Windmill Software Ltd
Data Acquisition Intelligence

RS232 Connections
January 2002

The Newsletter for PC-Based Data Acquisition and Control
Issue 42        January 2002
--------------------ISSN 1472-0221----------------------

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* New Year Survey
* Windmill News: More Help for Modbus Users
* The Ins and Outs of RS232 COM Ports

New Year Survey

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1. Which sections of Monitor do you find the most interesting or useful? Please rank from 3 (very) - 2 (fairly) - 1 (not very) - 0 (not at all) - New product announcements - Real world examples of how Windmill is being used - Tips on using Windmill - Excel corner - Exhibition dates - Major articles and tutorials 2. Are there any topics you would like us to cover in Monitor? 3. Have you downloaded either the free Windmill 4.3 data acquisition software suite, the free ComDebug trouble-shooting software, or the demo software? Windmill 4.3 ComDebug Windmill Demo Streamer Demo 4. Did you have any problems using the software and if so what were they? 5. We would like to list the instruments people are using with the free Windmill software. If applicable please could you let us know the make, model and type of instrument you are using? We'd also be very interested to know about the project for which you are using Windmill. 6. What is your job title and industrial sector or academic discipline? 7. Any other comments or suggestions on the Monitor newsletter, Windmill software or Windmill web site? E-mail: Many thanks. ________________________________________________________ ________________________________________________________ Windmill News: More Help for Modbus Users ________________________________________________________ As you probably know, our COMIML serial driver lets you read and control Modbus instruments. It can pass the acquired data to the free Windmill Logger and Chart programs, or to other Windows software like Excel. We've now provided more technical support for Modbus users with a new Modbus Settings web page. Plus we've updated the on-line Help file. You can download your copy from our Help page. For more information on the COMIML Modbus driver see the Modbus page. ________________________________________________________ ________________________________________________________ The Ins and Outs of RS232 ________________________________________________________ We discussed the RS232 standard in broad terms in Issue 15 of Monitor, back in October 1999. Today we're going into more detail about RS232 pin connections and wiring, concentrating especially on using the COM port to interface data acquisition instruments to the PC. Don't forget that Monitor subscribers can download free RS232 driver software and we also offer free RS232 trouble-shooting software (ComDebug). The RS232 standard defines how computers (it calls them Data Terminal Equipment or DTEs) connect to modems (it calls them Data Communication Equipment or DCEs). It says that computers should be fitted with a 25-way plug whilst modems should have a 25-way D socket. The interconnecting lead between a computer and a modem should be simply pin1-pin1, pin2-pin2, etc. However, the RS232 plug on the PC (known as the COM port) doesn't only let you connect modems: it is also used for interfacing a vast array of data acquisition instruments, including electronic balances, GPS receivers and temperature indicators. The PC's COM port has deviated from the standard and is now usually a 9-pin plug. The pin connections are: Pin DTE Signal Direction 1 Input DCD Data Carrier Detect 2 Input RXD Received Data 3 Output TXD Transmitted Data 4 Output DTR Data Terminal Ready 5 Signal Ground 6 Input DSR Data Set Ready 7 Output RTS Request To Send 8 Input CTS Clear To Send 9 Input RI Ring Indicator Each pin is either an input or an output - carrying information into or out from the computer. Note that a signal that is an output from a computer is an input to a serial instrument, and vice versa. For DCE (modem-type instruments), therefore, the signals are on the same pins but the directions are reversed. Pin 3, for example, is Transmitted Data (TXD) but is an input. This means that you can never tell from the signal name alone whether it is an input to or an output from a particular piece of equipment. Be careful when looking at signal names: we have encountered instrument manufacturers who change the names - after all it must be confusing to tell a user that Received Data is an output. To complicate things further, although the modems of the standard always have DCE connections, a data acquisition instrument might instead be configured as DTE (computer- type). In this case, instead of connecting wires pin1- pin1, pin2-pin2, etc, some will have to cross over. Pin 3 of the computer, Transmitted Data, will need to be linked to pin 2 of the instrument, Received Data. Similarly wires between pins 7 and 8, RTS and CTS, need to cross and those between pins 1 and 4, DCD and DTR. This is achieved either with a Null Modem Adaptor or a non-standard RS232 cable. * SIGNAL FUNCTIONS AND NAMES ========================== As we've seen, each signal has a name reflecting its function. TXD Transmitted Data This carries the serial encoded data from the computer to the serial instrument. It is an output from DTE (computer-type) and an input to DCE (modem-type). RXD Received Data This carries the serial data from the instrument to the computer. It is an input to DTE and an output from DCE. DSR Data Set Ready This may be set true by an instrument whenever it is powered on. It can be read by the computer to determine that it is on line. It is an input to DTE and an output from DCE. DTR Data Terminal Ready This may be set true by a computer whenever it is powered on. It can be read by the data acquisition instrument to determine that the computer is on line. It is an output from DTE and an input to DCE. RTS Request to Send This may be set true by a computer when it wishes to transmit data. It is an output from DTE and an input to DCE. CTS Clear To Send This may be set true by an instrument to allow the computer to transmit data. The standard envisaged that when a computer wished to transmit data it would set its RTS. The local modem would then arbitrate with the distant modem for use of the telephone line. If it succeeded it would set CTS and the computer would transmit data. The distant modem would use its CTS to prevent any transmission by the distant computer. It is an input to DTE and an output from DCE. DCD Data Carrier Detect In the standard, this is set true by a modem when it detects the data carrier signal on the telephone line. It is an input to DTE and an output from DCE. For data acquisition applications, only 3 signal lines are absolutely necessary: TXD, RXD and Signal Ground. If you suspect a wiring problem between your serial instrument and the PC, use the Windmill ComDebug software as a terminal emulator. This has several trouble-shooting features to help you identify and rectify communication problems. For example, it lets you see the state of the input signals to the computer and control the state of the output signals. To download your free copy go to * Further Reading =============== On the Windmill Site: Free RS232 Driver Software Free RS232 Troubleshooting Software Issue 15 of Monitor: What is RS232? Books: Serial Port Complete, Jan Axelson, Lakeview Research 1998. Available from and ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see 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 and an index of articles at 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]


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