Displayed Character Positions - Texas Instruments 990 Installation And Operation Manual

Jd75\ _ _ _ _ _ _ _
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945423-9701
Bit Number
o
Table 3-1. Model 911 VDT Controller Addressable Input Bits (Continued)
Description
Terminal Ready - normally indicates the status of the associated terminal. A logic 0
indicates the terminal is connected and available. A logic 1 indicates the terminal is
turned off or disconnected. Also, when self-test mode is selected, terminal ready is set
to logic 1.
Previous State Flag or Self-Test Signal - indicates the state of the word-select logic
before the last transfer to word 1. Logic 0 indicates word 0 was selected, and logic 1
indicates word 1 was selected. If self-test mode is selected, this signal provides one of
four test inputs.
The previous state flag permits interrupt-driven software to determine the controller
state prior to a keyboard interrupt. This permits the controller to process the interrupt
and restore previous conditions.
When test mode is selected, this bit has another function. In test mode the signal read
is determined by two display memory write data bits: CRU output bits 0 and 1 of
word O-VDT 0 and CRU outputs 10 and 11 of word O-VDT 1. The signals read during
test mode are video, audio "beep", horizontal sync, and vertical sync. The signals and
their characteristics are summarized in table 3-2.
Keyboard Parity Error - a logic 1 on this input indicates that a parity error occurred
on the previous keyboard data transmission. The error indication is reset by the output
signal, keyboard acknowledge. A logic 0 indicates the transmission had valid parity.
Keyboard Data Ready - Logic 1 indicates a character has been input at the display
keyboard and is available to be read by the CPU. This signal is reset to logic 0 by an
output of keyboard acknowledge.
NOTE
The VDT controller may generate two CRU interrupts. An interrupt signal
occurs as a result of the keyboard data ready signal from VDT 0 or VDT 1 if the
respective keyboard interrupt enable control signal is set to logic 1.
2 _ - _ _ - _ _ _ _ 4F
o
2 - - - - - - - - - 4F
50 - - - - - - - - - - - - - 9F 50 - - - - - - - - - - - - - - 9F
730 77F
370 3BF
I 92G-CHARACTER
960-CHARACTER
2048 8-BIT MEMORY (0-7FF)16
1024 8-BIT MEMORY (0-3FF) 16
(A)134319
Figure 3-3. Displayed Character Positions
3-5
Digital Systems Division