Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

294 AppendixA
Comparing MPE/iX Systems to MPE V Systems
Software Characteristics
For instance, XOFF ([CTRL]-S on terminals) is ignored in MPE V during
reads or when no I/O is pending on the device. (This means that nothing
is waiting to be printed on the device and no program is waiting for
input from the device.) With MPE/iX, the
[CTRL]-S character is not
ignored (except during a binary read).
Therefore, during a read, if [CTRL]-S is pressed, the read appears to be
suspended. Press
[CTRL]-Q to echo the characters typed to the screen.
Therefore,
[CTRL]-Q must be pressed to continue and complete the read.
The only exception is during binary mode. During a binary read, an
ASCII DC1 or DC3 can be a data character and not an XON or XOFF.
The ASC software does not react to them as if they are protocol
characters, but passes them through as data. Since these protocol
characters are accepted as data, they will be echoed. However, when an
XOFF is echoed, it will suspend the terminal as if an XOFF were sent to
it from the DTC. This means that echo should be disabled while in
binary mode.
Note that the XON/XOFF protocol characters are accepted as data only
when coming from the device. When an ASCII DC1 or DC3 is sent from
the DTC, the terminal will still react to the character as a protocol
character.
Using Smooth Scroll
A terminal uses its buffer to store incoming data and makes use of the
XON/XOFF protocol to keep the DTC from overflowing the terminal
buffer. Some supported devices (namely the HP150 and HP293x
terminals) support smooth scroll. When a device is in smooth scroll
mode, it transfers data to its screen more slowly (to make the output
look “smooth”) and therefore uses its buffer to store more incoming
data. This is especially true at higher speeds such as 9600 and
19,200 bps.
As a terminal user, use the XON/XOFF protocol to monitor what is
written to the screen by pressing
[CTRL]-S (XOFF) and [CTRL]-Q (XON).
When both the terminal and its user are sending XONs and XOFFs to
the DTC, the DTC has no way of knowing the source of the protocol
characters. Therefore, it can appear that the
[CTRL]-S and [CTRL]-Q
characters are not being recognized.
To avoid this problem, press
[Stop] instead of [CTRL]-S and [CTRL]-Q.
When
[Stop] is pressed, the terminal’s buffer stops processing data, so
no more will be printed to the terminal screen. When
[Stop] is pressed a
second time, the terminal will resume processing data in and out of its
buffer. (Therefore, the
[Stop] key functions as a toggle switch.) In this
way, the terminal controls which protocol characters are sent to the
DTC.