--- title: "Keyboard and printer interface in 8255 mode 1" subject: "Instrumentation II" chapter: 2 key: "semester_5/instrumentation_ii:keyboard-printer-mode1" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/keyboard-printer-mode1 site: BCT Engineering --- # Keyboard and printer interface in 8255 mode 1 Instrumentation II · Chapter 2: Parallel Interfacing With Microprocessor Based System - Asked in: 2071 Chaitra, 6 marks - Video: [Interfacing 8255 With Printer(हिन्दी )](https://www.youtube.com/watch?v=MYEsuzht7lg) — LEARN AND GROW - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/keyboard-printer-mode1 - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-2 ## Question Interface a keyboard and a printer in mode 1. Port A is designed as input for keyboard with interrupt I/O port B is designed as output for printer with status check I/O. Draw the mapping circuit and write the control word and address map. ## Short answer Port A mode 1 input (keyboard, interrupt-driven), port B mode 1 output (printer, status check). Control word B4H; enable keyboard interrupt with BSR 09H. Addresses: A=80H, B=81H, C=82H, control=83H (I/O mapped). Printer status checked via OBF_B' (PC1). ## Answer #### Assumptions 8085, 8255 I/O mapped at 80H-83H. Keyboard encoder on port A (interrupt driven, mode 1 input); printer on port B (status check, mode 1 output). #### Signals (mode 1) | Port C pin | Signal | Use | |---|---|---| | PC4 | STB_A' | Strobe from keyboard | | PC5 | IBF_A | Acknowledge to keyboard | | PC3 | INTR_A | Interrupt to 8085 (RST 6.5) | | PC1 | OBF_B' | Data ready to printer | | PC2 | ACK_B' | Acknowledge from printer | | PC0 | INTR_B | Not used (status read instead) | #### Mapping circuit ```text A7 ------------+ +--------------+ A6..A2 [NOR]---+--[AND]--[NOT]->CS' | IO/M(=1) ------+ | 8255A | A1, A0 ------------------------> A1, A0 | IOR' -> RD' IOW' -> WR' RESET OUT -> RESET | AD0-AD7 -> D0-D7 | PA0-PA7 <- keyboard PB0-PB7 -> printer | PC4 <- STB' PC5 -> IBF PC3 -> RST 6.5 | PC1 -> printer DATA READY PC2 <- printer ACK + ``` CS' is low when A7 = 1 and A6-A2 = 00000. #### Address map | A1 A0 | Port | Address | |---|---|---| | 00 | Port A (keyboard) | 80H | | 01 | Port B (printer) | 81H | | 10 | Port C (status) | 82H | | 11 | Control | 83H | #### Control word D7 = 1 (mode set), D6D5 = 01 (port A mode 1), D4 = 1 (A input), D3 = 0 (PC6, PC7 output, unused), D2 = 1 (B mode 1), D1 = 0 (B output), D0 = 0 = 1011 0100 = **B4H**. INTE_A enable (keyboard interrupt): BSR 0000 1001 = **09H** (set PC4). Printer status is checked by reading port C bit D1 (OBF_B' = 1 means printer took the previous byte). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which signal tells the keyboard that the 8255 input buffer is full? A. IBF_A on PC5 B. STB_A' on PC4 C. OBF_B' on PC1 D. INTR_A on PC3 Answer: A. IBF acknowledges the data to the keyboard; STB' is from the keyboard. (Key point: Mode 1 roles) 2. How is the printer on port B handled? A. By interrupt on RST 6.5 B. By DMA C. By ignoring handshake D. By status check, reading OBF_B' bit of port C Answer: D. INTR_B is unused; the program polls port C to see if the printer took the previous byte. (Key point: Interrupt vs status) 3. What is the control word for A mode 1 input, B mode 1 output? A. A6H B. B0H C. 94H D. B4H Answer: D. 1 01 1 0 1 0 0 = 1011 0100 = B4H. (Key point: Control word) 4. If CS' is low for A7=1 and A6-A2 = 0, what is the control register address? A. 80H B. 81H C. 83H D. 82H Answer: C. A1 A0 = 11 selects control, so 80H + 3 = 83H. (Key point: Address map) ## Related in Instrumentation II - [8255 mode 1 keyboard and printer with decoding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-keyboard-printer-mode1-fig) — 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 - [PCI bus; 8255 printer interface at 5500H](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Apci-printer-5500h) — 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 mode 0 DIP switches and LEDs](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-mode0-dip-leds) — 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 input and output ports](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-input-output-port-block-decoder) — Microprocessor, chapter 4 - [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 - [Address decoding for input port at 8050H](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-input-port-lda-8050h) — Microprocessor, chapter 4