titus-larsen-titus-1981-apple-interfacing

93427 OR EQUIV ROM
ADDRESS
BUS
1---- D7
..-----'''- 256 x 4 1---
ROM
1---- D4
TO DATA BUS
256 x 41---+-+-
ROM
D3
DO
R/W ---t :::o---+ ...._ _ __.
l/O SEL ---
Fig. 7-11. A 256-byte memory expansion circuit diagram.
oved the card to another interface slot, the addresses for the
USART would change, as noted in Table 7-3.
To use the USART interface, you must first initialize the chip with
me control information that is sent as two consecutive bytes to the
ontrol register. Don't worry about sending two bytes to the same
gister, the USART "knows" what to do with them. After the
SART has been initialized, you can use it to transmit and receive
synchronous-serial streams of information. The program shown in
xample 7-1 can be used to transmit an 8-bit byte of data, while the
rogram shown in Example 7-2 c be used to receive an 8-bit byte.
POKE 49328, TX
WAIT 49329, l
RETURN
WAIT 49329, 2
RX =
RETURN
Example 7-1. USART Transmitter Control Subroutine
Example 7-2. USART Receiver Control Subroutine
PEEK(49328)
e software checks the necessary flags so that the transmitter trans­
•ts its data only when it is ready, and the receiver only provides
ata when it has actually received some.
The main point here has been to develop a simple interface that
ses many of the Apple bus interface control signals, so that you can
e how they work. It is also nice to know that the interface example
ctually works, and that it can be used in some real applications. We
ope that you have seen how easy it is to develop an interface for
e Apple, based upon the concepts of port addressing, port control,
nd flags, that we introduced throughout the book.
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