Theory of Operation
STATUS INDICATOR (LED)
The ZT 8809A includes an LED near the extractor for general
purpose use. It is turned on by writing a logical 1 to bit 1 of the
printer port Control register at I/O address 037Ah, and is turned off by
writing a logical 0 to bit 1 of the same address. This bit controls the
printer port signal Autofeed (AFD), which is one of four signals at the
printer interface shared between ZT 8809A uses and printer uses.
Refer to Chapter 9, "Centronics Printer Interface," for a description of
this and other shared printer port signals.
AFD also controls write-protection to the 32 Kbyte static RAM on
board, provided jumper W17 is installed. The write signal to this
RAM is gated by the inversion of AFD. If jumper W17 is not
installed, the RAM may be written to regardless of the sense of AFD,
and the LED remains off. Refer to Appendix A for a complete
description of jumper W17.
If AFD is set to a logical 1, which turns on the LED, the 32 Kbyte
static RAM cannot be written. Therefore, setting the AFD bit to 1
protects valuable data in RAM from erroneous writes. STD DOS uses
this feature to protect the on-board RAM drive, which contains
valuable system configuration variables. Notice that upon power-up
or warm boot of an STD DOS system, the LED is turned on. STD
ROM development software does not access this bit, and the LED is
left in the reset state, which is off.
3-25
Содержание ZT 8809A
Страница 15: ...Tables Table B 9 J5 Pin Assignments B 17 Table B 10 J6 Pin Assignments B 18 Table B 11 J7 Pin Assignments B 19...
Страница 334: ...Jumper Configurations W49 W48 A W47 B A B W37 W66 A W50 B C J7 W67 Figure A 11 W37 W47 50 W66 W67 Jumper Blocks A 34...
Страница 346: ...Jumper Configurations W61 W60 W62 W63 W64 A B A B W65 Figure A 13 W60 W65 Jumper Block A 46...