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Wiring and Factory Communications

Wiring and Factory Communications. Big factories use lots of cable Some cables carry signals, some power Power sources and loads generate noise Noise in the signals causes havoc. Dealing with Electrical Noise. Strategies for combating noise reduce noise at source

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Wiring and Factory Communications

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  1. Wiring and Factory Communications • Big factories use lots of cable • Some cables carry signals, some power • Power sources and loads generate noise • Noise in the signals causes havoc

  2. Dealing with Electrical Noise • Strategies for combating noise • reduce noise at source • reduce coupling between sensitive circuits and noisy circuits • reduce sensitivity of signal circuits to noise

  3. Types of Noise • Low frequency • 60Hz • usually caused by grounding problems • High frequency • brush noise • PWM switching • arcing

  4. Types of Noise Coupling • Common Impedance • fixed by proper grounding • Electric Field • fixed by proper shielding and cable routing • Magnetic Field • fixed by twisted pair and proper cable routing

  5. Reducing Sensitivity of Circuits • Avoid high-impedance sources and loads • Use digital signaling rather than analog • Use differential signal transmission rather than single-ended

  6. Analog Signal Methods • Current Loop sensors • low impedance • zero level = 4mA • full scale level = 20mA • broken wire = 0mA, detectable!

  7. Digital Signal Methods • Discrete (on-off) signals • standard is 0v and 24v • Communication busses (low level) • serial bit stream • asynchronous: • start bit, data bits, “stop” bit • receiver and transmitter have to know bit rate • RS-232 common standard • RS-485 industrial standard (differential signalling) • synchronous: • clock signal sent with data (Ethernet)

  8. Digital Communications • Mid-level • data sent in packets • destination address, packet length, data, checksum (minimum number of fields) • Modbus, Profibus, DirectNet, etc. • TCP/IP taking over

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