High-speed connections require RS-485

-October 10, 2016

 

Six general guidelines should be applied:

 
  1. Use the smallest size possible for signal trace vias and connector pads to minimize their impact on differential impedance.
  2. Use solid power and ground planes for impedance control and for minimum noise on the transceiver’s power supply lines.
  3. Keep the trace electrical length between the RS-485 connector and the transceiver as short as possible to minimize attenuation and reflection.
  4. Place bulk capacitors (e.g., 10μF) close to power sources such as voltage regulators, or place them where the power is supplied to the PCB.
  5. Place small 0.1μF and 0.01μF decoupling capacitors at the transceiver VCC pin to provide a local source of charge for ICs requiring a significant amount of supply current in response to internal switching.
  6. If vias are required, use multiple vias when connecting Vcc, GND, decoupling capacitors, and TVS diodes to high-speed ICs.
 

In addition, the following recommendations on cable selection, bus termination and managing stubs help complete the design from a system perspective.

 

Transmission cable

 

RS-485 recommends the use of unshielded twisted pair (UTP) cable with a characteristic impedance of Z0 = 120Ω nominal. Cables commonly used are either dedicated single-pair RS-485 cables with Z0 = 120Ω or CAT-5 cable with four signal pairs and Z0 = 100Ω.

 

When using CAT-5 cable for single-pair applications, the three unused signal pairs should be properly terminated to ground with RT = 100Ω at both cable ends. This prevents noise coupling from the unused wires into the data pair.

 

Note that when selecting multi-pair cable other than CAT-5, so-called “no-skew cable” should not be used as it is designed for analog signals with low or zero harmonic content. When used with data signals, this type of cable will result in large crosstalk and data errors.

 

Bus termination

 

Because the RS-485 standard allows for two termination resistors, high-speed data lines always should use termination resistors, RT, at both ends of the data link. The value of RT should match the characteristic impedance of the cable, Z0, or of the controlled-impedance transmission lines on a circuit board: RT = Z0 = 100Ω.

 

Stub length

 

The connection between the transceiver and the main data cable, known as a stub, is left unterminated to avoid excessive bus loading. On the other hand, stub lengths should be kept short to prevent the build-up of signal reflections. Daisy-chaining cable connections effectively reduces stub length to the length of the PCB trace from transceiver to connector, as calculated using the simple equation or field-solver software used for node design

   

Conclusion

 

In conjunction with sound design principles applied at board and system level, transceivers that offer high output drive capability and minimal skew at both 3.3V and 5V supplies help designers overcome the signal degradation issues that challenge RS-485 high-speed communication links.

 

References

  1. Find out more about the ISL3179E.

Also see:

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