16 years ago
a Fry's, and found that they had them. Fortunately, they had them marked
incorrectly on their website, $29.90 rather than their normal $49.90, bonus
for me as they gave me the website prices when I said that's why I came.
There is no official support for the device under Linux, there is, however,
a fairly hackerish USB driver out there. It doesn't fully work, but it was
close enough to take a good look. I should be releasing a cleaned up
interface class on the LinuxPCRobot.org site sooner or later.
By replacing "usb_interrupt_[read|write]" with "usb_bulk_[read|write]" I was
able to get about 1200~1300 read-write cycles out of the unit. This will
clearly make the LinuxPCRobot less bandwidth limited on I/O over the
parallel port I2C. Also, since USB is handled in hardware, the
communication between computer and device takes very little CPU.
The D2A circuit utilizes PWM. An analog signal is generated by a simple R/C
circuit fed into an op/amp.
The A2D circuit utilizes a simple op/amp with programmable gain.
It uses a programmed PIC16C745 PIC chip with built in USB, PWM, and A2D
It is clear that Velleman doesn't utilize all that this chip can do, but it
is a pretty good start for short money.