562
Digital Electronics
for microcomputer systems. It has a priority logic that resolves the peripheral requests and issues a
composite hold request to the CPU. It maintains the DMA cycle count for each channel and outputs a
control signal to notify the peripheral that the programmed number of DMA cycles is completed.
13.14.5 Programmable Communication Interface
Programmable communication interfaces (PCIs) are interface devices that are used for data
communication applications with microprocessors. They basically convert the data from the
microprocessor into a format acceptable for communication and also convert the incoming data into a
format understood by the microprocessor.
8251 is a PCI device designed for Intel’s 8085, 8086 and 8088 microprocessors and is used in serial
communication applications. It is a 28-pin chip available in DIP and PLCC packages. It is basically
a universal synchronous/asynchronous receiver/transmitter (USART) that accepts data characters from
the CPU in parallel format and then converts them into a continuous serial data stream for transmission.
Simultaneously, it can receive a serial data stream and convert it into parallel data characters for the
CPU. The USART will signal the CPU whenever it can accept a new character for transmission or
whenever it has received a character for the CPU.
13.14.6 Math Coprocessor
Math coprocessors are special-purpose processing units that assist the microprocessor in performing
certain mathematical operations. The arithmetic operations performed by the coprocessor are floatingpoint operations, trigonometric, logarithmic and exponential functions and so on. Examples include
Intel’s 8087, 80287, etc. The 8087 numeric coprocessor provides the instructions and data types needed
for high-performance numeric application, providing up to 100 times the performance of a CPU alone.
Another widely used math coprocessor is 80287. The 80287 numeric processor extension (NPX)
provides arithmetic instructions for a variety of numeric data types in 80286 systems. It also executes
numerous built-in transcendental functions (e.g. tangent and log functions).
13.14.7 Programmable Keyboard/Display Interface
Programmable keyboard/display interfaces are devices used for interfacing the keyboard and the display
to the microprocessor. The keyboard section of the device debounces the keyboard entries and provides
data to the microprocessor in the desired format. The display section converts the data output of the
microprocessor into the form desired by the display device in use.
8279 is a general-purpose programmable keyboard and display I/O interface device designed for
use with Intel microprocessors. The keyboard portion can provide a scanned interface to a 64-contact
key matrix. Keyboard entries are debounced and strobbed in eight-character FIFO. If more than eight
characters are entered, overrun status is set. Key entries set the interrupt output line to the CPU. The
display portion provides a scanned display interface for LED, incandescent and other popular display
technologies. Both numeric and alphanumeric segment displays may be used. The 8279 has a 16 × 8
display RAM.
13.14.8 Programmable CRT Controller
The programmable CRT controller is a device to interface CRT raster scan displays with the
microprocessor system. Its primary function is to refresh the display by buffering the information
Digital Electronics
for microcomputer systems. It has a priority logic that resolves the peripheral requests and issues a
composite hold request to the CPU. It maintains the DMA cycle count for each channel and outputs a
control signal to notify the peripheral that the programmed number of DMA cycles is completed.
13.14.5 Programmable Communication Interface
Programmable communication interfaces (PCIs) are interface devices that are used for data
communication applications with microprocessors. They basically convert the data from the
microprocessor into a format acceptable for communication and also convert the incoming data into a
format understood by the microprocessor.
8251 is a PCI device designed for Intel’s 8085, 8086 and 8088 microprocessors and is used in serial
communication applications. It is a 28-pin chip available in DIP and PLCC packages. It is basically
a universal synchronous/asynchronous receiver/transmitter (USART) that accepts data characters from
the CPU in parallel format and then converts them into a continuous serial data stream for transmission.
Simultaneously, it can receive a serial data stream and convert it into parallel data characters for the
CPU. The USART will signal the CPU whenever it can accept a new character for transmission or
whenever it has received a character for the CPU.
13.14.6 Math Coprocessor
Math coprocessors are special-purpose processing units that assist the microprocessor in performing
certain mathematical operations. The arithmetic operations performed by the coprocessor are floatingpoint operations, trigonometric, logarithmic and exponential functions and so on. Examples include
Intel’s 8087, 80287, etc. The 8087 numeric coprocessor provides the instructions and data types needed
for high-performance numeric application, providing up to 100 times the performance of a CPU alone.
Another widely used math coprocessor is 80287. The 80287 numeric processor extension (NPX)
provides arithmetic instructions for a variety of numeric data types in 80286 systems. It also executes
numerous built-in transcendental functions (e.g. tangent and log functions).
13.14.7 Programmable Keyboard/Display Interface
Programmable keyboard/display interfaces are devices used for interfacing the keyboard and the display
to the microprocessor. The keyboard section of the device debounces the keyboard entries and provides
data to the microprocessor in the desired format. The display section converts the data output of the
microprocessor into the form desired by the display device in use.
8279 is a general-purpose programmable keyboard and display I/O interface device designed for
use with Intel microprocessors. The keyboard portion can provide a scanned interface to a 64-contact
key matrix. Keyboard entries are debounced and strobbed in eight-character FIFO. If more than eight
characters are entered, overrun status is set. Key entries set the interrupt output line to the CPU. The
display portion provides a scanned display interface for LED, incandescent and other popular display
technologies. Both numeric and alphanumeric segment displays may be used. The 8279 has a 16 × 8
display RAM.
13.14.8 Programmable CRT Controller
The programmable CRT controller is a device to interface CRT raster scan displays with the
microprocessor system. Its primary function is to refresh the display by buffering the information