Acces I/O products USB-FLEXCOM4 User Manual
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Contents
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Manual USB-FLEXCOM418Appendix A: Application ConsiderationsIntroductionWorking with RS422 and RS485 devices is not much different from working with standardRS232 serial devices and these two standards overcome deficiencies in the RS232standard. First, the cable length between two RS232 devices must be short; less than 50feet at 9600 baud. Second, many RS232 errors are the result of noise induced on thecables. The RS422 and RS485 standards permit cabling up to 5000 feet and since itoperates in differential mode, is more immune to induced noise.Connections between two RS422 devices (with CTS ignored) should be as follows:Device #1 Device #2Signal Pin No. Signal Pin No.Gnd 5 Gnd 5TX+ 2 RX+ 9TX- 3 RX- 1RX+ 9 TX+ 2RX- 1 TX- 3Table A-1: Connections Between Two RS422 DevicesA third deficiency of RS232 is that more than two devices cannot share the same cable. Thisis also true for RS422 but RS485 offers all the benefits of RS422 plus allows up to 32devices to share the same twisted pairs. An exception to the foregoing is that multipleRS422 devices can share a single cable if only one will talk and the others will all receive.SERIAL PORT CONNECTOR CABLE TO RS-485 DEVICESignal Pin No. Signal Pin No.Tx/Rx+ 2 Tx/Rx + 2Tx/Rx - 3 Tx/Rx - 3100 Ω toGround 5 100 Ω toGround 5Table A-2: RS485 Data Cable Wiring |
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