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Hardware Structure of the CDS _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 43
force all zeroes into the register. Instruction 69 is used
to read back the contents of the register for use by
in terrupt-handling
subroutines.
Users who are developing systems having one-level
110
should
remove the jumper on TLIO. The system
will then work as follows. A 61 instruction is used to
enter new selection data so long as the two-level
control flip-flop is
set.
This flip-flop is set when the
RUN U key is pressed. UT20 immediately writes a 01
to the
selection
register, enabling the terminal in-
t~rface.
The flip-flop is reset by the RESET
switch
or
'-by
execution
of the $P command, which additionally
: writes a 00 to the
selection
register. With the control
flip-flop reset, the
61
and 69 instructions are free for
use like any other instruction and cannot be used to
con trol the
selection
register.
Several
operational considerations exist for the two
options of TLIO grounded or not. These options are
summarized below.
With TLIO-N Grounded:
A.
RESET, RUN P Sequence
1. Starts execution atlocation 0000.
2.
Two-level
110
permanently enabled.
3.
Instructions 61 and
69
reserved for two-level
selection.
4. Program starts with P=X=O, IE=I,
Selection Register=OO.
B.
RESET, RUN U Sequence
UT20 starts and runs with selection
group
01
enabled.
$UCommand
1. Program
starts
at location
specified.
2. Two-level
110
permanently enabled.
3. Instructions 61 and 69 reserved for two-level
selection.
4.
Program
starts
with P=X=O, IE=I,
Selection Register= 0 1.
5.
If an interrupt is pending at start-up, UT20
will report it.
$PCommand
1. Program starts at specified location.
2. Two-level
110
permanently enabled.
3. Instructions 61 and 69 reserved for two-level
selection.
4.
Program
starts
with P=XO, IE=I,
Selection Register=OO.
5. If an interrupt is pending at start up, UT20
will attempt to report it.
With TLIO-N Open (High)
A.
RESET, RUN P Sequence
1. Starts execution at location 0000.
2. Two-level
110
is permanently disabled.
3. All
110
instructions are available to user.
4. Program starts with P=X=O, IE=I,
Selection Register=OO.
B.
RESET, RUN U Sequence
UT20 starts and runs with selection group 01
enabled.
$UCommand
1. Program starts at location specified.
2. Two-level
110
enabled.
3. Instructions 61 and 69 are reserved for two-
level selection.
4. Instruction 67 with data bit pattern
(xxxxxIxx) must be avoided if two-level
selection is to remain enabled.
5. Program starts with P=X=O, IE=I,
Selection Register=01.
6. If an interrupt is pending at start-up, UT20
will report it.
$PCommand
1. Program
starts
at location specified.
2. Two-level I/O permanently disabled.
3. All
110
instructions are available to user.
4. Program starts with P=X=O, IE=I,
Selection Register=OO.
5. If an interrupt is pending at start-up, UT20
will attempt to report it.
6. Starting a program in this case precludes the
use of the devices controlled by the Terminal
and Disk Interface modules.
NOTE: All RCA-supplied programs should be
started
with the $U command in all cases.
Interfacing Signals and
Custom I/O Modules
User devices
can
be interfaced to the CDS with
signals
available at the
110
bus and from the I/O
decoder. There
are
seven signals which are
sensed
by
the CPU, namely, EFI-N, EF2-N, EF3-N, EF4-N,
DMAIN-N, DMAOUT-N and INTERRUPT-N.
These
signals
are pulled high with 22-kilohm resistors
on
the CPU board and are brought to the
110
bus.
Control electronics for these signals should use a
transmission gate which pulls the signal lines low
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