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Latch16Bit Board: Circuit Sheet (Rev2)
The circuit is a pretty simple and straightforward design.
It uses 5 CMOS latches 74HC573 and some simple control
logic made using 4 AND and 4 NAND gates resulting in a
total of 7 cheap chips.
Download Latch16Bit board (rev2) electronic circuit sheet:
Medium-quality 1049x739 PNG image:
Latch16bit-rev2.png (26kb)
High-quality colored PDF:
Latch16bit-rev2.pdf (180kb)
Permission to copy and use this sheet is hereby granted provided credit is
given where it is due.
![Latch16Bit board (rev2) electronic circuit sheet [26kb]](Latch16bit-rev2.png)
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.
Furthermore, each IO latch has a
latch enable pin which enables you to either set the
latch transparent or make it keep its content so that you can access
the other latches.
In total, there are 8 control bits needed:
output enable and latch enable for
all 4 IO latches -- these are controlled by
the 8 lines of the control latch.
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.
For primitive diagnosis, a LED is available (use a 4mA low current LED for
the purpuse) which indicates active (i.e. LOW)
control select (thus also lights if you take the board
completely off the bus).
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.
For more explanations simply look at the circuit sheet and see
yourself -- it should not be hard to see what the components are doing.
Just if you wonder: "lp" stands for "line printer", thus the "lp port" is
the "parallel port".
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