--- title: "8255A at 80H: addresses, mode 0/1 words, BSR" subject: "Instrumentation II" chapter: 2 key: "semester_5/instrumentation_ii:ppi-80h-addresses-control" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-80h-addresses-control site: BCT Engineering --- # 8255A at 80H: addresses, mode 0/1 words, BSR Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System - Asked in: 2068 Baisakh (old course), 5 marks - Video: [Control Word and Modes of 8255: BSR Mode, IO Modes, and Detailed Mode 0, 1, & 2](https://www.youtube.com/watch?v=la-EoWUcSp4) — Engineering Funda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-80h-addresses-control - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2 ## Question An 8255A PPI connected to 8085 has a system base address of 80H. (i) What are the addresses assigned for Port A, Port B, Port C and control register? (ii) Write down the control word to initialize this card as follows: Port A mode 0 output, Port B handshake input, Port C upper output and remaining pin of Port C lower input. (iii) For above case, write down bit set/reset control word to initialize Port B interrupt request. ## Short answer Port A 80H, B 81H, C 82H, control 83H. Control word for PA mode 0 out, PB mode 1 in, PC upper out, PC lower in is 87H. To enable Port B interrupt, set PC2 using BSR control word 05H (PC2 set, INTE_B=1). ## Answer #### (i) Addresses Base address 80H, so A1 A0 select the register: Port A = **80H**, Port B = **81H**, Port C = **82H**, Control register = **83H**. #### (ii) Control word - Port A mode 0 output: D6D5 = 00, D4 = 0. - Port B handshake input: mode 1 (D2 = 1), input (D1 = 1). - Port C upper output: D3 = 0. - Remaining pins of port C lower input: D0 = 1. (In mode 1, PC0-PC2 are used for handshake, and the remaining pin PC3 is set as input.) - D7 = 1 (mode set). ```text D7 D6 D5 D4 D3 D2 D1 D0 1 0 0 0 0 1 1 1 = 87H ``` Control word = **87H**. #### (iii) BSR control word for Port B interrupt In mode 1 the interrupt enable of port B (INTE_B) is bit PC2 for both input and output. To set PC2: D7 = 0 (BSR), D6-D4 = 000, D3D2D1 = 010 (bit 2), D0 = 1 (set) = 0000 0101 = **05H**. ```text MVI A,87H OUT 83H MVI A,05H OUT 83H ; INTE_B = 1 ``` Now INTR_B (PC0) is raised when data arrives at port B. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For an 8255 at base 80H, which are the addresses of Port A, B, C and control? A. 80H, 81H, 82H, 83H B. 80H, 82H, 84H, 86H C. 83H, 82H, 81H, 80H D. 00H, 01H, 02H, 03H Answer: A. A1A0 = 00,01,10,11 select ports A, B, C and control starting from 80H. (Key point: Port addresses) 2. Port A mode 0 out, port B mode 1 in, PC upper out, PC lower in. Control word? A. 87H B. 83H C. 8FH D. 97H Answer: A. 1000 0111: D7=1, A mode 0 output (D6D5D4 = 000), D3=0, D2=1, D1=1, D0=1 = 87H. (Key point: Control word) 3. Which bit is INTE_B in mode 1, and what BSR word sets it? A. PC4, 09H B. PC2, 05H C. PC6, 0DH D. PC0, 01H Answer: B. INTE_B is PC2 for both input and output; bit 2 set is 0000 0101 = 05H. (Key point: BSR for INTE_B) ## Related in Instrumentation II - [8255A card at F3H with keyboard and display](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-f3h-keyboard-display) — Instrumentation II, chapter 2 - [ADC at 80H with 8255A: display on port B LEDs](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc-8255-80h-leds) — Instrumentation II, chapter 2 - [8255A at B0H: add port A and B, show on port C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-b0h-addition) — Instrumentation II, chapter 2 - [Keyboard and printer with 8255A at 4044H](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Akbd-printer-ppi-4044h) — Instrumentation II, chapter 2 - [Centronics printer with 8255A in mode 1](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acentronics-printer-8255-mode1) — Instrumentation II, chapter 2 - [8255 at A0A0H: interface and control words](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-a0a0h-control-words) — Instrumentation II, chapter 2 ## Related in other subjects - [Address decoding for 8 LEDs output port](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-output-port-8-leds) — Microprocessor, chapter 4 - [8086 ALP: words on lines and word count](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aalp-8086-words-lines-count) — Microprocessor, chapter 3