titus-larsen-titus-1981-apple-interfacing

puts. You must be careful in your design that you do not overload
these signals by expecting them to drive more chip inputs than they
can. If you need additional power from these signals so that they can
drive more inputs on an interface card, you must buffer the signals
with appropriate buffer chips. Just keep in mind that the buffers will
need some additional power from the power supplies, and there is
not a great deal of "extra" power at the interface connectors. Thus,
you must balance your needs for signal buffering with the available
power. You could always use an external power supply to power
some of the interface cards, but this defeats the purpose behind putting
the interface circuits in the Apple enclosure in the first place.
AN INTERFACING EXAMPLE
Now that most of the useful interface signals have been described,
let's take a close look at a typical interface circuit that can be used
with the Apple computer. In many applications, it is necessary for
the computer to communicate with other devices. These may include
printers, controllers, remote 'data acquisition stations, and maybe
even other computers. In most cases, a form of serial communication
is used, so that long lengths of multiconductor cables are not required.
Most serial communication schemes use three or four wires,
so that the information that is to be exchanged is transmitted in serial
fashion, bit by bit, over the wire. One set of wires is used for transmitting,
and the other set is used for receiving. Such communication
is usually called asynchronous-serial communication, since there is
no common clock signal, or referene, that connects the two systems.
Most of the microprocessor chip manufacturers have developed
some type of communication chip for their family of microprocessors.
In fact, you can even "cross" families, so that a communication chip
that was developed for the 8080A family can be used with a 6502
processor. In fact, that is exactly what we plan to do in this example;
an 8251 universal synchronous/ asynchronous receiver-transmitter
chip will be interfaced to the Apple computer, right at the interface
slot. We will not provide you with a great deal of detail about the
operation of the USART chip, since this has been covered in detail
in TRS-80 Interfacing, Book 2 (Howard W. Sams & Co., Inc., Indianapolis,
IN 46268) . A magazine article covering the subject is also
available. See "Cross-Pollinating the Apple," Byte, April, 1979, p. 24.
Since the 8251 USART chip is a bus-compatible chip, it should not
be too difficult to interface the Apple. A pin configuration and block
diagram for the USART are provided in Fig. 7JLYou should be able
to recognize the data bus inputs, the RD and WR control inputs and
a chip select input, CS. Since the USART contains two sets of registers,
there must be some way of distinguishing between them. The
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