In diagrams is upper presented by parallel port data register to apply family IC 74LS, even if now have been there is IC ASIC (Application Specific Integrated Circuits). Port non-bidirectional is to apply 74LS374. While port bidirectional uses bit control 5 interfaced to 74LS374 so that the output can be killed. At the time of parallel port we are set like that, data which we write to port parallel data register will not up to DB25 because 74LS374 in condition of doesn't send.
If parallel port didn't support mode bidirectional, we can enter in maximum 9 bit by one time. To do this we can apply 5 port line input Status and 4 input (open collector) from control port. With scheme like at diagrams circuit to status register bit functioned as 4 first MSB bit. Control register becomes 4 last LSB bit. Pin Busy is MSB from altogether and LSB 4 first bit is Select. For mode nibble, will be read 8 bit by using port parallel without changing it into modus back, mean not requiring setting of bit to 5 from control register. This mode was introduced by Hewlett Packard before IBM introduces parallel port which bidirectional.
