Latch16Bit Board: Circuit Sheet (Rev2)
The circuit is a pretty simple and straightforward design.
Download Latch16Bit board (rev2) electronic circuit sheet:
Little circuit description
The 8 data lines of the parallel port are connected to the input
lines of the two output latches and to the output
lines of the two input latches as well as to the
input of the control latch.
Each IO latch has an output enable
pin which allows you to set the output latches to
High-Z state if required and which is needed to take the
input latches off the data bus unless you want to
read data from the inputs.
Two separare control lines are present: control select and strobe. control select (active LOW) is used to tell the board that control data will be applied to the data bus. Hence, activating this line (by setting it LOW) will force the input latch's output lines to High-Z (taking them off the bus) and the make the output latches non-transparent (making them keep their content). After setting control select low, the control data byte is put on the data lines and strobe gets a (short) positive pulse to latch the control byte in the control latch. In case one of the output latches is transparent, the output data should be put to the bus before deactivating control select (setting it HIGH) and thus making the latch transparent again. See also the little wave form diagram on the right bottom of the sheet below.
In order to take the Latch16Bit board completely off the data bus, simply
take both control select and strobe
LOW. In this state, data on the bus will neither be interpreted as control
bytes nor forwarded to any of the output lines (because the
output latches are automatically set to non-transparent
state) and the input latches' output are set to High-Z.
The 220k resistors are used to force non-floating states of the lines but
can be left away if you feel that you do not need that. Note however that
the two single 220k resistors automatically take the board off the
bus and make the output latches non-transparent by forcing LOW when
leaving the control lines freely floating.