Bue
Interface
Loglc
Good design practice dictates most signaIs
to
and Iram the
system bus must be butlered so anly one
TIL
load per board
is presented
10
each nan-inverting buffer
lor
lhe Z-80 control
signais. This buHering
Is
accomplished by U32 and U33. (Noie:
The enables tor both these parts are lied low. allowing these
signais
10
be driven onlo the board
at
ail times. Open collector
devices are
use<!
10
drive the outputs
to
the bus which may be
driven by other boards, INTRO" and XFERRO.)
There is a basic problem with
using a Z-80/P10 with the
WD1791. The
PlO
has a nan-inverting dala bus white the
W01791 utilizes an inverting data bus. One extra stage ot
inversion is required
lar
the W01791. U19 and U20 accomplish
this extra inversion. These transceivers are normally receivlng
data but are enabled ta drive data toward the system bus when
an input operation (rom the ports assigned ta
the
FOC is in
progress.
Z-80/PIO
Interface
Loglc
The Z-80 parallel
110
(Pla)
interface controller is a genera
purpose, programmable, Iwo-port device which provides
m
compatible intertacing between peripheral devices and
thE
Z-80 CPU.
Any
al the follawing modes can be selected for either port:
byte output
byte Input
byte bi-directionaJ A(port Aonly)
byte or control mode
ln
addition, the PlO provides a elean and minimallogie methoc
for
generating mode 2 interrupts
ta
the Z-80 CPU.
Port A
is used in the control
mode,
which allows the eight
110
lines (A0 through A7)
la
be configured as either inputs
or
outputs. An eight·bit mask reglster and a Iwo-bit mask control
reglster
aHow
interrupts ta
he
generated. dependent on the
lagic states of the
1/0
Unes.
Port A is primarity
used
for slatus
checking and generating interrupts.
07
Printer Busy
0=
Not
Busy
1 = Busy
04
Primer Faull
0'"
Fault
1 =
Not
Fault
Table
3.
BIT
Allocation
Port
E'H.
Printer, FOO, FOC Interrupt
Statu'
06
Paper
Empty
" =
Paper
not
Empty
1 =
Paper
Empty
03·
PRIME
High ta Law
Transition
Resets
Printer
01
Two·Sided Oiskette
1'"
Two-Sided Oiskette
Preset
CJ
= Single-Sided Oiskette
05
Printer Select
0=
Selected
1 '"
Not
Selected
02·
Oisk
Change
0=
Ooor nol
OpenCfI
1 = Ooor Opened
Dt
FOC
lNT
REQUEST
1 = FOC
is
Interrupting
0'"
Not
Inlerrupting
°02
indicates that the selected drive
has
nad
its
dOOf
opene<!
since
it
was
tast
selected.
°03
is
an
output
which
resets
sorne printers.
3
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