Signal Types Quick Reference

From Appendix A of BAS Field Fundamentals: A Field Guide for the New Technician

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Analog and discrete signals

Signal	Expected range	Meter setting	Where to measure	Common failure modes	Caution
0–10 VDC	0–10 V linear	DC volts	Signal to signal-common, at controller and at the device	Voltage drop on long runs; loose terminal; noise from nearby line voltage	Confirm scaling is truly linear; measure at both ends
2–10 VDC	2–10 V	DC volts	Same	Same as above	2 V is a valid zero; 0 V means a dead wire
4–20 mA	4–20 mA	mA DC, meter in series	Break the loop, or across a known loop resistor	Broken conductor; failed transmitter; lost loop power	0 mA is a broken loop, not a zero. Know if loop-powered or externally powered
0–20 mA	0–20 mA	mA DC in series	Same	Same	Cannot distinguish true zero from a break — avoid on anything critical
Dry contact	Open / closed	Continuity or ohms	Across the contact, power off	Corroded contacts; vibration dropout; wrong NO/NC expectation	Confirm whether the input expects NO or NC
Thermistor	Per curve; 10K types ≈ 10K Ω at 77 °F	Ohms, power off, one leg lifted	Across the sensor leads	Drift; open element; wrong type configured	10K Type II vs III differ badly at range ends
RTD	Pt100 = 100 Ω, Pt1000 = 1000 Ω at 0 °C	Ohms, power off	Across the sensor; 3-wire cancels lead resistance	Open element; lead resistance on long 2-wire runs	Nickel and Balco 1K have different curves
Thermocouple	Millivolts DC	mV DC	Across the junction leads	Junction corrosion; extension wire of the wrong alloy	Poor resolution on a general-purpose DMM
24 VAC control	~24 VAC	AC volts	To transformer common, loaded	Overloaded transformer; open fuse; shared-common errors	Unloaded voltage proves nothing
Triac output	Switched AC	AC volts, LoZ	Across the load, load connected	Leakage voltage when off; failed triac	A high-impedance meter reads full voltage on an off triac
Pulse / contact pulse	Contact closures	Continuity, or pulse count	Across the contact	Bouncing contacts; wrong scaling in software	Units-per-pulse lives in software, not the device

Communication buses

Once points arrive over a bus, there is nothing to measure with a meter and the questions change.

Bus	Physical layer	Typical use	What replaces the meter test
BACnet MS/TP	RS-485 twisted pair, daisy chain	Field bus to terminal controllers	MAC address, baud rate, max master, termination, biasing, polarity
BACnet/IP	Ethernet	Building controllers to supervisor	IP, subnet, BBMD, device instance
Modbus RTU	RS-485 twisted pair	Meters, VFDs, generators, PLCs	Slave address, baud, parity, register map, byte order
Modbus TCP	Ethernet	Same, over IP	IP, unit ID, register map
LonWorks	FT-10 twisted pair	Older installed base	Neuron ID, bindings, commissioning records
EtherNet/IP, PROFINET	Ethernet	PLCs and industrial equipment	Vendor tooling; usually reached through a gateway

RS-485 basics: it is a daisy chain, not a star. Every device is polarity-sensitive. It needs termination at each physical end and biasing somewhere on the segment. One miswired device, one star branch, or one missing terminator can destabilise an entire bus in ways that look like random dropouts.