RS232 software handshaking
- an overview of the way in which codes such as X-ON
and X-OFF sent over an RS232 communication link can be used for handshaking.
When using RS232, the most usual method for handshaking
between two equipments is to use hardwired lines between the two equipments to
control the data flow. Unfortunately when using RS232, it is not always possible
to use these additional lines. For example when data is being sent along a
telephone line, where only the signal lines are available, it is not possible to
utilise additional lines for RS232 handshaking. Additionally, there are
instances where it is more convenient to use software methods to effect the
handshaking. With the rise of processor control, this is often a more attractive
technique than introducing extra hardwired lines.
To implement RS232 software handshaking, it is necessary to
send codes along the lines to establish communications and control the data
flow. There are a number of control codes used for RS232 (and other) links under
these circumstances. However they are normally used with older legacy equipment.
Two of the most commonly used code systems used with RS232
are X-ON / X-OFF and ETX / ACK. However there are other codes that can also be
used to provide other facilities as well.
X-ON / X-OFF
The X-ON X-OFF operates by sending the control characters along the data line
from the equipment receiving the data. When X-ON is received at the transmitting
end data transmission is started. Once the input buffer on the terminal or
printer starts to become full the X-OFF character is sent to stop the data. When
sufficient space appears in the buffer X-ON can be sent to the transmitter to
resume the data flow.
EXT / ACK
A second method is called EXT/ACK. Using this method the data is separated into
blocks and after each block has been sent the control code ETX is transmitted to
show the end of this block of text. Once the data has been accepted and there is
sufficient space in the input buffer the ACK or acknowledgement control code is
sent. Once this has been received, the next block of data is sent.
Other codes
Other codes are also used within what is effectively software control of an
RS232 communications link. These codes are used for a variety of purposes from
indicating tabs, form feeds as well as providing audible 'bell' warnings. These
codes are naturally focussed on some of the older teletypes that used two wire
RS232 links.
Control Codes Names
| NUL |
Null (blank) |
| SOH |
Start of header |
| STX |
Start of text |
| ETX |
End of text |
| EOT |
End of transmission |
| ENQ |
Enquiry |
| ACK |
Acknowledgement |
| BEL |
Bell (Audible) |
| BS |
Backspace |
| HT |
Horizontal tab |
| LF |
Line feed |
| VT |
Vertical tab |
| FF |
Form feed |
| CR |
Carriage return |
| SO |
Shift out |
| SI |
Shift in |
| DLE |
Data link escape |
| DC1 |
Device control 1 |
| DC2 |
Device control 2 |
| DC3 |
Device control 3 |
| DC4 |
Device control 4 |
| NAK |
Negative acknowledgement |
| SYN |
Synchronous idle |
| ETB |
End of transmission block |
| CAN |
Cancel |
| EM |
End of medium |
| SUB |
Substitute |
| ESC |
Escape |
| FS |
File separator |
| GS |
Group separator |
| RS |
Record separator |
| US |
Unit separator |
| DEL |
Delete |
Summary
Although not all of the codes are used these days because many are intended for
use by the older teletype equipments, the same processes are valid for modern
equipment. This set of codes are known as control codes, which even gives rise
to the 'CTRL' key on today's keyboards. Additionally the idea of software
handshaking, although it was not so widely used in the early days, is now well
established and usually the preferred method of implementing control of
communications systems in view of the flexibility offered by processors.
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