LTC2876 datasheet(16/32 Pages) LINER

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LTC2876/LTC2877

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Driver Output Requirement Differences from RS485

The driver requirements for PROFIBUS are specified dif-

ferently than how the RS485 standard specifies them. A

key difference is the terminated driver output voltage, VOD,

as described below.

The PROFIBUS driver output levels are required to meet

the following condition as stated in the “Test Specification

for PROFIBUS DP Masters” and “Test Specification for

PROFIBUS DP Slaves”:

• The differential voltage between A- and B-line shall be

a minimum of 4V and a maximum of 7V, peak-to-peak

differential.

• This measurement shall be taken at the far end of the

cable in use, with termination at each end.

On the other hand, RS485 specifies the following:

• The differential voltage between A- and B-line shall be

a minimum of 1.5V and a maximum of 5V, peak dif-

ferential.

• This measurement shall be taken at the driver terminals

with a 54Ω resistor between A and B.

Clearly, these requirements are quite different. A common

misunderstandingisthatifanRS485driverdevelopsmore

than 2.1V across a 54Ω RS485 resistive load, then it will

meet PROFIBUS requirements when used with a PRO-

FIBUS termination network. This is not always the case.

Furthermore, the strength of the driver can be too high,

exceeding the upper limit of the PROFIBUS Specification

(7VP-P). The best way to ensure PROFIBUS compliance

is to test the device with a PROFIBUS load.

TheLTC2876andLTC2877aretestedwithaPROFIBUSload

and with extra resistance added to represent cable losses

for 100m and 200m to ensure they meet the PROFIBUS

VOD requirement. The devices are also fully tested with

RS485 loads to ensure they meet RS485 specifications.

See the Electrical Characteristics section for details.

DRIVER OPERATION

The driver is enabled when the LTC2876/LTC2877 is

powered up, DE is high, and there are no thermal faults.

The polarity of PB–PA follows that of DI. That is, when DI

is high, PB drives to a voltage that is greater than PA. If DI

is low, PA is higher than PB. When the driver is disabled

with DE low, both outputs are high impedance and the

overall pin resistance is dominated by the receiver inputs

sharing pins PA and PB.

DRIVER OVERVOLTAGE AND OVERCURRENT

PROTECTION

The driver outputs PA and PB are protected from short

circuits to any voltage within the absolute maximumrange

of –60V to +60V, with a maximum differential voltage

of –120V to +120V. The maximum short-circuit current

to any voltage within this range is ±250mA. The driver

includes a progressive foldback current limiting circuit

that continuously reduces the driver current limit with

increasing output short circuit voltage to better manage

power dissipation and heating effects.

The LTC2876/LTC2877 also features thermal shutdown

protection that disables the driver and receiver in case of

excessive power dissipation (see Note 3).

RECEIVER

The receiver provides full PROFIBUS and RS485 compat-

ibility. When enabled, the state of RO reflects the polarity

of (PB–PA). When the receiver is disabled, the output is

high impedance and RO weakly pulled high through an

internal 250k pull-up resistor.

High Receiver Input Resistance Permits 200 Nodes

The RS485 and PROFIBUS specifications allows for up to

32 receivers, each contributing one unit load, to be con-

nected together in one network. The input resistance of

the LTC2876/LTC2877 is guaranteed to be at least 6.25

times higher, and drawing proportionally less current,

than a standard RS485 load, permitting a total of 200

receivers per contiguous network. The input load of the

receiver is unaffected by enabling/disabling the receiver

or by powering/depowering the device.

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