Sadly, you will often find that the arbitrary byte ordering used by any particular Modbus slave device manufacturer is poorly documented, if at all. This means you may have to experiment with di erent byte orderings before achieving success reading or writing floating-point data. Some Modbus device manufacturers are thoughtful enough to actually provide configurable options for how their 32-bit floating-point values will be represented within the slave device. Likewise, many Modbus master device manufacturers (e.g. PLCs and HMIs) provide options for how to read and write floating-point values in their Modbus read and write instructions, because they do not know whose Modbus slave device you may be communicating with.

15.12Review of fundamental principles

Shown here is a partial listing of principles applied in the subject matter of this chapter, given for the purpose of expanding the reader’s view of this chapter’s concepts and of their general interrelationships with concepts elsewhere in the book. Your abilities as a problem-solver and as a life-long learner will be greatly enhanced by mastering the applications of these principles to a wide variety of topics, the more varied the better.

•Analog vs. digital signals: analog signals have infinite resolution but are susceptible to corruption by noise. Digital signals have limited resolution but are tolerant of any noise measuring less than the di erence in thresholds between the high and low states.

•Superposition theorem: any linear, bilateral electrical network with multiple sources may be analyzed by taking one source at a time (while replacing all other sources with their internal impedance values) and analyzing all voltages and currents, then superimposing (summing) those voltage and current values to obtain the voltages and currents with all sources active. Relevant to analyzing DC and AC signals in HART instrument circuits.

•Transmission lines: short-duration (pulsed) electrical signals travel along a cable at nearly the speed of light, reflecting o the end of that cable if not properly terminated. Relevant to signal cables carrying high-frequency signals.

•Self-balancing opamp circuits: all self-balancing operational amplifier circuits work on the principle of negative feedback maintaining a nearly zero di erential input voltage to the opamp. Making the “simplifying assumption” that the opamp’s di erential input voltage is exactly zero assists in circuit analysis, as does the assumption that the input terminals draw negligible current.

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