LTC2858 datasheet(15/32 Pages) LINER

15 / 32 page

LTC2876/LTC2877

28767fa

For more information www.linear.com/LTC2876

Note:Specificationsinthissectionrepresenttypicalvalues

unless otherwise noted.

PROFIBUS-DP AND RS485

PROFIBUS-DP can communicate over a variety of media,

including copper wires, fiber optics, and even air in an

infrared communicator. By far, the most commonly used

media is a twisted pair of wires connecting devices that

communicate with TIA/EIA-485-A (RS485) transceivers.

RS485 offers high speed differential signaling that is

robust for communication between multiple devices over

long distances in noisy environments such as factory

applications.

Not All RS485 Transceivers Are Suitable for PROFIBUS

Although the PROFIBUS standard specifies the use of

RS485 devices at the physical layer, there are differences

in the cable, termination, and driver requirements from

RS485. A device meeting RS485 specifications may not

applicaTions inForMaTion

be compliant to PROFIBUS requirements. The LTC2876/

LTC2877wasdesignedspecificallytomeetPROFIBUSand

RS485requirementsandistestedinawaythatensuresthis.

Cable and Termination Differences from RS485

The cable and termination network used for PROFIBUS is

different than for RS485 as illustrated in Figure 10. The

PROFIBUS network includes bus biasing resistors that

are used in conjunction with the differential termination

resistors on each end of the bus. The cable is a shielded

twisted pair with an impedance of 150Ω. Oddly enough,

theeffectivedifferentialresistanceofthespecifiedtermina-

tion network is 172Ω, which is not a perfect match for the

150Ω cable, resulting in a slightly underdamped network.

This manifests itself as a small bump, or increase in the

signal voltage, at the receiving end of the cable, lasting

twice as long as the cable propagation delay.

In contrast, the RS485 network shows the preferred

configuration with only differential termination resistors

at each end of the bus, matching the 120Ω characteristic

impedance of the cable.

Figure 10. Cable and Termination Differences in RS485 and PROFIBUS Multi-Node Networks.

PROFIBUS Type A Cable and Termination Shown in PROFIBUS Example (Top)

Proﬁbus Multi-Node Network

Twisted Pair Cable (ZO = 150Ω)

RS485 Multi-Node Network

Twisted Pair Cable (ZO = 120Ω)

390

220

390

PROFIBUS

MASTER

PROFIBUS

STATION

PROFIBUS

STATION

PROFIBUS

STATION

390

220

390

120

RS485

MASTER

RS485

NODE

RS485

NODE

RS485

NODE

120

28767 F10

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