A-20
A2
PC or
Computer
Modem
Modem
DDN or Leased Line
2303
Interface
Digital
Device
Digital
Signals
RS-232
RS-232
Chassis
Figure A-8 Long Distance Communication using Modems
Another aspect of timing is the control of data transmission to avoid
data overrun. The two methods used are control signals and X-on/X-
off characters.
For the control signal method, extra wires are provided in the cable for
handshake signals that enable or inhibit data flow. The more common
control signal pairs are:
Request-to-send / Clear-to-send
Data-terminal-ready / Data-set-ready
All signals must be high to enable data transmission. Dropping any
line normally means the receiving device's buffer is full or it is busy
with the last message.
Another method of controlling the data flow is to imbed X-on/X-off
characters in the data message. At turn on, both devices are initially
in the X-on state. When one device becomes full, it sends the other
an X-off character to inhibit future data transmission. X-on is then
sent to restart the data transmission when there is room in the receiving
device's buffer for additional data.
