You've plugged your instrument into your computer's COM
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For example, when there is no hardware handshaking in place the connections are as follows.
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When the PC is connected to a DTE instrument - called a null modem arrangement - some of the cable wires must cross over. In this case you need an adaptor or the RS232 cable wired differently to normal. Instead of being a DCE device (see above for details), a data acquisition device might be configured as DTE.
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This is set true by a modem when it detects the data carrier signal on the telephone line.Ī nine pin D plug has become the standard fitting for the serial ports of PCs, although it's nothing to do with the RS232 standard.
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The distant modem would use its CTS to prevent any transmission by the distant computer.ĭCD Data Carrier Detect, Pin 8 of 25 Way D If it succeeded it would set CTS and the computer would transmit data. The local modem would then arbitrate with the distant modem for use of the telephone line. The standard envisaged that when a computer wished to transmit data it would set its RTS. This is set true by a device to allow the computer to transmit data. This is set true by a computer when it wishes to transmit data. It can be read by the device to determine that the computer is on line. This should be set true by a computer whenever it is powered on. It can be read by the computer to determine that the device is on line.ĭTR Data Terminal Ready, Pin 20 of 25 way D This should be set true by a device whenever it is powered on. This is the serial encoded data received by a computer from a device. This is the serial encoded data sent from a computer to a device. Also, instead of being a DCE device, a data acquisition device might be configured as DTE. This means that you can never tell from the signal name alone whether it is an input or an output from a particular piece of equipment. It is important to note that a signal which is an output from a computer is an input to a modem and vice versa. The main signals and their direction of flow are described below. The interconnecting lead between a computer and a modem should be simply pin1-pin1, pin2-pin2, etc. The standard says that computers should be fitted with a 25 way plug whilst modems should have a 25 way D socket. The standard defines how computers (it calls them Data Terminal Equipment or DTEs) connect to modems (it calls them Data Communication Equipment or DCEs). The authors of the standard had in mind the situation below: Any other use is outside of the standard. The standard was defined to connect computers to modems. The answer is that the RS232 standard was created for just one specific situation and the difficulties come when it is used for something else. If RS232 is a standard why can't I just use a standard lead to connect together two RS232 ports and expect them to talk to one another? That's a good question. The standard defines the electrical and mechanical characteristics of the connection - including the function of the signals and handshake pins, the voltage levels and maximum bit rate. Over the years this has been updated: the most commonly used version of the standard is RS232C (sometimes known as EIA232) the most recent is RS232E. In the 60's a standards committee now known as the Electronic Industries Association developed an interface to connect computer terminals to modems. The Ins and Outs of RS232 | Testing Serial Port Communication | Status of the COM Port Lines | DTR High: Removing Handshaking | Further Reading | Comments or Questions? Finally there are suggestions for further reading and you are invited to send us your comments or questions. The next section gives a step-by-step guide to getting communications working: testing the different elements of the system. It starts by discussing the RS232 standard and continues with the various pin connections available.
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This article tells you just about everything you need to know to get a RS232 connection working between your computer and your measuring device or instrument.