Q. Illustrate control and timing signals?

The requirement of I/O from different I/O devices by processor is quite unpredictable. In fact it relies on I/O needs of particular programs and generally doesn't follow any pattern. Because the I/O interfaces also shares memory and system bus for data input/output, control and timing are required to coordinate the flow of data to/from external devices from/to processor or memory. For illustration control of the transfer of data from a peripheral device to processor might include subsequent steps:

1. The processor enquires from I/O interface to check status of attached device. Status can be busy, ready or out of order.

2. I/O interface returns device status.

3. If device is operational and ready to transmit then processor requests transfer of data by a command that is a binary signal to I/O interface.

4. I/O interface attains a unit of data (for example 8 or 16 bits) from external device.

5. Data is transferred from I/O interface to processor.