Electrical output

Just as the Measure activity allows direct input for sensors, it is useful to have a direct electrical output for robotics, LED's, and other purposes.

Possibilities:

• Audio port: This would theoretically support a highly gradated, relatively precise, d/a output, at low currents. No word on whether this works.

• You can blink LEDs by connecting them to modem control signals. Advantages: the relevant connector (phone cord) is widely available and cheap (or even scavengable)

• USB to serial adapters are readily available. They work fine for simple things, but aren't great for microsecond level timings. A quick search finds prices under $20.

• ntp uses one of the modem signals to catch the PPS (pulse per second) timing. Many kernels have a hack to grab the time when a modem signal changes state. USB adds a lot of jitter. (this is actually input, not output.)

• USB to printer port adapters are also available. I've never played with one. Prices are under $40. Advantages: number of parallel outputs (how many?)

• There are also things like this with 24 GPIO lines.
  • USBIO24R http://www.elexol.com/ US distributor: http://www.orteches.com/ $75

• OLCP just had a summer intern, Arjun Sarwal, who worked on the Measure/oscilliscope activity. We also talked about an OLPC digital gadget prototyping dongle with a USB-equipped microcontroller like those available from, for example, Atmel. Those chips cost a dollar or two and Arjun can get all the other parts really inexpensively in India where he lives.

• For applications requiring a higher level of current/power, there is the power line of the USB ports. Would it be possible to separately power up/down the USB ports on the two sides? Hack them to power up/down programmatically, against the USB spec?