ZBasic Forum

I am building an interface to my home alarm system (Ademco VISTA 50P). It requires 32 I/O, and 2 rs232s. I looked at Don's AN-203 (I/O Expansion techniques) which uses a PCF8574A as an example. I have been looking at the PCF8575C (16 bit version) for my application. It appears to me programming would be just like the 8574A except two data bytes vs one. I would like to separating the CPU (ZX-44 based) complex from the I/O boards by about a meter using i2C. Any gotcha's I should watch out for? -- Jay

I haven't used the 16-bit expander myself. I suspect that you are correct that you simply read/write two bytes instead of just one.

A longer cable will present a higher bus capacitance to the I2C drivers. You will undoubtedly have to use smaller pullup resistors on SDA/SCL with the longer cable than you would for a shorter cable. The Atmel documentation gives formulae for the minimum and maximum value for the pullup resistors as follows for speeds up to 100KHz: This suggests that the maximum allowable bus capacitance, Cb, is about 650pF. The specifications for your cable may include a capacitance value per length of cable.

There are different specifications for operating above 100KHz. Generally, the longer the bus, the slower will be your maximum operating speed.

Earlier in the spec it says: ".. limited by the bus capacitance limit of 400pF .. ". Appreciate the pointer.
Jay