Mode 1—strobed I/O

We call mode 1 as the strobed Input Output or handshake Input Output. We use this mode when the data is supplied by the input device to the microprocessor at irregular interval of time. A port which is functioned to program in mode uses three handshake signals. These handshake signals are provided by Port C. Only port A and B works in mode 1. The pins PC₂, PC₁, and PC₀ provides handshake signals for port B when we configure it for Input port or Output port. Moreover, the pins PC₇, PC₆, and PC₃ provides handshake signals for port A. The point to be noted that PC₃ pin is a handshake line for Port A in the input terminal and also performing the output operations. If port A and B both works in mode 1 the remaining two pins of port C are used for performing the simple Input Output operations in mode 0. If either of Port A or B works in mode 1 then the rest 5 pins of port C are completely free.

Example 1: Configure Port A as strobed input port, Port B as strobed output port, and PC7, PC6 as output lines.

The required mode definition control word is shown in the figure below −

It is to be noted that the LS bit is a don't care bit. This is because all the four lines of Port C

lower are used for handshaking purposes some of which will be input pins and some others outputs. The 8255 will automatically configure these handshake pins, and so it is unimportant whether this LS bit is a 0 or a 1.

Bit 3 must be a 0 to indicate that PC₇ and PC₆ are output lines. In this case it does not mean that

the entire Port C upper is output, as PC₅ and PC₄ are used for handshaking purposes. The instructions to achieve this requirement assuming the chip select circuit are as follows.

MVI A, B4H ; Treating X as 0
OUT 23H