Topic
Analog & I/O
Getting signals in and out: 4-20 mA scaling, sinking and sourcing inputs, data types, addressing, and wiring I/O so it reads what the field is doing.
4 articles
Analog scaling: turning 4-20 mA into something you can read
Scaling is a straight-line map: subtract the raw minimum, multiply by the engineering span, divide by the raw span, then add the engineering minimum. The part that goes wrong is the raw range, which differs by platform: 4 mA is 3277 counts on a 1769-IF4 and 5530 on a Siemens analog input, not zero.
6 min readSinking and sourcing, settled
Sourcing supplies current, sinking accepts it. A sourcing output pushes current into a sinking input, and a sinking output pulls current from a sourcing input. The pairing rule is the only thing you have to remember: a sourcing device must connect to a sinking one, never to another sourcing device.
6 min readRTD or thermocouple: choosing a temperature sensor
An RTD measures temperature as a change in resistance and is more accurate and stable over a moderate range. A thermocouple generates a small voltage from two dissimilar metals and covers a far wider range, including very high temperatures. Use an RTD below about 600 degrees where accuracy matters, and a thermocouple above it or where response speed matters more than precision.
6 min readWiring I/O without creating problems for later
Keep signal and power segregated, ground shields at one end only, use interposing relays for anything inductive, and label both ends of every wire with the same identifier used in the program. The single highest-value habit is making the label on the wire, the terminal, the drawing and the tag name identical.
6 min read