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Theory of Operation
3–54
1780R-Series Service Manual
Diagram 19 UART & A/D
The Universal Asynchronous Receiver/Transmitter (UART) has both RS-232 and
RS-422 buffers for communication between the Master MPU (Diagram 16) and
external peripherals. By using two buffers and internal terminations it is possible
to use a single rear-panel 9-pin, D-type connector for both interface standards.
The A/D Converter changes the 16 analog potentiometer signals into 8-bit digital
bytes for use by the Master MPU.
Also included on this diagram are the Remote Sync and RGB Input Amplifiers.
UART. U330 is a dual UART that runs at a clock frequency of 3.6864 MHz. It is
configured as an 8-bit peripheral activated whenever the Master MPU addresses
the UART address space. The chip select /UARTCS (from the Master MPU) is
the chip enable (CEN) for the UART. The /RD and /WR determine the type of
communication between the Master MPU and the connected external peripheral.
The clock signal is generated by U430, a CMOS oscillator.
The remainder of the circuit consists of U495, an RS-232 buffer, and U395, an
RS-422 buffer. The receive outputs from these parts are NANDed (by U595C)
and input to the UART RXDA (Read External Data). This allows either an
RS-232 or RS-422 device to be connected to rear-panel J595, without having to
reconfigure the UART input. R393, R391, and R392 are a 120 W termination for
the RS-422 line which bias it to the inactive state when there is no connection to
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Circuit Theory