[JonS]

Stretching several rusty skills at once, I have got an optical (limit) switch driving a CMOS inverter.

I have got the output (+5V) of the CMOS inverter attached to a pin on JP5 of Kflop. I am happy that this is working.

I have a loop reading the inputs and displaying them:
0 0 1 1 1 1 0 0
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0 0
1 1 1 1 1 1 0 0
1 1 1 1 0 1 0 0
1 1 1 0 0 0 0 0
1 1 0 0 0 0 0 0
1 1 0 0 0 0 0 1.......

I have used SetBitDirection for the 8 pins.

I was hoping to see the switch voltage output on either pin1 or pin8 Readbit 36 or 43... but the output just cycles on all 8 regardless of switch output voltage.

What current does the input need? Why are digital inputs floating?

Could it be an earth issue?

N.B. this is the first thing I have done with the Kflop, it is only powered by USB and has no option cards plugged in.

Please presume that I am stupid and have missed something basic that I really should know about!

Thanks,

Jon

[TomKerekes]

Hi Jon,

I'm not sure I fully understand your issue, but yes the inputs on JP5 are floating and if not connected to anything will float to any level and may read random states or often follow the state of an adjacent pin. The inputs are CMOS so are very high impedance (meg ohms).

All KFLOP inputs are 3.3V LVTTL FPGA inputs with diode protection clamping to the 3.3V power rail. Some pins (all JP5 pins and some JP7pins) have a series 47 ohm resistor which allows the pin to be driven to 5V without damage. However when this happens the protection diode draws a fair amount of current and bleeds some small current into an adjacent pin in some cases. This doesn't cause any major problems but should be avoided if possible by not driving the pin above 3.8V. Most 5V TTL outputs do not drive above 3.8V.

IO pins on KFLOP JP4 and JP6 are 3.3V only and will cause damage if driven to +5V. But the first 8 of 10 IO on each connector have strong 150ohm termination resistors (pull downs) that may work better for you.

HTH