Jonathan A. Titus

Microcomputer Pioneer

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Lesson 9: Bus and Input-Output

by David G. Larsen, Peter R. Rony, and Jonathan A. Titus

Microcomputer I/O Devices

IN THE DISCUSSION of the structure of a microcomputer, we described the various data paths in a microcomputer, including data input, data output, external device addressing, in and out function pulses, and interrupt signals. These are the vital lines of communication between the microcomputer and the "outside world," i.e., those signal lines that are necessary to interface the microprocessing unit (MPU) to the input/ output (I/O) devices that you would like to control.

Some useful definitions pertaining to I/O devices are given in the table. The traditional view of an I/O device is that it is somewhat large or complex. Card readers, magnetic tape units, cathode-ray tube displays, and Teletypes certainly fit such a description. However, a single integrated circuit chip, such as a latch, shift register, counter, or small memory, can also be considered an I/O device to a computer.
Read More: Adobe Acrobat PDF FileMicrocomputer IO Devices.pdf [33.0KB]

Microcomputer Output Instructions
IN A PREVIOUS COLUMN, we d.iscussed. different types of SImple input/output devices and provided a listing of general principles of interfacing that apply to a wide variety of computers. This month, we would like to explain how computer instructions, i.e., software, cause an I/O device to operate.

The I/O device that we shall choose for our discussion is the optically isolated solid-state ac relay. These relays can control any ac power device within the output current ratings of the relay. Typical solid-state relays are shown in Figures la-b. Such relays permit a single TTL output signal of logic 0 or logic I to control up to lOA of 220-v ac power, as is possible with the Hamlin model 7522 relay shown at the top middle part of Figure I a. Internally, each relay contains a light-emitting diode, a light-sensitive transistor, a power triac, and a transparent dielectric optical path that isolates the digital and power circuitry and can itself withstand a voltage difference of at least 1000 v.
Read More: Adobe Acrobat PDF FileMicrocomputer Output Instruction.pdf [99.9KB]

Accumulator I/O vs Memory I/O
W H EN DATA ARE transmitted between a microcomputer and an input/ output device, three actions must occur sim ultaneously:
1. The microcomputer must select the specific input/output device that will either receive or transmit eight bits of data. 2. The microcomputer must indicate to the specific input/output device when the bidirectional data bus is available for data transmission.
3. The data must be transmitted between the microcomputer and the input/output device in ·a very short period of time, typically of the order of microseconds.
In previous columns, we have discussed accumulator I/O, in which data are exchanged between the accumulator and an external I/O device. A significant disadvantage is associated with such an interfacing technique in that only a single origin or destination for data exists. In addition to the accumulator, a typical microprocessor chip, such as the Intel 8080, has a variety of internal general-purpose registers that can exchange information with memory. These registers include the B, C, D, H, and L registers, each of which is an 8-bit register. From a programming standpoint, it would be very useful to be able to exchange data between any of these registers and any external I/O device. This is the subject of this month's column.
Read More: Adobe Acrobat PDF FileAccumulator IO Vs Memory.pdf [48.7KB]