta

ard

S-

232

is a

standard that defines the characteristics of point-to-point communication. It is suitable for transferring

Figure 2-1 Modem Function in Data Transfer

Figure 2-2 Simplex/Duplex Modes