Parallel port is dead, and USB microcontrollers are really the future IMO. If you use the HID or CDC profile you don't even need any driver on your PC. Eg the Teensy for $18, you get 25 I/O, of which all can be used as digital I/O, or up to 12 of them as analog inputs and 7 of them PWM channels. The Teensy is more general purpose, if you want something specifically to provide communication between your PC and electronics, there are other devices targeted more towards that, like the Bus Pirate. Or, if you don't need too many I/O's, you could use the standard, fairly simple FT232R USB chip in 'bit-bang' mode, and get 8 digitial I/O out of it.
If you absolutely refuse to listen to reason, then what you need is a PCI card which adds a parallel port, like this one. You'll need a PCI slot (not PCI-Express), most motherboards still have one these days, but they are starting to get phased out.
USB to Parallel adaptors will have all sorts of problems. The main one is latency...standard parallel port latency is measured in microseconds, whereas USB latency is measured in milliseconds. The other is that many USB to Parallel adaptors are designed only for printers, and lack the extra circuitry needed to individually control the address lines in the way you want.
Short answer: Putting power supplies in parallel is rarely a good idea.
Why?
Because the power supplies will have slightly different output voltages, one will supply more, and one less. It is next to impossible to determine in advance how the current will be divided.
About your circuit, clearly, it contradicts the product sheet of the charger. The charger has to be parallel to the battery and the load, not in series.
The charger IC is programmed to a certain charging current, max 2A. This is only indirectly related to the supply current of the solar panels. If they can supply 4A, only 2A will be charged to the battey, the rest should be drawn by the load. If the load doesnt draw >2A at all times, problems may (will) arise.
Best Answer
The PC parallel port is not designed to power external equipment.
While it xcan be used for his the power levels obtainable are low.
Better, if available, is USB.
Even a serial port MAY have greater power capability.
Parallel port pinouts are given here and here and here and here
I provide several pages as, when you are trying to do something non standard, seeing what various people say can help.
Below is a typical pinout table.
Any one of the output pins may supply some current.
If you take all outputs, connect a 1N4148 diode from each facing "outwards" (Anode to port, cathode to output) , connect all Cathodes and add a capacitor o ground (say 10 uF) you will get some voltage. How much and at what level is to be determined.
This page reports that Linksys, who should know better [tm] have drawn power from some pins for some of their equipmenty. eg
"Another area that might be of interest in your document would be some comment on the parallel port extenders. I have a xmit/rcv pair from LinkSys that I bought from Fry's Electronics for about $70. They convert the parallel signal to a serial data stream, using the signal and control lines for power. My set was working fine until I added a hardware dongle for an expensive Windows application. Then, printing ceased to work reliably. I took the transmitter apart and partially traced the schematic.
They have used 7 diodes to suck power from pins 13, 14, 15, 17, 1, 2, and 3.
Also, they connected pins 15 (ERR) to 16 (INIT). The strobe line is coupled in to a flip-flop, which starts clocking the parallel loaded data."