------------------------Monitor------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 32 www.windmill.co.uk March 2001 --------------------ISSN 1472-0221---------------------- Welcome to the March issue of Monitor. This month we're pleased to announce the launch of two new serial communications programs, one of them free! Our main article continues the serial communication theme and covers the differences between RS232, RS485, RS422 and Modbus. We only send this newsletter to people who have subscribed - should you wish to cancel your free subscription please visit http://www.windmill.co.uk/newsletter.html CONTENTS ======== * Windmill News - Windmill launch new driver for RS232, RS422, RS485 and Modbus devices - Also released today, free serial trouble-shooting software * Serial communications - RS422 and RS485 - Modbus ________________________________________________________ Windmill News: Windmill Launch New Serial Driver and Free Trouble-Shooting Software ________________________________________________________ For some time we have been offering you free software, LabIML, to read data from instruments connected to the PC over RS232. Today we're pleased to launch two new serial communications programs: COMIML and ComDebug. COMIML is our new serial driver. It improves upon our free LabIML driver as it: - can read binary as well as ASCII data - can accept RS485, RS422, Modbus and RS232 instruments - lets you send ASCII values, twos complement integers, unsigned integers, single bits, etc, to your instrument For more details of COMIML visit http://www.windmillsoft.com/daqshop/rs232-modbus.html Our second new program, ComDebug, makes it extremely easy to interact with your instrument. What's more it's free! ComDebug lets you see exactly what messages your instrument is sending and quickly extract the necessary data. If you have problems eliciting replies from your instrument, ComDebug lets you see what's happening at the COM port, and provides several trouble-shooting facilities. When you've established communication, you can save your ComDebug settings in a file. The new COMIML serial driver will automatically read this file and control your instrument accordingly. However, you don't have to have bought COMIML to use ComDebug: you can use it simply as a terminal emulator. If you are having trouble reading data with LabIML, for example, you can use ComDebug to investigate the problem. For more about ComDebug see http://www.windmill.co.uk/serial.html ________________________________________________________ Serial Communications: RS422, RS485 and Modbus ________________________________________________________ In serial communications, the instrument sends one bit after another to the computer. Eight of these bits make a byte, and the computer translates the bytes into characters. The instrument and the computer must agree on such things as: - At what rate will the data be arriving (baud rate)? - What, if any, error checking is used (parity)? - What signifies the start and end of a byte of data (start and stop bits)? In the Windmill news section we talked about COMIML, the new serial device driver. This can read data from instruments connected to the PC's COM port communicating over RS232, RS422, RS485, Modbus or TCP/IP. The COM port was designed to accept RS232 signals. The RS232 standard, initially conceived in the 60's, specifies electrical and mechanical characteristics of the connection, including the function of the pin connections, the voltage levels and maximum bit rate. RS422, RS485 and Modbus were designed later. So, how is it that RS422, RS485 and Modbus devices can also be attached to the RS232 COM port? RS422 and RS485 =============== The short answer for RS422 and RS485 is with an adaptor plugged into the COM port. This takes the RS485 or RS422 voltage signals and converts them to RS232 signals. RS422 and RS485 use differential data transmission, giving them two major advantages over RS232. 1. Cables can be much longer and instruments thus much further away from the PC. 2. Several instruments can be connected to one RS422 or RS485 cable. Length of Cable and Voltage Levels Instruments connected to an RS485 cable can be around 1200 m away from the computer, as opposed to around 50 m for those connected to an RS232 cable. The reason for this increase in distance is due to the reduction of noise errors brought by the differential connections and shielded wires. In a differential system each signal is transmitted over two wires. On wire carries a negative voltage signal and one a positive. As the wires are twisted together, any external noise interference should apply equally to each wire. Subtracting the reading of one wire from the other eliminates any common noise. But in the introduction we talked about sending a series of bits, not voltage signals. Well, a bit is set to 1 or 0 depending on the voltage level on the cable. The baud rate sets the time span before the next bit arrives. Number of Instruments RS232 uses one-to-one communications. A PC is at one end of the cable and one data acquisition instrument at the other. To connect more instruments over RS232 you need to install more COM ports. RS485, on the other hand, is multidrop. It allows 32 devices or stations to be attached to one cable. The earlier RS422 standard allows 10 devices to be attached to a cable. For a channel on RS485 or RS422 device, you need to include an address in your request for data. What about Modbus? ================== Unlike the Recommended Standards (RS232, RS422 and RS485), Modbus is a protocol that doesn't insist on voltage levels, connection pins, etc. Modbus devices can use a variety of cables or networks, including RS485 or RS232. The Modbus protocol defines a message structure - how the PC addresses and requests information from a Modbus device, how the device will respond and how errors are detected and reported. Modbus has two modes: ASCII and RTU (remote terminal unit). In ASCII mode each 8-bit byte in a message is sent as two ASCII characters. It allows intervals of up to 1 second between characters, without causing an error. Messages start with a colon and end with a Carriage Return followed by a Linefeed. In RTU mode each 8-bit byte in a message contains two 4- bit hexadecimal characters. Greater character density allows better data throughput than ASCII for the same baud rate. Each message is transmitted in a continuous stream. The final part of a RTU message is a cyclic redundancy check, CRC. This calculates its value based on all earlier bytes in the message, it then adds its 2 bytes into the message. The computer therefore knows when it has received a corrupted message and can ask the instrument to resend its data. * Further Reading: We covered RS232 in more detail in Issues 42 and 15 http://www.windmill.co.uk/monitor42.html http://www.windmill.co.uk/rs-232.html Help on using the free Windmill ComDebug trouble-shooting software with Modbus devices is at http://www.windmill.co.uk/modbussettings.html For more information on the Modbus protocol see http://www.modicon.com/TECHPUBS/intr7.html We've ignored any discussion of the newer Universal Serial Bus, as you cannot plug USB instruments into the COM port. For a discussion of the USB see Issue 24 of Monitor http://www.windmill.co.uk/monitor24.html ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see http://www.windmill.co.uk We are happy for you to copy and distribute this newsletter, and use extracts from it on your own web site or publication, providing the above notice is included and a link back to our website is in place. An archive of previous issues is at http://www.windmill.co.uk/newsletter.html and an index of articles at http://www.windmill.co.uk/newsletter.html Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 833 2782 Facsimile: +44 (0)161 833 2190 E-mail: [email protected] http://www.windmill.co.uk/ http://www.windmillsoft.com/
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