Texas Instruments 990 User Manual

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CRA9Q
0
BTL
NTRL
CRAOQ
0
0
Table 1-6. Test Data Selection
BKBINL BBTSL
TKBL PREVWIQ
KBTL BTSL
Table 1- 7. Selected Test Inputs
CRAlQ
0
0
BBTSL
VIDEOQ
VSQ
HSBQ
AQ
NLEDL
NLEDL*
0
The computer acknowledges Keyboard Data Ready by setting Keyboard Acknowledge which sets
the keyboard acknowledge latch (KBACKQ). The keyboard acknowledge latch holds KBACKQ
high until the keyboard interrupt reset latch is set to ensure proper input conditions are provided
to the keyboard acknowledge synchronizer latch (KBASQ). The keyboard acknowledge syn-
chronizer latch and the keyboard interrupt reset latch provide double-buffered synchronization
to ensure recognition of Keyboard Acknowledge by the UART. KBAKQ, the double-buffered
synchronized version of Keyboard Acknowledge, clears the keyboard acknowledge latch and the
keyboard data ready latch (Keyboard Data Ready) KBRDYL, and the parity error signal from
the UART, and removes the device interrupt, KNBINT, from the interface.
Figure 1-3 5 shows flowcharts for the transmitter and receiver portions of keyboard logic.
1.4.4.4 CRU Interface Logic. CRU interface logic provides the circuitry to input and output
data, control, and status signals to and from the computer. Figure 1-36 shows CRU interface
logic, and the following discussions describe the circuits in that diagram.
Input Buffers. The input buffers provide buffering for all the CRU interface signals from the
computer.
Instruction Decoder. The instruction decoder monitors CRUBIT, 12-15 and decodes those signals
to provide the instructions contained by bits 8
16
through F
16
of each CRU output word.
Table 1-8 lists the instructions provided by the instruction decoder.
Word Select Latch. The word select latch is a flip-flop that latches the Word Select signal from
the CRU interface. When NCR15P from the instruction decoder makes a high-to-low-to-high
transition, the word select latch's Q output goes to the value of CRDL, the buffered
CRUBITOUT from the computer. The word select latch maintains its last state until cleared by
the computer (NRLI).
l-52
Digital Systems Division

Texas Instruments 990 Specifications

General

Brand	Texas Instruments
Model	990
Category	Desktop
Language	English