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SECTION
IV
DIRECT
DATA CHANNEL
OPTION
The purpose
of
the
Direct Data
Channel (DDC)
is
to
provide an
I/O
path
for
high-speed
data transfers
between
the controller
and
I/O
device.
Communication
is
via the
DDC
bus
and
programmed
I/O
bus.
The
transfer
is
entirely
under
the
control
of
the
external device
and can occur
at
the
end
of
any
instruction.
The
external device controls
address
of the
memory
location, the
direction
of
transfer
and
the
number
of
cycles
to
be taken
at
one time
.
The hardware
in
the
controller
performing
the
transfer
is
called a channel.
There
are
two
channels
in
the
DDC
option (A
and
B).
The
device or device adpater
hardware
is
designated
as
the
subchannel.
This device
hardware
is
not
included as part
of the
DDC
option.
In
the
case
of
simultaneous
requests, channel
A
has
priority.
SIGNALS
All signals
on
the
DDC
bus are
active,
true or
represent
a logical
ONE
when
at
ground.
They
are passive,
false or
represent
ZEROs
when
at
+6
volts.
Signal
timings are guaranteed
at
the
device adapter (subchannel)
when
the
standard
cable
is
used
for
interconnections.
Each
subchannel
is
connected
to
a
channel
by
a separate
cable; the
cables are not daisy- chained.
The
length
of
each
cable
may
be no longer than
10 feet.
Each
subchannel
may
load a signal
line
on
the
DDC
bus with up
to
3
loads.
A
load
is
to
consist
of
a
standard
gate input
in
parallel with a
6.
8K ohm
resistor
to
+6
volts,
and
the
cathode
of
a
grounded
diode.
The
controller
uses
a type
932 integrated
circuit
power
amplifier
circuit
to
drive
the
output signals
on
the
DDC
bus.
A
subchannel
must
drive
the
channel
via
the
DDC
bus with standard
drive
circuits
used
on
the
programmed
I/O
bus.
This
circuit
is
a
type
944 integrated
circuit
power
amplifier
(uncommitted
collector)
with
a
6.
8
Kilohm
pull
up
resistor
to
+6
volts
in
parallel
with
the
cathode
of
a
grounded
diode.
Standard driver and receiver
circuits
are contained on
the
CS-517, CS-520, CS-521
and
CS-548
PACs
(see
Appendix
A).
Those
signals
on
the
programmed
I/O
bus
which
are
actively
used
during
DDC
transfers
must
be
received and driven
by standard
I/O
circuits.
All rules
for the
I/O
bus apply.
DDC
Interface
The
interface
between
the
channel and
a
subchannel
consists
of the
signals
shown
in
Figure 4-1.
All
of the
signal
names
which end
in
"A"
apply
to
subchannel
A
and
those
that
end
in
"B"
apply
to
subchannel B.
4-1
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