Programmed I/O Data Transfer scheme of 8085 microprocessor
Programmed I/O Data Transfer scheme of 8085 microprocessor is a simple parallel data transfer scheme. This method of data transfer is generally used in the simple microprocessor systems. It is obvious that where speed is unimportant. This method uses instructions to get the data into or out of the microprocessor. Programmed I/O Data Transfer scheme of 8085 microprocessor can be work on synchronous or asynchronous mode. The data transfer can be synchronous or asynchronous it completely depends upon the type and the speed of the I/O devices.
Synchronous type of data transfer
Synchronous type of data transfer can be used when the speed of the I/O devices matches with the speed of the 8085 microprocessor. So for synchronization established between I/O device and microprocessor we need common clock pulse. This common clock pulse synchronizes the microprocessor and the I/O devices. Synchronous type of data transfer scheme because of the matching of the speed, the microprocessor does not have to wait for the availability of the data. The microprocessor immediately sends data for the transfer as soon as the microprocessor issues a signal.
The asynchronous data transfer
The asynchronous data transfer method is used when the speed of the I/O devices is slower than the speed of the microprocessor. Because of the mismatch of the speed, the internal timing of the I/O device is independent from the microprocessor. That is why two units are said to be asynchronous to each other. The asynchronous data transfer is normally implemented using ‘handshaking’ mode. Now question is what is handshaking mode? In the handshaking mode some signals are exchanged between the I/O device and microprocessor before the data transfer takes place.
By this handshaking the microprocessor has to check the status to the input/output device. Now if the device is ready for the data transfer or not.
- First step of microprocessor is initiates the I/O device to get ready.
- Then status of the I/O device is continuously checked by the microprocessor.
- This process remain continues until the I/O device becomes ready.
- After that microprocessor sends instructions to transfer the data.
Flow chart for this mode of data transfer is shown above.
Now form this bellow figure, the microprocessor sends a ready signal to I/O device. When the device is ready to accept the data, the I/O device sends an ‘ACK’ (Acknowledge) signal to microprocessor. By sending ACK, it indicating that the I/O device has acknowledged the ‘Ready’ signal. Now finally it is ready for the transfer of data.
Again in bellow figure shows the asynchronous handshaking process to transfer the data from the I/O device to microprocessor. In this case I/O device issues the ready signal to microprocessor indicating that I/O device is ready to send the data to microprocessor. In response to this signal, valid data signal is sent by the microprocessor to I/O device and then the valid data is put on the data bus for the transfer.