Modbus Comms RTU

RTU is the binary version of Modbus (not ASCII) Modbus RTU over serial Byte Function0 Addrerss1 Function code2:# Data# CRC   Modbus RTU over serial – Function 3 See here Modbus RTU over serial – Function 6 See here

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Antenna separation

A basic rule: The horizontal distance between antennas should be greater than 1/4 of its wavelength (absolute minimum separation), but it should not be located at the exact multiples of its wavelength (maybe avoid the first 3-4 multiples). If multiple frequency antennas are near each other, then use the spacing distance of the lower frequency […]

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Terminating resistor

A RS485 bus needs a 120ohm resistor placed at each end of the cable run. Software switched terminating resistor This circuit works well for digitally switching in a terminating resistor:

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DMX Values

16 bit values 16 bit values are usually sent MSB first, or as course/fine where course = channel, fine = channel+1

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SPI Mode

Mode CPOL CPHA 0 0 0 Clock idle low, data is clocked in on rising edge, output data (change) on falling edge 1 0 1 Clock idle low, data is clocked in on falling edge, output data (change) on rising edge 2 1 0 Clock idle high, data is clocked in on falling edge, output […]

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MOV (Metal Oxide Varistor)

A MOV is normally open circuit. If the voltage across it exceeds its nominal rated voltage it will very quickly clamp it by becoming very low impedance, shunting an overload through it for instance. When shunting a MOV is dissipating all of the energy across itself and therefore it’s often used in combination with a […]

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PTC / Thermistors

PTC stands for Positive Temperature Coefficient and is simply another name for a thermistor. A PTC device will have a nominal resistance, for instance 1kohm, that it is normally at. If it gets hot then its resistance increases. PTC’s are often used to protect inputs for instance in multimeters but not they are no fast-acting. […]

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Resistors voltage tolerance

Increasing voltage capability Place resistors in series. For example, 5x 200kohm 100V resistors is equivalent to 1 x 1Mohm 500V resistor. Using multiple lower voltage resistors is often cheaper than a single high voltage resistor. Also multiple resistors provides increased creepage distance against a single resistor of the same footprint (can be handy to avoid […]

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Multimeter Input Stage

Input Impedance Voltage Measurement Most multimeters will have an input impedance of 10Mohm. This means it will effectively apply a resistance across what you measure of 10Mohm, potentially affecting the measurement if the source has a high impedance Current Measurement Will depend on the meter, but it might be say 1ohm for the mA range […]

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Collector-Emitter Sustaining Voltage

Transistors are often specified with two collector-emitter max voltages: (Vces) collector-emitter voltage (Vbe=0V) (Vceo) collector-emitter voltage (Ib=0A) Vceo(sus) is the Collector-Emitter Sustaining Voltage and its confusing. It’s basically the the collector-emitter breakdown voltage with the base open circuit, which is the most vulnerable state for a transistor. This diagram from Electrical Ratings and Characteristics of […]

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