Controls › Instrumentation
Signals
What actually travels on the wire, and what corrupts it.
Written guides in this section
Full references, procedures, and diagnostic guides published under Signals.
4-20 mA Signals from the Instrument Side
How a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
9 min readUpdated Sep 5, 2026
HART
Digital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
10 min readUpdated Sep 5, 2026
Pulse Signals
Pulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
7 min readUpdated Sep 5, 2026
Frequency Signals
Frequency as a process signal: the instruments that produce it, how frequency differs from pulse counting, the K-factor and the conversion to flow, frequency inputs and period measurement at low rates, converters to 4-20 mA, noise, and how to verify one.
7 min readUpdated Sep 5, 2026
Digital Signals
Discrete signals between instruments and controllers: dry and wetted contacts, sinking and sourcing transistor outputs, two-wire sensor leakage, NAMUR sensors with wire-break detection, contact ratings at low current, and wiring conventions.
8 min readUpdated Sep 5, 2026
Signal Isolation
Isolating analog signals: loop isolators and their variants, splitters that feed two receivers, converters, the common-mode problem isolation solves, ratings, accuracy and response costs, and when an isolated input module makes the isolator unnecessary.
8 min readUpdated Sep 5, 2026
Topics in this area
4-20 mA
The current loop from the instrument side: wiring, power, and loop checks.
HART
Digital data riding on the 4-20 mA loop.
Pulse
Pulse — practical reference notes for controls engineers, integrators, and technicians working with signals.
Frequency
Frequency — practical reference notes for controls engineers, integrators, and technicians working with signals.
Digital Signals
Digital Signals — practical reference notes for controls engineers, integrators, and technicians working with signals.
Signal Isolation
Signal Isolation — practical reference notes for controls engineers, integrators, and technicians working with signals.
Ground Loops
Two grounds, one loop, and a reading that will not sit still.
Surge Protection
Surge Protection — practical reference notes for controls engineers, integrators, and technicians working with signals.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.
References to manufacturers, products, model numbers, software platforms, technical specifications, companies, standards organizations, or other organizations are provided for informational purposes and do not imply sponsorship, endorsement, authorization, or affiliation unless expressly stated. Manufacturer specifications must be independently verified against current manufacturer documentation.