--- title: "Gas chamber STP monitoring with 8255 and ADCs" subject: "Instrumentation II" chapter: 4 key: "semester_5/instrumentation_ii:gas-chamber-stp" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/gas-chamber-stp site: BCT Engineering --- # Gas chamber STP monitoring with 8255 and ADCs Instrumentation II · Chapter 4: Interfacing A/D And D/A Converters - Asked in: 2079 Baisakh, 8 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/gas-chamber-stp - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-4 ## Question Consider yourself as a fluid dynamic engineer, who has been assigned the task of designing a hardware circuit that keeps a gas chamber under standard temperature and pressure (STP) conditions. The circuit should be an MBI system, an 8255 PPI, two 10-bit ADCs and appropriate temperature and pressure sensors that constantly monitor the pressure and temperature inside the gas chamber. An alarm LED should be lighted to notify the operator when either the temperature exceeds zero degree Celsius or the pressure exceeds hundred Kilopascals. Calibrate your temperature and pressure sensors accordingly. Sketch your design, show the necessary control words, and draw an appropriate flowchart to show the logic of your software algorithm. ## Short answer The MBI system uses an 8255 PPI (control word 93H) with two 10-bit ADCs sharing ports A and B. Start conversion via PC4, read EOC on PC0/1, and enable output on PC5/6. Alarm LED (PC7) lights if temperature >0°C (reading >0156H) or pressure >100 kPa (>0201H). ## Answer #### Assumptions STP: 0 deg C and 100 kPa (IUPAC). The alarm LED turns on when the temperature is above 0 deg C or the pressure is above 100 kPa. The system uses an 8085 with the 8255 I/O mapped at **80H (A), 81H (B), 82H (C), 83H (control)**. Two identical 10-bit ADCs with tri-state outputs share the data lines. #### Sensor calibration - **Temperature:** the range -20 to +40 deg C (60 deg C span) is mapped to 0-5 V by the conditioner (RTD/LM35 + offset + gain): V = 5 x (T + 20)/60. At 0 deg C: 1.667 V = (1.667/5) x 1024 = 341 = **0155H**. - **Pressure:** the range 0-200 kPa is mapped to 0-5 V (pressure transducer + amplifier). At 100 kPa: 2.5 V = 512 = **0200H**. - Resolution: temperature 60/1024 = 0.059 deg C per count; pressure 200/1024 = 0.195 kPa per count. - The alarm is given when a reading is above its threshold: compare with 0156H (T) and 0201H (P). #### Design ```text Temp sensor -> cond. -> ADC-T (10 bit) --+ D0-D7 --> PA0-PA7 Press sensor-> cond. -> ADC-P (10 bit) --+ D8,D9 --> PB0,PB1 PC4 -> START of both ADCs PC0 <- EOC of ADC-T, PC1 <- EOC of ADC-P PC5 -> OE of ADC-T PC6 -> OE of ADC-P PC7 -> transistor -> ALARM LED 8085 AD0-7 -> D0-7 of 8255; IOR', IOW', A0,A1, CS' (A7-A2 = 100000) ``` Data from the selected ADC (only one OE is active at a time) appear on port A (low 8 bits) and PB0-PB1 (high 2 bits). #### Control word Port A input, port B input, port C upper output (PC4-PC7), port C lower input (PC0-PC3), mode 0: 1001 0011 = **93H**. The bit set/reset (BSR) control words written to 83H are: PC4 (START) = 09H set / 08H reset; PC5 (OE-T) = 0BH / 0AH; PC6 (OE-P) = 0DH / 0CH; PC7 (LED) = 0FH / 0EH. BSR is used so that changing one control line never disturbs the LED bit. #### Algorithm and flowchart ```text START -> Init 8255 (93H), LED off -> Start conversion of both ADCs (PC4 pulse) -> Wait until EOC-T and EOC-P are both 1 -> Enable OE-T, read 10-bit T, disable OE-T -> T > 0156H ? --yes--> LED ON (alarm) | no -> Enable OE-P, read 10-bit P, disable OE-P -> P > 0201H ? --yes--> LED ON | no -> LED OFF -> repeat ``` #### Program ```text MVI A,93H OUT 83H ; mode word MVI A,0EH OUT 83H ; LED (PC7) off LOOP: MVI A,09H OUT 83H ; START = 1 (PC4) MVI A,08H OUT 83H ; START = 0 WAIT: IN 82H ANI 03H ; EOC-T and EOC-P CPI 03H JNZ WAIT MVI A,0BH OUT 83H ; OE-T = 1 (PC5) IN 80H MOV L,A IN 81H ANI 03H MOV H,A ; HL = 10-bit temperature MVI A,0AH OUT 83H ; OE-T = 0 MOV A,L SUI 56H MOV A,H SBI 01H ; HL - 0156H JNC ALARM ; no borrow : T > 0 deg C MVI A,0DH OUT 83H ; OE-P = 1 (PC6) IN 80H MOV L,A IN 81H ANI 03H MOV H,A ; HL = pressure MVI A,0CH OUT 83H ; OE-P = 0 MOV A,L SUI 01H MOV A,H SBI 02H ; HL - 0201H JNC ALARM ; no borrow : P > 100 kPa MVI A,0EH OUT 83H ; LED off JMP LOOP ALARM: MVI A,0FH OUT 83H ; PC7 = 1 : alarm LED on JMP LOOP ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. With -20 to +40 deg C mapped to 0-5 V on a 10-bit ADC, what code corresponds to 0 deg C? A. 0155H B. 0200H C. 03FFH D. 0100H Answer: A. V = 5 x 20/60 = 1.667 V, and 1.667/5 x 1024 = 341 = 0155H. (Key point: Temperature threshold) 2. With 0-200 kPa mapped to 0-5 V on a 10-bit ADC, which code corresponds to 100 kPa? A. 0155H B. 0100H C. 0200H D. 03FFH Answer: C. 100 kPa is half scale: 2.5 V = 512 = 0200H. (Key point: Pressure threshold) 3. Ports A and B input, upper C output, lower C input, mode 0 gives which control word? A. 90H B. 93H C. 99H D. 83H Answer: B. 1001 0011: A in, C upper out, B in, C lower in = 93H. (Key point: Control word) 4. Why are the OE signals of the two ADCs enabled one at a time? A. To save power only B. To start conversion C. To set the alarm D. The ADC outputs share the same data lines, so only one tri-state output may drive them Answer: D. Two active outputs would conflict on the shared lines; one OE is enabled while reading. 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