Information Technology Reference
In-Depth Information
Table 13.1
Main pin connections used in 25-pin connector
Pin
Name
Abbreviation
Functionality
1
Frame ground
FG
This ground normally connects the outer sheath of the
cable and to earth ground.
2
Transmit data
TD
Data is sent from the DTE (computer or terminal) to a
DCE via TD.
3
Receive data
RD
Data is sent from the DCE to a DTE (computer or ter-
minal) via RD.
4
Request to send
RTS
DTE sets this active when it is ready to transmit data.
5
Clear to send
CTS
DCE sets this active to inform the DTE that it is ready
to receive data.
6
Data set ready
DSR
Similar functionality to CTS but activated by the DTE
when it is ready to receive data.
7
Signal ground
SG
All signals are referenced to the signal ground (GND).
20
Data
terminal ready
DTR
Similar functionality to RTS but activated by the DCE
when it wishes to transmit data.
13.2.5 Frame format
RS-232 uses asynchronous communication which has a start/stop data format (Figure 13.4).
Each character is transmitted one at a time with a delay between them. This delay is called
the inactive time and is set at a logic level high (-12 V) as shown in Figure 13.5. The trans-
mitter 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'. Next, 5, 6 or 7 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. Figure 13.5 shows a
frame format and an example transmission of the character 'A', using odd parity. The timing
of a single bit sets the rate of transmission. Both the transmitter and receiver need to be set to
the same bit-time interval. An internal clock on both sets this interval. These only have to be
roughly synchronised at approximately the same rate as data is transmitted in relatively short
bursts.
Tx Character
Tx Character
Tx Character
Inactive condition
Figure 13.4
Asynchronous communications
