Serial I/O Lines in 8085 Microprocessor

The 8085 Microprocessor has Serial Input/Output lines consisting of two pins as follows:

1. Serial Output Data (SOD)
2. Serial Input Data (SID) 

They both are specially made for Input/Output which is further controlled by software. The transfer of data is controlled with the help of two instructions, i.e, SIM and RIM.

Now, let’s see both the pins one-by-one:

  1. Serial Output Data (SOD):
    The SIM Instruction should be initiated in order to output data in serial manner. This is done through the SOD Line.



    MVI A, 80H : Set D7 in the accumulator=1
    RAR : Set D6 = 1 and bring carry into D7
    SIM : Output D7

    In the above set of instructions, the serial output line is enabled by rotating 1 into bit position D6; the instruction SIM outputs the carry bit through bit position D7.
    It can be represented as:

    Figure – Interpretation by SIM Instruction

  2. Serial Input Data (SID):
    In SID, the RIM Instruction is initiated to input data in a serial manner. This is done through SID line.
    It can be represented as:

    Figure – Interpretation by RIM Instruction

In a software controlled Input/Output system, the SID and SOD lines eliminate the need of an input and output port respectively.

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