Velleman K8000 - [K8000 FAQ]
Can I connect other circuits to the K8000 I²C (interIC) bus?
Yes, the I²C bus is available at the 5P screw connector at the border of the card. You can find an SCL (Serial Clock), SDA (Serial Data) signal, and the ground. But also a DIS (Disable) signal is available. It is used to disable the communications with the computer, when DIS is "high".
I don't like to switch the IC-IN and IC-OUT all the time, to configure a channel as an input, or an output. Is it possible to use IC-IN and IC-OUT together?
No, this isn't possible. If you use the I/O port as an input, and apply a certain DC voltage, the phototransistor of IC-IN will conduct, but at the same time the internal LED of IC-OUT will light up. This will cause the phototransistor of IC1-OUT to conduct, and the input voltage will become short cutted by IC-OUT.
I saw in the I²C specs only 2 lines are used for communications between systems. Why does the K8000 have 3 opto couplers and 3 data lines?
Normally, I²C requires only 2 lines: SCL and SDA. But Velleman designed the K8000 with opto couplers to protect your computer against damage via the K8000. An opto coupler can only pass information in 1 direction. This is nothing for the SCL line: only the computer has to generate clock signals, and doesn't have to receive information over the SCL line. But the SDA line is bidirectional, so 2 opto couplers are used: 1 for each data direction.
Can I raise the limit of 4 K8000 cards?
You could do this for the I/O channels, so you can have a maximum of 128 I/O channels! (I didn't test this setup, maybe you get troubles with impedances ...). Principle: connect 8 K8000 cards together. On the first 4, use the PCF8574AP as I/O IC'c. For the last 4, use the PCF8574P as I/O IC's. Both IC's have the same function, but a different I²C address. There is a disadvantage: the PCF8574P is conflicting with the DAC IC's, so you can't use them at the same time! You will also need to modify the program code to support those extra I/O chips!