Radio Shack TRS-80 Service Manual page 31

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Service Manual
7/Theory of Operation
System Overview
The DT-1 Data Terminal consists of a state of the art single
chip microcomputer, video RAM, keyboard interface, CRT
controller, and two printer ports.
The microcomputer has 4K bytes of ROM, and 255 bytes of RAM.
Video RAM is made up of four
1~24
byte by four bit memories,
structured into
2~48
eight bit words. The display is
formatted to have 24 lines and each line holds up to 8'
characters.
The Data Terminal can emulate four of the "standard"
terminals on the market today, and has certain common
features that apply to all emulations. The two printer
ports, one serial, and one parallel, allow the Data Terminal
to be used with just about any printer available on the
market.
The DT-1 is an RS-232 serial communications device,
operating at Baud rates from 75 to
19,2~~.
The serial
printer must operate at the same baud rate as the terminal,
while any speed parallel printer may be used. XOFF and XON
protocol is used to control printer feed rates.
While most terminals use Dip Switches to set user features
and functions, the Data Terminal utilizes a state of the art
non-volital memory, that is programmable from the keyboard.
This memory retains settings even when power is off.
Detailed Theory of Operation
Microprocessor
The 8~51 microprocessor, U-12, contains over 4K bytes of on
chip ROM that has been coded to operate as a terminal. It
contains a full duplex serial interface that allows this
microcomputer to be easily used as a terminal controller.
U-12 controls all sections of the Data Terminal. Operating
at 7.3728 MHZ using crystal oscillator Y-2, the
microprocessor receives data from the Host system and routes
this data to the video RAM. It also scans a nine by eight
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