Centronics Printer Interface
Status Port
The Status Port register has five read-only status bits:
1.
Busy (BUSY)
2.
Acknowledge (ACK*)
3.
Paper Error (PE)
4.
Printer Selected (SLCT)
5.
Error (ERROR*)
None of these bits in the Status Port register are inverted from
connector J6, with the exception of the BUSY signal. This is not
immediately obvious from the signal names, of which ACK* and
ERROR* show an active low state. The asterisk ("*") after the signal
names represents the active logical state at the printer interface rather
than inversion in the printer port. If not used by STD DOS as a printer
port, these status bits may be used for general purpose inputs. One
exception exists with the ERROR* signal.
The ERROR* signal is used by STD DOS to monitor the Interrupt
Request (INTRQ*) signal from the backplane. If the INTRQ* line is
still active after one source of interrupt has been serviced, then
another must be pending.
This allows for the edge-triggered
backplane interrupt to be shared by two or more sources.
To
understand more about interrupts, refer to Chapter 12, "Interrupt
Controller (8259A)."
It follows that the ERROR* signal is required for DOS usage only if
sharing backplane interrupts. It is best to leave this signal available to
STD DOS for future system expansion. Since most printers do not
require use of this signal, STD DOS assumes it may use ERROR*.
However, if a printer in use does require this signal, then changes
must be made as described in "Disabling Sharing of Printer Port
Signals" on page 9-12.
9-7
Содержание ZT 8809A
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