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| RS-485 Troubleshooting Guide-1/3 © 2002 by B&B Electronics. All rights reserved. | |||
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| co od co | troubleshooting guide for rs-485 B&B Electronics You can avoid many pitfalls and save troubleshooting by paying attention to some fundamentals. Avoiding Pitfalls The best approach is preventive: Determine a device's communications characteristics before system design is complete—and then ask the right questions during system design: 1. Is this device configured for two-wire or four-wire systems? 2. Is a signal ground connection available? 3. Is the device isolated? Does it include any surge suppression devices? 4. What value bias resistors, if any, are used in the device—and are they accessible for modification? 5. Is the device terminated—and accessible for modification? 6. What is the device's response time ("turnaround delay")? 7. What is the programmable address range of the device? 8. What baud rate or range of baud rates is supported? Tristate Control Using RTS When you use an RS-485 converter with RTS control, set the RTS high before data is sent. Also, set the RTS line low after the last data bit is sent. When an RS-485 network is connected in a two-wire, multidrop, party-line mode, connect the receiver at each node to the line. Configure the receiver to get an echo of its own data transmission. This setup is desirable in some systems, but troublesome in others. So check the data sheet for your converter to determine how the receiver "enable" function is connected. Check the requirements of your application software to see if it supports RTS control. If in doubt, choose a converter with Send-Data control. Using Send-Data Control If Send-Data control is used, the converter automatically disables the transmit data line at a fixed interval after the last bit, typically one character length. Interval length is important. If too short, you can miss parts of each character being sent. If longer than the turnaround delay of the responding devices, your system may try to switch the data line from transmit to receive before the node with the Send Data converter is ready to receive data. In the latter case, you will miss portions, even complete characters, in the response. Is it a Two-Wire or Four-Wire System? RS-485 systems can be either two-wire or four-wire systems. The two-wire configuration—with the additional ground conductor here—reduces cabling cost, but is restricted to half-duplex communications (cannot receive and transmit at the same time). The majority of RS-485 devices are two-wire configurations. Devices configured for four-wire communications bring out A and B connections for both transmit and receive pairs. You can connect transmit lines to receive lines to create a two-wire configuration. The latter type device gives the most configuration flexibility. | ||
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| B&B Electronics Ltd Westlink Commerc ial Park Oranmore Co. Galway Ireland +353 91 792444 Fax+353 91 792445 www.bb-europe.com orders@bb-elec.com support@bb-europe.com | |||
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