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5.4
THEORY OF OPERATION
Communication
The CPU, in addition to controlling placement of generated
characters, following instructions generated by control
codes,
and placing data into the appropriate memory location, also
handles communication with external
devices.
The instructions
are stored in the program ROM(s) and executed as a result of
keyboard entry or a command sequence. Routing of transmit-
ted and received code is handled by the CPU in a manner simi-
lar to storing a new
character.
If one or more stored characters are to be sent to either a host
computer or auxiliary printer, the CPU will make use of the bus
control circuitry to locate and retrieve data on a character-by-
character basis from the video character and attribute
memories. Each character is then converted to serial data and
transmitted with the selected protocols (baud rate,
handshak-
ing,
and parity characteristics) out the appropriate port.
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