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RS-485 Application Cheat Sheet

10/22/2018

Use It Anywhere 

The EIA/TIA RS-485 communications standard, an upgrade of RS-422, supports 32 devices (driver/receiver pairs) in a party line or multi-drop mode, on a cable of up to 1220 meters (4000 ft) for balanced differential signal transmissions at a common-mode voltage (Vcm) of -7 to +12 V. 

You can internally or externally configure RS-485 devices. Four-wire connections, which require an additional ground, require a “master” node (e.g., a PC) that communicates to all others, called “slaves,” each which, in turn, can only communicate with the master. 

To Terminate, or Not to Terminate? 

The RS-485 specification advices using termination. For high baud rates and long cable runs, this is true. In most equipment though, with maximum speeds of 115kbps, it is unnecessary. Adding termination dramatically increases power consumption and requires that the network be rebiased, which is rarely done. Termination complicates system design and rarely solves problems when used in the kilobit data range. 

Extend the Network Easily 

By adding repeaters, you get longer distances and each “refreshed” signal can drive another 1220 meters (4000 ft) of cable – and 31 more RS-485 loads (driver/receiver pairs) per repeater. 

Isolation Peace of Mind

Long networks are especially vulnerable to grounding and surge problems. This is easily addressed by isolating the nodes. Use optically isolated repeaters and isolated converters to attach the nodes of your network and you will have reliable long-distance applications. 

Tips for Best Performance 

  • Check the converter’s data sheet to see how the receiver’s “enable” function is connected. 
  • Test the interval after the last bit is transmitted to ensure complete transmission. A too-short interval causes missed parts of each character being sent. A too-long interval may cause the system to switch the data line from Transmit to Receive.
  • Select appropriate isolation or shunting for protection against surges. When in doubt – isolate! Add a fuse-type device to shunting-type suppression to protect against short circuits to power conductors. 
  • Use appropriate signal grounds (a must-have) and shielded cable (desirable) for safety. 
  • Check signal types and related issues before writing or purchasing software protocols. 
  • Device communication characteristics must be checked before completing system design. 
  • Get a schematic of each serial port to assist in troubleshooting and repairs.

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