--- title: "8255A card at F3H with keyboard and display" subject: "Instrumentation II" chapter: 2 key: "semester_5/instrumentation_ii:ppi-f3h-keyboard-display" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-f3h-keyboard-display site: BCT Engineering --- # 8255A card at F3H with keyboard and display Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System - Asked in: 2076 Asoj, 1+3+2+2+2 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ppi-f3h-keyboard-display - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2 ## Question An 8255A PPI card is connected to 8085 microprocessor has system as shown in figure below in which control word is stored in address of F3H. (a) What are the addresses captured by 8255A PPI card? (b) Draw the minimum interfacing circuit. (c) Write down the control word to initialize the 8255A PPI card. (d) Write down the status word format for 8255A PPI card for the system. (e) Write down BSR control word to initialize port A interrupt request. [Figure: 82C55A. Port A (PA0-PA7) takes inputs R0-R5, SHIFT and CONTROL from a fully decoded keyboard. PC4 is STROBE (input from the keyboard) and PC5 is ACK (output to the keyboard). PC3 is an INTERRUPT REQUEST line. Port B (PB0-PB7) drives B0-B5, BACKSPACE and CLEAR of a Burroughs self-scan display. PC1 is DATA READY (output), PC2 is ACK (input), PC6 is BLANKING (output), PC7 is CANCEL WORD (output). PC0 is an INTERRUPT REQUEST line.] ## Short answer Addresses: A=F0H, B=F1H, C=F2H, Control=F3H. Control word B4H sets port A mode 1 input for keyboard and port B mode 1 output for display. Status word at port C includes IBF_A, INTR_A, OBF_B'. BSR word 09H enables INTE_A for keyboard interrupt. ## Answer #### (a) Addresses The control register is F3H (1111 0011), so A1 A0 = 11 and the base is F0H (1111 0000). The card captures **F0H-F3H**: Port A = F0H, Port B = F1H, Port C = F2H, Control = F3H. Decoding: A7-A2 = 111100. #### (b) Minimum interfacing circuit (8085, I/O mapped) ```text A7,A6,A5,A4 --------+ A3,A2 --[NOT]-------+--[NAND]--> CS' (low for F0H-F3H) A1 -> A1 A0 -> A0 IOR' -> RD' IOW' -> WR' AD0-AD7 -> D0-D7 RESET OUT -> RESET PA0-PA7 <- keyboard (R0-R5, SHIFT, CONTROL) PC4 <- STROBE PC5 -> ACK PC3 INTR_A -> 8085 RST 5.5 PB0-PB7 -> display (B0-B5, BACKSPACE, CLEAR) PC1 -> DATA READY PC2 <- ACK PC0 INTR_B -> RST 6.5 PC6 -> BLANKING PC7 -> CANCEL WORD ``` #### (c) Control word Port A: mode 1 input (keyboard with STROBE, ACK, INTR). Port B: mode 1 output (display with DATA READY, ACK, INTR). PC6, PC7 output. D7 = 1; D6D5 = 01; D4 = 1; D3 = 0 (PC7, PC6 output); D2 = 1; D1 = 0; D0 = 0 = 1011 0100 = **B4H**. #### (d) Status word (read from port C) | D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 | |---|---|---|---|---|---|---|---| | CANCEL WORD (I/O) | BLANKING (I/O) | IBF_A | INTE_A | INTR_A | INTE_B | OBF_B' | INTR_B | #### (e) BSR word for port A interrupt request INTE_A is PC4 (input mode). D7 = 0, D3D2D1 = 100, D0 = 1: 0000 1001 = **09H**. Sending it to F3H (`MVI A,09H` and `OUT F3H`) enables INTR_A. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The control register is at F3H. Which addresses does the card capture? A. F0H-F3H B. F3H-F6H C. F0H-FFH D. 00H-03H Answer: A. Control = F3H means A1A0 = 11 so base is F0H; ports are F0H, F1H, F2H and F3H with A7-A2 = 111100. (Key point: Address range) 2. Port A mode 1 input, port B mode 1 output, PC6/PC7 output. What control word? A. A4H B. B4H C. B6H D. 9CH Answer: B. 1011 0100: D7=1, mode 01, A input, PC upper output, B mode 1, B output, PC lower D0=0 = B4H. (Key point: Control word) 3. In the status word read from port C (A input, B output), which bit is IBF_A? A. D3 B. D4 C. D5 D. D1 Answer: C. The order is D7 I/O, D6 I/O, D5 IBF_A, D4 INTE_A, D3 INTR_A, D2 INTE_B, D1 OBF_B', D0 INTR_B. (Key point: Status word bits) 4. Which BSR word enables the port A (input) interrupt request? A. 0DH B. 05H C. 09H D. 08H Answer: C. INTE_A is PC4 for input mode; set PC4 = 0000 1001 = 09H. (Key point: BSR for INTR_A) ## Related in Instrumentation II - [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 - [8255A at 80H: addresses, mode 0/1 words, BSR](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-80h-addresses-control) — 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 - [Interfacing ADC0808 with 8085 using 8255A](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc0808-8255-interface) — Instrumentation II, chapter 4 - [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 - [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