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The SBS-1 System-Peripheral Interfaces

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Peripheral Interfaces: 

Programmable Peripheral Interfaces (8255) 
It is a general-purpose programmable I/O device with 24 pins. There are two 8255 on the board, so we can have 48 I/O parallel lines. These I/O lines are extended to two 40-pin connectors. 

Programmable Interval Timer (8253) 
It is a programmable counter/timer integrated circuit with 3 independent 16-bit counters, each separately can be programmed in any of five different modes. 

Programmable Communication Interface (8251) 
It is designed for programmable serial synchronous/asynchronous data communication. It accepts dada characters from the CPU in parallel format and then converts them into a continuous serial data stream for transmission. Simultaneously it can receive serial data stream and convert them into parallel data character for the CPU. The asynchronous baud rate required for this may be programmed in 8253 and the clock output from 8253 may be connected to TxC & RxC of 8251. System clock may also be used. 

Programmable Interrupt Controller (8259)
It handles up to eight vectored priority interrupts for CPU. It has several programmable modes permitting optimization for a variety of system requirements. The priority modes can be changed or reconfigured dynamically at any time during the main program. Each of the eight-interrupt requests is connected to the outputs of two TTL inverters (7400) and the corresponding inverter inputs are connected with eight PTH terminals. 

Programmable Keyboard/Display Interface (8279) 
It is a keyboard and display controller integrated circuit that handles the interface between 8085 and the keypad and LED display on the board. The 8279 refreshes the display from an internal memory while scanning the keyboard to detect any key pressing. The interrupt request output is connected to RST 5.5 of 8085. 

Eight Channel Analog to Digital Converter (ADC0809) 
It is 8-bit monolithic A/D converter and the conversion is done using successive approximation technique. It has eight channels for connecting eight different analog signals and they can be sampled regularly in time-multiplexed fashion. Its maximum frequency is 800 KHz. 

 Serial Data Interface Through SID/SOD of 8085
SID and SOD pins are available for computing VDU or TTY. Extra interface for 20mA current loop or RS232C is required.

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