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Theory of Operation
3–62
1780R-Series Service Manual
Diagrams 22, 23, & 24 MPU Annex, Front Switch Panel, & Touch Panel
Circuits on the Annex circuit board (Diagram 22) tie the 1780R-Series data bus
to the front-panel switches, touch screens, indicators, and the Optical Encoder.
Data transactions to and from the Master MPU are through a bi-directional
transceiver. Two-way communication with the front-panel switches and
indicators along with the CRT touch panels is through the Keyboard Display
driver and non-latched interface.
Optical Encoder output is decoded into a digital data stream and put on the bus.
Audio feedback, when the Optical Encoder is turned or when some of the switch
functions are activated, is provided by a speaker on the Annex circuit board.
Data Transceiver. U409 is the I/O to the External Data (ED) Bus (MD0MD7). It
is a tri-state device conducting A-to-B, B-to-A, or nonconducting depending on
the /G and DIR inputs. /G (/ANDBE from U305 on the Interconnect, Diagram
21) is low for the transceiver to conduct in either direction. When DIR (DDIN/
from the Master MPU, Diagram 16) is low it conducts from B-to-A, and when
high it conducts from A-to-B.
Keyboard Display. U437 is a Programmable Keyboard/Display Interface tying the
data bus, from the MPU, to the front-panel drivers. Chip Select (CS) from the
Overview
Circuit Theory