--- title: "ADC with 8085 using 8255: fan and heater control" subject: "Instrumentation II" chapter: 4 key: "semester_5/instrumentation_ii:adc-8255-fan-heater" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/adc-8255-fan-heater site: BCT Engineering --- # ADC with 8085 using 8255: fan and heater control Instrumentation II · Chapter 4: Interfacing A/D And D/A Converters - Asked in: 2068 Chaitra, 8 marks - Video: [Temperature control using microprocessor](https://www.youtube.com/watch?v=Nxe8D3UHaTg) — Engineering Basics - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/adc-8255-fan-heater - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-4 ## Question You have to interface ADC with 8085 using 8255A ports. Interface a fan and a heater using opto couplers to derive the I/O devices. If the temperature is less than 10°C, turn on the heater and if the temperature is higher than 35°C, turn on the fan. Use port A of 8255 for transferring digital data output of ADC to the CPU and part C for control signals. Assume that an analog input is present at second input of the multiplexer and a clock input of suitable frequency is available for ADC. Also write an appropriate flow chart and algorithm to facilitate your design. Draw the diagram of your design. ## Short answer 8255 at 80H–83H, control word 91H: port A input (ADC data), port B output (PB0 heater, PB1 fan via optocouplers), PC4 START/ALE, PC5 OE, PC6 ADD A=1, PC0 EOC. ADC channel IN1 (001). Heater on for code <1AH (10°C), fan on for code >5AH (35°C). ## Answer #### Assumptions 8085 I/O mapped; 8255 at **80H (A), 81H (B), 82H (C), 83H (control)**. ADC0808 (8 channels, 8 bit), analog temperature signal at **channel IN1** (ADD C B A = 001). Sensor LM35 (10 mV/deg C) with amplifier gain 5 so that 0-100 deg C = 0-5 V; 1 count = 0.39 deg C. Heater and fan are driven through opto-couplers (MOC3021 + triac or transistor + relay) from port B. #### Threshold codes 10 deg C -> 10/100 x 255 = 25.5, take **1AH (26)**. 35 deg C -> 35/100 x 255 = 89.25, take **5AH (90)**. #### Control word and port use - Port A: input (ADC data). Port B: output (PB0 = heater, PB1 = fan, via opto-couplers). Port C upper output: PC4 = START/ALE, PC5 = OE, PC6 = ADD A (high). Port C lower input: PC0 = EOC. - Control word: 1 00 1 0 0 0 1 = 1001 0001 = **91H**. #### Diagram ```text LM35 -> amp -> IN1 --+ | ADC0808 -- D0-D7 --> PA0-PA7 CLK (640 kHz) -------+ START,ALE <-- PC4 OE <------- PC5 ADD A <---- PC6 (=1), ADD B, C = GND EOC ------> PC0 8085 <-> 8255 (AD0-7, IOR', IOW', A0, A1, CS') PB0 -> [Opto-coupler] -> Heater driver -> Heater PB1 -> [Opto-coupler] -> Fan driver -> Fan ``` #### Algorithm 1. Initialise the 8255 (91H); turn heater and fan off. 2. Select channel 1, give START/ALE pulse. 3. Wait for EOC = 1. 4. Enable OE, read port A (temperature code), disable OE. 5. If code < 1AH: heater ON, fan OFF. 6. Else if code > 5AH: fan ON, heater OFF. 7. Else both OFF. 8. Repeat from step 2. ```text START -> Init -> Start conversion -> EOC? -no-> wait -> Read data -> T < 10 ? -yes-> Heater ON -no-> T > 35 ? -yes-> Fan ON -no-> both OFF -> back to Start conversion ``` #### Program ```text MVI A,91H OUT 83H MVI A,00H OUT 81H ; heater, fan off LOOP: MVI A,40H ; ADD A=1, START=0 OUT 82H MVI A,50H ; START/ALE = 1 OUT 82H MVI A,40H ; START = 0 : conversion begins OUT 82H WAIT: IN 82H ANI 01H ; EOC (PC0) JZ WAIT MVI A,60H ; OE = 1 OUT 82H IN 80H ; read temperature MOV B,A MVI A,40H ; OE = 0 OUT 82H MOV A,B CPI 1AH JC HEAT ; below 10 C CPI 5BH JNC FAN ; above 35 C MVI A,00H ; between: both off OUT 81H JMP LOOP HEAT: MVI A,01H OUT 81H JMP LOOP FAN: MVI A,02H OUT 81H JMP LOOP ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. With 0-100 deg C mapped to 0-255 counts, what code corresponds to 10 deg C? A. 5AH B. 0AH C. 1AH D. FFH Answer: C. 10/100 x 255 = 25.5, taken as 26 = 1AH. (Key point: Threshold code) 2. Port A input, port B output, upper C output, lower C input (EOC on PC0), mode 0 gives which word? A. 91H B. 99H C. 81H D. 93H Answer: A. 1001 0001: A in, C upper out, B out, C lower in, giving 91H. (Key point: Control word) 3. What does the program do after reading the temperature code? A. Fan on if code below 1AH B. Heater on if code below 1AH, fan on if above 5AH, otherwise both off C. Both on if above 5AH D. Heater on if above 5AH Answer: B. Below 10 deg C turns the heater on, above 35 deg C the fan, and in between both are off. (Key point: Decision logic) 4. Why are opto-couplers used between port B and the heater and fan? A. To amplify the digital data B. To increase clock speed C. To convert analog to digital D. To isolate the low-voltage 8085 side from the high-power loads Answer: D. Opto-couplers give electrical isolation so the loads cannot damage the logic and noise is not fed back. (Key point: Opto-coupler) ## Related in Instrumentation II - [8-channel ADC interface with 8255 and 8085](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc8-8255-problem) — Instrumentation II, chapter 4 - [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 - [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 - [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 - [Interfacing ADC0808 with 8085 using 8255A](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc0808-8255-interface) — Instrumentation II, chapter 4 ## Related in other subjects - [3-phase IM principle; speed less than sync](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Athree-phase-im-principle-slip) — Electrical Machine, chapter 5 - [Detection gain of DSB-AM is less than 1](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adetection-gain-dsbam-less-than-1) — Communication Systems, chapter 9 - [When terminal voltage is less or more than emf](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-less-or-more) — Electric Machines II, chapter 1 - [8085 ALP: transfer data with more than 3 ones](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aalp-8085-ones-greater-than-3) — Microprocessor, chapter 2