Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90051)

Chapter 9 189
Describing Asynchronous Devices
How to Control the Flow of Data
The Read Trigger Character
The read trigger character tells an asynchronous device when it can
send data to the computer. It prevents a device from sending data
before the computer is ready to receive it. Data sent before the read
trigger character is received is not accepted unless typeahead is
enabled. If typeahead is enabled, data can be entered at any time and
the read trigger character is not necessarily sent to the terminal. After
the computer sends the read trigger character, the input of data can
proceed.
The ASC software generates the read trigger character at the beginning
of each read when the computer is ready to accept data. The read
trigger character is the ASCII DC1 character (the same as XON). If the
device is operating in character mode, the device can begin
transmitting without further exchange of protocol characters.
However, if the data is sent through block mode, the device informs the
DTC by sending an ASCII DC2 character in response to the read trigger
character. The DTC, informed that a lot of data will be coming, sends
another read trigger character when it is ready to accept the block of
data. This second read trigger character is called a block read trigger
character.