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4 6 ___ User Manual for Microboard Computer Development System CDP18S693 & CDP18S694
Note that if the remote device is passive, such as a
display or a set of points, handshaking is not necessary.
The output port may be loaded at any time to change
data without acknowledgment from the remote device.
Similarly, the input port may be read at any time to
store the current state of the input lines.
Using the INTERRUPT Lines for
Data Synchronization
If link LK4l A:B is jumpered, INTA and INTB
generate INT to the CPU. INT is not conditioned by
the group select. INT is set by the remote device
sending STB to acknowledge an output port and is
reset by loading an output port. Similarly, INT is set by
the remote device sending STB to load an input port
and is reset by reading the input port. Table XV
summarizes the actions of READY and INT for input
and output modes.
The software can find the source of the interrupt by
setting the group select 08, and then, either testing the
RDY lines or reading the status byte. The low-order
two bits of the status byte are: bit 0 = INTB; bit 1 =
INTA.
Bidirectional Mode
Port A may be programmed to be directional. In this
case, Port B must be programmed to be in the bit-
programmable mode, to be described later. In the
bidirectional mode, ARDY and ASTB become A
INPUT RDY and A INPUT STB; BRDY becomes A
OUTPUT RDY, and BSTB becomes A OUTPUT
STB. Each of the eight lines AD0—AD7 maytransmit
data in both directions, using the input handshaking
lines to synchronize inputs and the output hand
shaking lines for the output data. Operation is much
the same as for independent input and output ports
except that output data is gated into ADO—AD7 only
when the OUTPUT STB line is raised. In summary,
Port A in the bidirectional mode is an output port and
an input port sharing the same eight data lines, each
having a set of handshaking lines.
Bit-Programmable Mode
Both Port A and Port B are capable of being
programmed to be in the bit programmable mode.
Port B must be in this mode if Port A is in the
bidirectional mode. In the bit-programmable mode,
each line in AD0-AD7 and B0-B7 is programmed to be
either input or output. In addition, the handshaking
lines are programmed to be input or output lines unless
Port A is bidirectional, in which case it uses all four
handshaking lines. The handshaking lines, when used
as data lines, are accessed by a write control for output
lines and read status for input lines. The other eight
lines in each port are accessed by the usual read and
write data instructions.
Interrupts are generated when an input line goes true
except that the former handshaking lines cannot
generate interrupts. The bits may be individually
masked so as not to generate interrupts. The interrupt
control word selects one of the two interrupt rules,
AND or OR. The AND rule results in an interrupt only
when all unmasked lines are true. The OR rule results
in an interrupt when any unmasked line is true. The
interrupt control word also defines the input lines as
logically true when high or logically true when low.
I/O Expansion
RCA I/O Modules
A num ber of m odules are available in the
Microboard product line that allow expansion of the
I/O capabilities of the MCDS. Included are:
UART Interface CDP18S641
D /A Converter CDP18S642
A /D Converter CDP18S643
A /D and
D / A
Converter CDP18S644
Parallel I/O Module CDP18S646
D /A Converter CDP18S647
A /D Converter CDP18S648
Floppy Disk Interface Module CDP18S651
A /D and D /A Converter CDP18S654
D /A Converter CDP18S657
A /D Converter CDP18S658
Combination Memory and I/O Module CDP18S660
Video-Audio-Keyboard Interface CDP18S661
PIO Opto 22 Module Interface CDP18S662
Custom I/O Modules
The constraints on custom memory modules are
also applicable to custom I/O modules. The physical
size must be compatible if the custom board is to plug
into the backplane, and the backplane signals should
be buffered. Each module must latch the group-select
byte and decode the I/O addresses. The user must
Table XV—READY and INTERRUPT Actions for Input and Output Modes.
Output Port
Input Port
READY
Set by
Loading data
Reading data
Reset by
STB leading edge
STB leading edge
INTERRUPT
Set by
STB trailing edge
STB trailing edge
Reset by
Loading data
Reading data