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Keyboard and printer with 8255A at 4044H

Short answer

Base 4044H: port A=4044H, B=4045H, C=4046H, control=4047H. Control word A6H configures port A mode 1 output (printer) and port B mode 1 input (keyboard). Enable port A interrupt with BSR 0DH (set PC6, INTE_A).

In a microprocessor based system, an 8255A PPI card is used to interface a keyboard and a printer to the processor. The 8255A PPI is interfaced with the 8085 microprocessor in the system such that the base address of 8255 A PPI is 4044 H. (a) What are the addresses captured by the card? (b) Draw the complete interfacing circuit of 8255A PPI with 8085 for the given system. (c) If the printer is interfaced to port A and the keyboard is interfaced to port B of the PPI generate the control word to initialize the 8255A PPI with proper explanations. Both printer and keyboard use 8-bit parallel data transfer with handshaking. (d) Derive the control word to enable interrupt request to the microprocessor by port A of 8255A PPI in above system, with proper explanations.

(a) Addresses

Base 4044H = 0100 0000 0100 0100. The 8255 uses A1 A0 for the register, so A15-A2 = 01 0000 0001 0001 are decoded: Port A = 4044H, Port B = 4045H, Port C = 4046H, Control = 4047H.

(b) Interfacing circuit (memory mapped)

 8085                                8255A
 AD0-AD7 -> 74LS373 (ALE) -> A0,A1 ---> A0, A1
 AD0-AD7 ------------------------------> D0-D7
 RD' ---------------------------------> RD'
 WR' ---------------------------------> WR'
 RESET OUT ---------------------------> RESET
 A15 [NOT], A14, A13 [NOT], A12 [NOT], A11 [NOT], A10 [NOT],
 A9 [NOT], A8 [NOT], A7 [NOT], A6, A5 [NOT], A4 [NOT], A3 [NOT], A2
 IO/M ---------------------> all through gates ---> CS' (low when 4044H-4047H, IO/M = 0)
 PA0-PA7 <-> printer data    PB0-PB7 <- keyboard data
 PC7 (OBF_A') -> printer STROBE / DATA READY    PC6 (ACK_A') <- printer ACK
 PC3 (INTR_A) -> RST 5.5 of 8085
 PC2 (STB_B') <- keyboard STROBE    PC1 (IBF_B) -> keyboard ACK
 PC0 (INTR_B) -> RST 6.5 of 8085 (optional)

(c) Control word

Printer on port A: output with handshake = mode 1 output. Keyboard on port B: input with handshake = mode 1 input. D7 = 1 (mode set); D6D5 = 01 (A mode 1); D4 = 0 (A output); D3 = 0 (PC4, PC5 free, output); D2 = 1 (B mode 1); D1 = 1 (B input); D0 = 0 (PC0-PC2 are used by the group B handshake, so this bit has no effect) = 1010 0110 = A6H.

(d) Control word to enable interrupt from port A

In mode 1 output the interrupt enable INTE_A is bit PC6. BSR word: D7 = 0, D3D2D1 = 110 (PC6), D0 = 1 (set) = 0000 1101 = 0DH. When INTE_A = 1, INTR_A (PC3) goes high after the printer sends ACK' (OBF_A' high), and the 8085 can be interrupted to send the next character.

✓ Answer checked by Claude Sonnet 5.5

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