Regarding HART475 communication issues
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1. Instruments such as voltage transformers and flow meters that use Hart for communication – is it necessary to supply them with a 24V power source? (Yesterday, a voltage transformer was connected; a 10V voltage was applied using a signal generator, and communication was still possible with 475.) 2. For the voltage transformer (which wasn’t connected to the DCS circuit and was kept in the control room), communication was possible using 475 – why wasn’t it possible either with a 250-ohm resistor? I would appreciate it if experts could provide some guidance
Is it the same voltage transformer? 475 can communicate
Is a resistor already connected in series inside the power supply?
It’s the 10V voltage provided by the signal generator
Did your power supply arrive first at the 475, and then is it the 475 that connects to the instrument? It’s strange that communication is possible at just 10 volts
1. Most two-wire instruments can operate at 12V; it’s not unusual to find ones that work at 10V, and there are also those that can function at 7–8V. With a normal 24V supply, a portion of the voltage is dropped across the load and the wires; if the total load resistance is 500 ohms, the voltage at the transmitter will be 14V at 20mA. 2. When a 250-ohm resistor is connected to the communicator, it is used to convert digital signals in the form of current pulses into voltage signals. So it doesn’t have to be 250 ohms; as long as the converted voltage is higher than the sensitivity of the communicator, communication is possible. 250 ohms is merely a parameter that applies under specific experimental conditions such as wire distribution parameters and transmission distance; its failure to be met does not necessarily mean it is unusable ； The conversion resistor does not have to be a separate component; all resistors in the signal circuit (including the power supply’s internal resistance) can serve this purpose. Therefore, connecting a field communicator usually does not require an additional resistor. In most cases mentioned by the poster, it is due to a relatively high internal resistance of the power supply, such as in the power supply for certain testing equipment.