Group A is associated with one of three modes, mode 0, mode 1 and mode 2,
and group B with one of two modes, mode 0 and mode 1.The modes are
determined by the contents of the control register whose format is given
in Fig.9-22.These modes are:
Mode 0 - If a group is in mode 0, it is divided into two sets.For group A
these sets are port A and the upper 4 bits of port C,and for group B they
are port B and the lower 4 bits of port C.Each set may be used for inputting
or outputting, but not both.Bits D4,D3,D1 and D0 in the control register
specify which sets are for input and which are for output.These bits are
associated with the sets as follows:
D4 - Port A
D3 - Upper half of port C
D1 - Port B
D0 - Lower half of port C
If a bit is 0,then the corresponding set is used for output;if it is 1, the
set is for input.
___
PC4 STBa A 0 applied to this pin causes PA7-PA0 to be latched,
or "strobed", into port A.
PC5 IBFa Indicates that the input buffer is full.It is 1 when
port A contains data that have not yet been input to
the CPU.When a 0 is on this pin the device can input
a new byte to the interface.
PC6,PC7 May be used to output control signals to the device
or input status from the device.If D3 of the control
register is 0, they are for outputting control signals;
otherwise, they are for inputting status.
For outputting:
PC4,PC5 Serve the same purpose as described above
___ for PC6,PC7.
PC7 OBFa Indicates that the output buffer is full.It outputs
a 0 to the device when port A is outputting new data
___ to be taken by the device.
PC6 ACKa Device puts a 0 on this pin when it accepts data
from port A.
In mode 1, PC3 is denoted INTRa and is associated with group A.It is used
as an interrupt request line and is tied to one of the IR lines in the
system bus.When inputting to port A, this pin becomes 1 when new data are
put in port A (i.e., it is controlled by PC4) and is cleared when the CPU
takes the data.For output, this pin is set to 1 when the contents of port A
are taken by the device and is cleared when new data are sent from the CPU.
If group B is in mode 1, port B is input to or output from according to
bit D1 of the control register (D1=1 indicates input).For input,
PC2 and PC1 are denoted \STBb and IBFb, respectively, and serve the same
purposes for group B as \STBa and IBFa do for group A.Similarly, for output
PC1 and PC2 are denoted \OBFb and \ACKb.PC0 becomes INTRb and its use is
analogous to that of INTRa.The interrupt enable for group A is controlled by
setting or clearing internal flags.Setting or clearing these flags is
simulated by setting or clearing PC4 for input, or PC6 for output, using
a Set/Reset instruction.Similarly, the interrupt enable for group B is
controlled by set/clear of PC2 for both input and output.
___
PC4 STBa A 0 on this line causes the data on PA7-PA0 to be
"strobed" into port A.
PC5 IBFa Becomes 1 when port A is filled with new data from
lines PA7-PA0 and is cleared when these data are
___ taken by the CPU.
PC6 ACKa Indicates that the device is ready to accept data
___ from PA7-PA0.
PC7 OBFa Becomes 0 when port A is filled with new data from
the CPU and is set to 1 when data are taken by the
device.
While group A is in mode 2, group B may be in either mode 0 or mode 1.
However, if group B is in mode 0, only PC2-PC0 can be used for input or
output because group A has borrowed PC3 to use as an interrupt request
line.Normally, if group A is in mode 2, PC2-PC0 would be connected to
control and status pins on the device attached to the port A lines.Port B
may also be used for this purpose.
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