Watkins-Johnson Company WJ-8718A/MFP Instruction Supplement page 108

WJ-8718A/MFP
CIRCUIT DESCRIPTION
3.4.1. 7.4
Chip Enable Input (CS)
The CS signal must go low to enable the USART. The signal is generated by
decoder U4 (paragraph 3.4.1.2) when one of two USART addresses has been placed on the bus by
the microprocessor and latched by U2 (paragraph 3.4.1.1). The two addresses, hexadeCImal 1020
and 1021, differ only in the logic level of the LSB which determines the level of the C/D signal,
described in the following paragraph.
3.4.1. 7.5
Control/Data Signal
(Cm)
The
c/D
line is logic high when the USART address 1021 is used. The micro-
processor uses this address to indicate to the USART that the data to follow are mode or
com mand instructions. Following a USART reset, the first control word the mLc-roproc.essor
must issue is a mode instruction which defines the general operating characteristics of the
USART. The second control word is a command instruction which defines a status word used to
control the operation of the USART.
The USART address 1020 places a logic low level on the least significant bit (the
C/D line), and is used by the microprocessor to inform the USART that a read or write data
transmission is to follow.
3.4.1. 7.6
Read/Write Signals (RD/WR)
A
R'I')
or WR signal is pulled low by the microprocessor when a read or write
operation is to be initiated at the addressed device. The RD and WR signals have no effect on
the USART unless the CS (enable) signal is low. Refer to paragraph 3.2.3.4 and Figures 3-7 and
3-8 for information concerning read and write operations.
3.4.1.7.7
Data Input/Output Lines (DO Through D7)
The DO through D7 input/output ports of Ul are connected to the lower-order
address/data lines on the microprocessor data bus. Data are loaded on the bus by the
microprocessor and enter the D ports of Ul during a write operation. During a read operation,
data are loaded on the bus from the D ports of the USART.
3.4.1.7.8
Receiver Ready (Rx RDY)
The Rx RDY signal, generated by the USART, goes high to inform the micro-
processor that an entire character has been received from the remote controller and is ready to
be fetched by the microprocessor. The microprocessor tr·eats the Rx RDY signal as an interrupt
request, halts its main program, and branches to a subroutine to execute the instructions there.
During the' course of these instructions, the data in the USART are read and the Rx RDY
automatically resets (goes low).
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