3/22/2023 0 Comments 1734 vhsc24 remoter rack![]() ISA 100 is primarily designed for wireless communication between battery powered process field instruments to a powered gateway, using wireless mesh tactics. The serial communication parameters of both devices must be identical - word size (8 bits for RTU), parity and stop bit. Once you're convinced that you're sending a request to the correct slave node address at the right baud rate and parity, but not getting a response, it pays to swap the A/B or (-)/(+) lines and try it again. If the lines are connected backwards, the line drivers are not damaged, but communications doesn't work. Some manufacturers label one way, others the reverse. 2) RS-485 has no standard for which line is (-) and which line is (+). You need to make sure the PLC can act as a Modbus master, unless you find that the meter is, in fact, a Modbus master. The PLC is the Modbus master, which requests the data. Meters are typically Modbus slaves that reply to a Master's request for data. A Master can write data to a slave, but it is highly unlikely that a meter is a Modbus master. The function code the master uses determines whether whether it is a read or a write operation. Typically, a master requests specific data from specific slave register locations and the slave replies to the request with the specific data. ![]() I'm a Modbus guy, not an Omron guy, but I can provide some insight into Modbus basics 1) The concept of a device 'sending' data is not a Modbus concept. The Modbus spec (MODBUS over serial line specification and implementation guide V1.02) has info on LRC calculation Dan If the formats are the same, then you're missing the byte count register and the data bytes all of which contribute to the LRC calculation. The slave node ID is not included in the example. The graphic below is an example from the Modbus spec. I think the RTU/ASCII message formats are common, but at this point, it's foggy. timing differences that I can't recall. Only allowable data characters are hexadecimal 0–9, A–F (ASCII coded). An ASCII message must start with a 'colon' ( : ) character (ASCII 3A hex), and end with a 'carriage return – line feed' (CRLF) pair (ASCII 0D and 0A hex). Do you REALLY mean Modbus ASCII, not ASCII characters as data over Modbus RTU? I assume you do because you have the colon delimiter, but it's important because - Modbus ASCII uses 7 bit words, RTU uses 8 bit words - Modbus RTU uses CRC check and Modbus ASCII does NOT use CRC, it uses a one byte LRC value for error checking (does not include delimiters, colon or LRCF). I so seldom run into Modbus ASCII that I have to stop and think. Better to limit too much current than too little. If it were me, I'd start high and work down. ![]() A 1200 ohm resistor in a 24V circuit would limit the current to 20mA. ![]() I'm guessing that this statement says 25mA is the maximum current rating (page 25) "For continuous currents up to 25 mA (Imax = 250 mA / 20 ms)" but it's not crystal clear to me. I know 10K ohms is common for pull-up/down resistors, limiting current to 2.4mA. This is E&H's minimalist documentation: If you put A and A return across the pull down resistor, then you should have the voltage drop across the input. ![]() But I suspect you already know that from what you've explained, since you're wired to A/Areturn and source 24Vdc from the card. National Instruments has a 2 or 3 page document on frequency/quadrature input I/O, which is what your board is. The indication of open circuit at 0mA is a great feature of 4-20mA but it wasn't the driving factor, 2 wire loop power was.Ī nice reference set of wiring diagrams for open collector, specifically for Signet, but really universal. The 20mA span followed the the ratio of the pneumatic predecessors, 3-15psi, where the span to elevated zero ratio is 5:1. All field instruments offer a choice between fail-low or fail-high indication, so the region around 3.6 to 3.8mA is used for fail-low indication, as the graphic from NAMUR shows. In 2 wire loop powered applications, the transmitter consumes about 3.6mA for its sensing and operation. Running a 3rd or 4th wire for power would have been a deal breaker for anyone contemplating 3 or 4 wire field instruments. Those industries were central control room panel board oriented and home run wiring to the control room (or marshalling panels) was the norm. The demand for two wired loop powered instrumentation came from the process industries that dominated the Instrument Society of America (ISA) at the time: refineries, steel mills, power plants, and paper mills. This is known as 'two wire loop powered'. The purpose of 4mA as a minimum is to allow the transmitter to draw its power from the loop itself to avoid the requirement for power wiring. ![]()
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