Powering devices from PC parallel port
Data lines
In ordinary parallel port implementations the data outputs are 74LS374 IC totem-pole TTL outputs
which can source 2.6 mA and sink 24 mA Usually the output of the chis is followed by 22 ohm resistors
and 2.2-10 nF capacitor, but some implementations don't use that RC circuit.
The outputs are resigned so that they give at least 2.4V at 2.6 mA load.
This 2.6 mA figure is for ordinary LS-TLL circuits used, the LSI implementations used in many computers
can gvie more or les,. For example quite popular (few years ago) UM82C11-C parallel port chip can only source 2 mA.
Control outputs
You can also use the Control Out pins. They can't source much of anything (about 1 mA through the 4.7K
resistors to +5), and can only sink about 7mA. (The LS TTL gate actually sinks 8 mA, but one is taken up
by the 4.7 K resistor to +5).
Again, check on clones with different electrical specs.
For example very popular (few years ago) UM82C11-C chip implements the control outputs as standard TTL outputs
with lower (1.5 mA) current capacity instead of traditional open collector circuit with pull-up resistors.
82C11 os quite popular IC for implementing parallel ports few years ago, but it was very sensitive to
ESD and CMOS latchup effects. After facing problems with this the manufacturer started to use other solutions
(nowadays the whole I/O card is usually integrated to one chip).
Comparision of different parallel port types
....................................Normal...... UM82C11-C..... IEEE 1284 level II
Data output (>2.4V)...... 2.6 mA........ 2 mA............. 14 mA
Data line sink (<0.4V) .... 24 mA....... 24 mA............. 14 mA
Control output (>2.4 V) .. 0.5 mA*... 1.5 mA.............. ?
Signal lines (short circuit) ...1 mA ...... ?.................... ?
Control line sink (<0.4V).... 7 mA........ 7 mA.............. 14 mA
* This value is caluculated from the known data available from port.
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Power from parallel port