Thread: Port power

Parallel I believe uses 5V, and Serial uses 3V. I'm not sure. I was trying to make my own IR circuit attached to the parallel port earlier. I was using IBM's specifications. Look online and i'm sure you can find information.

Bubba: That information may be in your A+ book(I doubt USB is, but Parallel / Serial may be) but it should not be asked on your A+ exam, as it is not Coriolis criteria.

Parallel depends a lot on the technology. It was not at all designed for power, only signaling, so it's designed to drive a very high impedance load. If you can get it to source 2.5 mA @ 2.5 volts (6.25 mW), you're getting a lot out of the port. They can often sink about 20 mA, though.

Serial ports of the RS232 variety can vary between +5 and +15 for the logical zero state, and -5 to -15 for the logical 1 state (yes, the "1" is negative in voltage). At the receiver's end, you can expect at least 3V under 3kohm load, so 3V @ 1 mA is the minimum that serial is required to provide. That's pushing the limits, though; most serial devices are designed to draw less than 3 mW.

Older serial devices are more likely to be workhorses. Some very old-school ISA serial cards can provide 10V @ 10 mA, which is more than 30 times the required power. Devices made in the 1970s and 1980s are probably going to be more powerful. The same is not necessarily true of parallel ports; older models often can sink a ton of current but they're often poor at sourcing it because the output driver is essentially an open collector BJT with a weak pullup (10 kohms or so).

USB, however, WAS designed to power devices in addition to moving data, so it will be guaranteed to be able to source more power than serial or parallel.

If you want to work on various devices, I recommend you pick up a book on embedded systems. Programming and Customizing PICMicro Microcontrollers, by Myke Predko, is a good one. There's a printed circuitboard that comes with the book that will let you use a parallel port to program many PICMicros, including all mentioned in the book. You need to buy and solder components onto the board (components are reasonably cheap, be sure to look at his website for a revised bill of materials).

I'd also recommend becoming familiar with soldering before you attempt it, and also get a decent soldering iron. A temperature-controlled soldering station that keeps the tip at about 750 F is a good buy; Radio Shack has a decent unit for $50-$65 or so that fits the bill (digital temperature controlled station), but be warned, the only size tip available is too big for really tiny soldering (like surface mount parts). Weller is probably the most well-known brand, but you pay about double for the ability to buy many different sizes and shapes of tips.

Be warned, if you have poor tools, you're very liable to damage the PCB that comes with the book I mentioned. I am quite experienced at soldering, but I was using a bad iron and ripped a pad clean off the board.

If you want practice, go to radio shack, pick up an empty PCB with copper pads on one side, and pick up some DIP sockets and a solder sucker and solder wick. Practice putting the sockets on the board and removing them.

Surface mount soldering is actually quite easy, once you get the hang of it.

Also, don't go and try to steal port power from the get-go. Just buy an AC adapter or "borrow" one from something else. I have a single 15V/500mA one that I use for most things. You can use 5V regulators to get +5 from anything higher, although be careful you get regulators rated for the work you plan to do. You could just get a decent 6V supply, and regulate that to 5V, which will help you in 90% of cases. EEPROM programming, though, often needs +12V so be forewarned you often need to power with +15.