Top 10 RS-485 Design Tips
Top 10 RS-485 Design Tips Делiрr R RE DE D R RE DE D Determine maximum cable length Make stub length no longer than LStub < 3-10-4 . tr v LStub = stub length (m) tr= driver rise time (ns) V = signal velocity (%) 5V 3V 6 with Chart A Master Node 1 Slave Node n Use twisted-pair cable with Zo = 1200 or 1002 AJB AB GPD AB JAJB %3D 7 For +7V GPDS use Lstub RFs 1 Connect bus nodes via 4 standard transceivers daisy-chain RT23 RT1 For +20V GPDS use SN65HVD17XX For higher GPDS use isolated transceivers 4. Terminate one cable end with RFS: LBus RT1=ZO Apply failsafe biasing to the other end 8 Terminate unused conductors with R RE DED Vor min CC Res= +1 •k R-Z, to their local grounds VAR-Idle k=27.80 for Zo =1200 k=23.40 for Zo = 1000 AB 104 5% 8. Jitter pooooo0 RT RT 103 5V For ESD, EFT and surge protection use SM712 RT RT Terminate this end with 10 2Rs Z FS 10 10 10 10 10° 10° 107 2Rs-Z, 10 Limit clamping current into the transceiver with 100 pulse-proof or MELF resistors Data Rate - bps Chart A You can operate 3V and 5V devices on the same bus I TEXAS INSTRUMENTS A standard of many names RS-485 is now deemed EIA/TIA-485, but you may also see TIA-485-A, ANSI/TIA/EIA-485 or EIA-485. Keep that straight! Why is RS-485 so cool? It enables data transmission speeds of 35 Mbit/s up to 10 m and 100 kbit/s at 1,200 m and can span relatively large distances – up to 4,000 feet! Purpose-built for industrial Applications include: programmable logic controllers, motors, welding equipment, building autonmation and video security. Rule of thumb The speed in bit/s multiplied by the length in meters should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. For more industrial analog design resources, please go to ti.com/analog. 10 2. Cable Length - m
Top 10 RS-485 Design Tips
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