Functions of Control Pins:
• IO/M’ – It is a status signal which determines whether the address is for input-output or memory. When it is high (Logic 1) the address on the address bus is for input-output devices. When it is low (Logic 0) the address on the address bus is for the memory.
• RD’ – It is a signal to control READ operation. When it is low the selected memory or input-output device is read.
• WR’ – It is a signal to control WRITE operation. When it goes low the data on the data bus is written into the selected memory or I/O location.
3:8 Decoder A 3 to 8 line decoder IC 74138 is used. It has 3-inputs and 8-outputs. Three signals from the 8085 microprocessor namely read (RD’), write (WR’) and Input Output/Memory (IO/M’) are connected as input to the decoder. Four output signals are taken from output pins of decoder.
Control Signals The table list input conditions required or different inputs. • S1, S0 – These are status signals. They distinguish the various types of operations such as halt, reading, instructions fetching or writing. • RD’, WR’ – Read and write are two basic control signals for reading and writing operations respectively.
Functions of the control signals
• MEMR’ – It indicates Memory Read operation • MEMW’ – It indicates Memory Write operation • IOR’ – It indicates Input Read operation • IOW’ – It indicates Output Write operation
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