The 8251 BlockDiagramin Microprocessor has a set of control inputs and output’s that can be used to simplify the interface to almost any modem. It provides control circuitry for the generation of RTS and DTR and the reception of CTS and DSR. In addition, a general purpose inverted output and a general purpose input are provided. 8251 Block Diagram in Microprocessor | Control Word of 8251 Was this helpful? Intel 8255A - Pin Description - Tutorialspoint Intel 8255A - Pin Description - Let us first take a look at the pin diagram of Intel 8255A − When the signal is low, the microprocessor reads the data from the selected I/O port of the 8255. A 0 and A 1. These input signals work with RD, WR, and one of the control signal. Following is the table showing their various signals with their result.