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Unread 21-01-2003, 09:57
Lloyd Burns Lloyd Burns is offline
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The inputs are standard TTL or CMOS digital inputs. Their thresholds are listed in Data Sheets on manufacturers websites. No mention has been made (re IFI equipment) of Schmitt trigger inputs, which have hysteresis, and which would change things only a little.

The highest threshold they would have is CMOS's half of Vdd, with the IFI system, about 2.5 V. TTL and CMOS with TTL-compatibility will switch at around 1.25 to 1.5 V. I haven't tested which IFI has, as it seems unimportant.

Any of these inputs acts like a high impedance input, with a 1 to 100 kohm resistor (depending on logic family) pulling it up to the +5V rail. Basically, the input voltage is either 5 V (but high resistance) when open, or zero when the switch is closed to ground. If you put a resistance from input to ground that is in the range of the pull-up, the state of the output would be practically indeterminate.

When the input is grounded, the fact is reported in the rc/oi_sw bytes as a one.

Analog (slowly varying) signals cause jittery outputs when near the threshold of non-schmitt trigger inputs, and the output snaps to +5V or 0V when the input voltage is further from the threshold. For large (steep slope) analog signals, the output appears as a square wave. Depending on your analog output impedance and voltage, you could have a low, a high, some jitter, or even a serious oscillation [possibly RF] (at the threshold, where the gate is biased to linearity, the gain of ttl gates is very, very high).

Since the IFI eqipment reads the values only every 26 ms (a value posted by others elsewhere in CD), you would not read the jitter near threshold, but the value would not be reliable.

Feeding a pot or a gyro into such a digital input is a poor substitute for using the analog inputs on the other connector. They have ADCs to read the voltage. Once you have a number, your program can compare to other values.

If you apply a positive voltage, say via a switch connected to +5V, you would be taking an input already pulled up to +5V, and switching it to +5V. You'd see no change, so it won't tell you about the switch closure.

BUT, if the +5V is more than half a volt higher than the input IC's +5V, say form a separate (external to IFI) power supply that fails and goes high (eg, to 12V), or a badly adjusted op-amp, you could cause a current flow into the IC's substrate, which would damage the IC permanently, so DON'T. Yes, there are series input saftey resistors in good practice, but good practice says don't depend on them.

So... don't put your daughter on the stage, Mrs Worthington (the outcome is predictably bad), and don't apply analog signals to digital inputs (the output is unpredictably bad).

Sorry for the length.
 


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