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PSI BOX Reference
E)
When ready to receive another byte from the PSI BOX the
printer sets BUSY low and toggles the ACKNOWLEDGE
line low. The PSI BOX will maintain valid data on the
data bus for a minimum of 1 microsecond after the
STROBE line goes high. Later the printer sets BUSY low
and toggles the ACKNOWLEDGE line low, for a
minimum of 1 microsecond.
F)
The handshake starts again.
The Centronics Input Handshake.
_____________________________ _
The Centronics port input handshake, which has been
improved to cope woth screen dumps via Shift-PrtScr from the
latest breed of fast 386 & 486 PCs, is as follows.
A)
If the PSI BOX is busy or if its Centronics input buffer is
full, it will raise the BUSY line true. Any data that is sent
to the PSI BOX while the BUSY line is high is LOST!
Even though the Centr i/p LED may flash.
B)
When the Centronics device senses that the PSI BOX has
set the BUSY line low, it puts data on the data bus.
C)
The Centronics device signals the PSI BOX by toggling
the STROBE line low, for a minimum of 1 microsecond.
D)
The PSI BOX will respond by setting the BUSY line true,
high, and will read the data off the data bus.
E)
When ready to receive another byte the PSI BOX sets
BUSY low and toggles the ACKNOWLEDGE line low,
for a minimum of 1 microsecond. If the PSI BOX input
buffer becomes full then it will keep BUSY high and will
not sent ACKNOWLEDGE until the buffer is emptied.
F)
The handshake starts again.
Page 34
Parallel Port
Chapter 4
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