krisl@hpindwa.HP.COM (Kris Livingston) (07/20/89)
I am trying to design a board that plugs into the Printer or Modem port on a Mac Plus or later, and I'd like to know how to get 5 volts from the port. (Apple removed the 5 volt supply pin from the serial ports beginning with the Mac Plus.) Apple does it with their MIDI interface, and Farallon does it with MacRecorder, so I know it can be done. Do any of you EE-types know exactly what is being done on the board that extracts 5 volts from the serial ports? Or is this all proprietary? I'd really rather not have to use an external power supply! As usual, email responses--if there's sufficient interest, I'll post a summary. Thanks! Kris Livingston krisl@hpindwa.hp.com Disclaimer: HP is oblivious to any opinions that I might have expressed above.
bcase@cup.portal.com (Brian bcase Case) (07/22/89)
>Do any of you EE-types know >exactly what is being done on the board that extracts 5 volts from the serial >ports? Or is this all proprietary? I'd really rather not have to use an >external power supply! In the MIDI interface: The way it is done is pretty clever: the power comes from the serial lines by using diodes to steer the current from the wires to the power terminals of the few chips correctly no matter what is the polarity of the serial lines. The key is the realization that the serial lines are differential: if one is high, the other is low. Thus, two diodes per polarity will guarantee that current is always flowing, *except* when the polarity is changing. A cap will smooth out the pulses. Obviously, this scheme will only supply power to a couple ICs at most. A friend and I figured this out by looking at the MIDI circuit board; there are no schematics of which I know. If you can get them, that would make figuring it out easier.
kent@sunfs3.camex.uucp (Kent Borg) (07/24/89)
In article <20657@cup.portal.com> bcase@cup.portal.com (Brian bcase Case) writes: >>Do any of you EE-types know >>exactly what is being done on the board that extracts 5 volts from the serial >>ports? Or is this all proprietary? I'd really rather not have to use an >>external power supply! >The way it is done is pretty clever: the power comes from the serial >lines by using diodes to steer the current from the wires to the power >terminals of the few chips correctly no matter what is the polarity of >the serial lines. The key is the realization that the serial lines >are differential: if one is high, the other is low. Thus, two diodes >per polarity will guarantee that current is always flowing, *except* >when the polarity is changing. A cap will smooth out the pulses. I think it might be too clever. First, don't expect to get much power out of the serial port no matter how you do it. Second, do it the way (I think) MacRecorder does: Pull a few mA's off of pin #1. Tell the SCC to assert *DTR (an excersise left to the reader). This gets inverted and sent through a few dozen ohms of rf filtering to become Output Handshake (See Inside Macintosh Vol. IV, p. 248). Viola! No filtering or rectifying required. Now, maybe you can get a bit more power by rectifying the data lines, but if you only need a little power for a couple CMOS chips, try the simpler way first. Kent Borg kent@lloyd.uucp or ...!husc6!lloyd!kent