Technical information

Read This First. OPTO DIO Reference
Power Off And Reset Output Levels.________________________________
Due to the difference in the nature of internal circuitry of
the sink and source driver chips there are systematic differences
in the power off and reset voltage levels of the output lines
when using these two driver types.
Typically the the source drivers keep the output lines at
zero when the PC is switched off, whilst the sink drivers allow
the outputs to float to a high voltage level determined by the
external circuit.
Figure 1-11. Sink Versus Source Outputs.____________________________________
DRIVER PC STATE EXTERNAL BUFFER PORT PORT
POWER USED SETTING LINES
SOURCE POWER OFF ON EITHER N/A OFF
SOURCE POWER RESET ON EITHER RESET =low OFF
SOURCE ON ON 74LS541 0 =low OFF
SOURCE ON ON 74LS541 1 =high ON
SOURCE ON ON 74LS540 INV 0 =low ON
SOURCE ON ON 74LS540 INV 1 =high OFF
SINK POWER OFF ON EITHER N/A ON
SINK POWER RESET ON EITHER RESET =low OFF
SINK ON ON 74LS540 INV 0 =low OFF
SINK ON ON 74LS540 INV 1 =high ON
SINK ON ON 74LS541 0 =low ON
SINK ON ON 74LS541 1 =high OFF
Note: By default Source driver use 74LS541 non inverting buffers,
74LS540 INVERTING buffers will invert the output signals.
Note: By default Sink driver, since they are inverting drivers
themselves, use 74LS540 inverting buffers to give a non inverting
output. Using 74LS541 buffers will invert the output signals.
Safety First.
If the power off and power reset state of the output lines is
critical to the safe operation of your circuit, examine Figure 1-11
and check the actual operation of the card when the external
circuit is connected. Perform this check both with and without
Chapter 1 Page 9