. DTR - Data Terminal Ready. DSR - Data Set Ready. RTS - Request To Send. CTS - Clear To SendThere are multiple ways of doing things because there were never any protocols built into the standards. You use whatever ad-hoc 'standard' your equipment implements.Just based on the names, RTS/CTS would seem to be a natural fit. However, it's backwards from the needs that developed over time.
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This is the binding for the DSC PowerSeries Alarm System, utilizing either the EyezOn Envisalink 4/3/2DS interface or the DSC IT-100 RS-232 interface. It provides connectivity to the DSC Alarm panel via a TCP socket connection to the EyesOn Envisalink 4/3/2DS interface or a RS-232 serial connection to the DSC IT-100 interface.
These signals were created at a time when a terminal would batch-send a screen full of data, but the receiver might not be ready, thus the need for flow control. Later the problem would be reversed, as the terminal couldn't keep up with data coming from the host, but the RTS/CTS signals go the wrong direction - the interface isn't orthogonal, and there's no corresponding signals going the other way. Equipment makers adapted as best they could, including using the DTR and DSR signals.EDITTo add a bit more detail, its a two level hierarchy so 'officially' both must happen for communication to take place. The behavior is defined in the original CCITT (now ITU-T) standard V.28.The DCE is a modem connecting between the terminal and telephone network. In the telephone network was another piece of equipment which split off to the data network, eg. X.25.The modem has three states: Powered off, Ready ( Data Set Ready is true), and connected ( Data Carrier Detect)The terminal can't do anything until the modem is connected.When the modem wants to send data, it raises RTS and the modem grants the request with CTS. The modem lowers CTS when its internal buffer is full.So nostalgic!
An important difference is that some UARTs (16550 notably) will stop receiving characters immediately if their host instructs them to set DSR to be inactive. In contrast, characters will still be received if CTS is inactive.
I believe that the intention here is that DSR indicates that the device is no longer listening and so sending any further characters is pointless, while CTS indicates that a buffer is getting full; the latter allows for a certain amount of 'skid' where the flow control line changed state between the DTE sampling it and the next character being transmitted. In (relatively) later devices that support a hardware FIFO it's possible that a number of characters could be transmitted after the DCE has set CTS to be inactive.
SolutionThe Modbus I/O Server allows you to easily link Modbus register values to that you can use in your LabVIEW VIs. Modbus I/O Servers provide a simple way to access and write Modbus data in LabVIEW. Modbus I/O Servers is included in the and the. In exchange for ease of use, Modbus I/O Servers do not have the fine control that the supported LabIVEW Modbus API has over the Modbus protocol that can be required for some applications.(boxed in blue in the screenshot above)The supported Modbus API provides fine control over the Modbus protocol, with high performance. Everything must be done manually when using this low-level API. The supported Modbus API is included in the LabVIEW DSC Module and the LabVIEW Real-Time Module.NI OPC Servers and DataSocketsIf you are using LabVIEW on Windows, you can also use to communicate with an OPC Server that can communicate with Modbus. If you do not have an OPC Server, NI OPC Servers provides an interface to many protocols, including Modbus.
To learn more about using DataSockets to connect to OPC Servers, take a look at the tutorial on.Unsupported Modbus Library (boxed in green in the screenshot above). LabVIEW 2012 and laterThe is the latest release of the free LabVIEW Modbus Library. It is intended as a replacement and expansion of the free Modbus library functionality that supports versions of LabVIEW before 2012. LabVIEW 7.1 and laterThe is a wrapper for VISA commands. Programming with it requires some knowledge of Modbus function calls to program.
This library may not contain as many features you need as the or the supported API included with LabVIEW DSC Module.Note: The unsupported Modbus Libraries are not supported by National Instruments. You may get a response if you post your questions on the NI Community Forums.
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