03 - Analog

The units

A channel reads and writes in the unit it is wired for.

So 5 V is 5.0 and 12 mA is 12.0. A reading below the range reads 0.

Over range: a saturated input reads 22 mA / 11 V

The converter tops out just above the range's full scale. An input driven beyond
what it can measure — a shorted 4-20 mA transmitter, a sensor failed high —
saturates the converter, and the reading is then a clear over-range value
rather than a plausible full scale:

So transmitter-failed-high detection works: NAMUR NE43 counts anything from
21 mA up as a failed transmitter, and 22.0 is there. Saturation is judged on the
converter count (AI01_RAW at 32765 or more) before calibration, so no offset
can hide it. Short of saturation, a channel calibrated so that full scale sits
below the converter's limit can read a little above it (20.5 mA, an NE43
over-range), up to the same 22 mA / 11 V.

The mode pin is not the wiring

This is the thing to get right, because nothing will tell you when it is wrong.

Whether a terminal accepts 0-10 V or 0-20 mA is a solder bridge on the
board, decided when the board is built. The mode pin only tells the software
which arithmetic to use.

The board leaves the factory with the inputs bridged for 0-20 mA and the
outputs for 0-10 V — cut the bridge to swap either. The mode pins default to
UIO_VOLTAGE, so an input on a factory board needs UIO_CURRENT set
explicitly.

Set UIO_CURRENT on a channel bridged for voltage and you get a number that
looks entirely reasonable and is wrong. There is no error and no way for the
software to detect it. Check the bridges against what your program says.

Raw counts

AI01_RAW and friends are the converter's own count, 0 to 32767, before
scaling and before calibration. AO01_RAW and AO02_RAW are the same on the way
out, after scaling.

You need them for two things: calibrating a channel, and working out whether a
reading that looks wrong is the sensor or the scaling.

How often a reading changes

The four inputs share one converter and are read in turn, so any one channel
refreshes roughly every 20 ms no matter how fast your task runs. Reading
AI01 twice in one scan gives the same number twice, and reading it every
millisecond does not make it change every millisecond.

The output converter is only written when the value changes, so holding a
setpoint costs nothing on the I2C bus.

Calibrating

Two points and an offset per channel:

• LOW_COUNT is the raw count you see with the bottom of the range applied.
• HIGH_COUNT is the raw count with the top applied.
• OFFSET is added to the scaled result afterwards.

Leaving LOW_COUNT and HIGH_COUNT equal falls back to a straight ratio,
which is what an uncalibrated channel does.

To calibrate AI01:

1. Apply a known 0 V (or 0 mA) and read AI01_RAW. That is LOW_COUNT.
2. Apply a known 10 V (or 20 mA) and read AI01_RAW. That is HIGH_COUNT.
3. Set CHANNEL := UIO_AI01, the two counts and OFFSET := 0, then pulse APPLY.
4. Check AI01 now reads what you applied.
5. Pulse SAVE.

APPLY takes effect at once but only in RAM. Nothing reaches flash until
SAVE, so you can try values freely.

CHANNEL names the channel — UIO_AI01 to UIO_AI04 for the inputs,
UIO_AO01 and UIO_AO02 for the outputs.

RESET puts every channel back to its default and saves that — worth knowing
about when a calibration has gone wrong and you want a known starting point.