LTC2876 datasheet(22/32 Pages) LINER

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LTC2876/LTC2877

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BUS PINS PA & PB NAMING CONVENTION

PROFIBUScommunicateswithRS485signalingthrougha

differential signal interface. These wires are labeled A and

B. The PROFIBUS standard specifies that the bus wire B

takes on a positive value with respect to bus wire A when

no station is transmitting data (during the idle periods).

However, the polarity convention of most RS485 devices

uses the opposite convention. That is, with no transmis-

sion on the bus, the receiver reports a logic value that

would result if A were higher than B—in this case a high

on RO. From a practical standpoint, this means that if a

general RS485 transceiver is connected to a PROFIBUS

network, the transceiver’s A pin must connect to the B

wire and the B pin connect to the A wire. Certainly this

can be confusing!

Since the LTC2876/LTC2877 was designed specifically

for PROFIBUS applications, the pin naming convention

was made to match the PROFIBUS specification. To avoid

confusion with other RS485 transceivers, the prefix “P”

was added, meaning “PROFIBUS.” If driver and receiver

are enabled, a high level on DI, will drive the bus lines so

that PB is higher than PA and the receiver will report a

high level on RO.

In PROFIBUS installations, connect PB to the B wire (red)

and PA to the A wire (green). For non-PROFIBUS RS485

applications,thePBpinshouldbeconnectedtotheAsignal

and PA pin should be connect to the B signal to match the

convention of most other RS485 devices.

PROFIBUS CABLES

It is recommended that PROFIBUS installations use cable

designed for PROFIBUS applications. Typically, Type A

cable and termination is used. This is a shielded twisted

pair with the following properties:

Table 1. PROFIBUS Type-A Cable Properties

PROPERTY

VALUE

Impedance

135Ω to 165Ω

Capacitance

< 30pF/m

Loop Resistance

< 110Ω/km

Conductor Area

≥ 0.34mm2

Color of Sheath (Non-IS)

Violet

Color of Inner Conductor A

Green

Color of Inner Conductor B

Red

The three resistors that make up the termination network

should be placed at both ends of the bus and must be

powered during operation. If there are multiple nodes

communicating on the bus, only the nodes at the ends

should be terminated.

The cable shield helps to improve electromagnetic com-

patibility (EMC). It is recommended to ground both ends

of the shield, through the case of the connector, to the

chassis of the connected station. In applications where

ground potential differences exist between stations, for

example long distance transmission between buildings,

the shield should be grounded only at one end of the cable.

If the potential difference exceeds several volts, galvanic

isolation is recommended at one or more of the con-

nectedstations.Inthiscase,considerusingtheLTM®2892

µModuleisolator(see3500VRMSIsolatedPROFIBUSNode

with Termination on the last page).

If the shield cannot be grounded through the connector

case, pin 1 of the D-sub connector can be used as an

alternative, although the added inductance makes this

sub-optimal. In such a case, it is better to bare the cable

shield at an appropriate point and ground it with a short

cable or clamp to the metallic structure of the station.

Unshielded cable can be used in PROFIBUS installations

if there is no severe electromagnetic interference (EMI).

Do not use cables that are untwisted pairs.

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