I would like to parallel 2each SF1008 on a parallel port connection. After
looking at the schematic, it appears that the output on these cards cannot
be 3-state’d without modifying the hardware … would someone verify this
for me??
Also, is there a demand for these devices? or should I pursue my own design?
Although not really tri-stated, they are done in a wired OR arrangement and the outputs of several SF1008’s can be tied together, as well as data inputs, clocks and power/ground connections. The only connection that cannot be shared is the /CS or active low chip select line. This line controls which SF1008 would be selected. The selected device would turn the driver on its SO output on, which would enable the inverting output driver transistor on the board. Since the other boards would should not be selected at the same time, their output transistors would not be driving, allowing the line to pull up whenever the selected output driver was not seeing a logic level of ‘1’.