COM20051 datasheet(21/82 Pages) SMSC

21 / 82 page

Since the Backplane Configuration encodes data

differently

than

the

traditional

Hybrid

Configuration, nodes utilizing the Backplane

Configuration cannot communicate directly with

nodes

utilizing

the

Traditional

Hybrid

Configuration.

The Backplane Configuration does not isolate

the node from the media nor protect it from

Common Mode noise, but Common Mode Noise

is less of a problem in short distances.

The COM20051 supplies a programmable output

driver

for

Backplane

Mode

operation.

push/pull or open drain driver can be selected by

programming the P1MODE bit of the Setup

The COM20051 defaults to an open drain output.

The Backplane Configuration provides for direct

connection between the COM20051 and the

physical medium in open drain configuration of

the output driver. Only one pull-up resistor (for

open drain only) is required somewhere on the

media (not on each individual node).

The

nPULSE1 signal, in this mode, is an open drain

or push/pull driver and is used to directly drive

the media. It issues a 200nS negative pulse to

transmit a logic "1". Note that when used in the

open-drain mode, the COM20051 does not have

a fail/safe input on the RXIN pin.

The nPULSE1 signal actually contains a weak

pull-up resistor. This pull-up should not take the

place of the resistor required on the media for

open drain mode.

In typical applications, the

serial backplane is terminated at both ends and a

bias is provided by the external pull-up resistor.

The RXIN signal is directly connected to the

cable via an internal Schmitt trigger. A negative

pulse on this input indicates a logic "1". Lack of

pulse indicates a logic "0".

For typical single-

ended

backplane

applications, RXIN

connected to nPULSE1 to make the serial

backplane data line.

A ground line (from the

coax or twisted pair) should run in parallel with

the signal.

For applications requiring different

treatment of the receive signal (like filtering or

squelching), nPULSE1 and RXIN remain as

independent

pins.

External

differential

drivers/receivers

for

increased

range

and

common mode noise rejection, for example,

would require the signals to be independent of

one another. When the device is in Backplane

Mode, the clock provided by the nPULSE2 signal

may be used for encoding the data into a

different encoding scheme or other synchronous

operations needed on the serial data stream.

Differential Driver Configuration

The Differential Driver Configuration is a special

case of the Backplane Mode. It is a DC coupled

configuration recommended for applications like

car-area networks (CAN) or other cost-sensitive

applications

which

not

require

direct

compatibility with existing ARCNET nodes and

do not require isolation. Figure 7 illustrates this

configuration.

The Differential Driver Configuration cannot

communicate directly with nodes utilizing the

Traditional

Hybrid

Configuration.

the

Backplane Configuration, the Differential Driver

Configuration does not isolate the node from the

physical medium.

The Differential Driver interface includes a

RS485 Driver/Receiver to transfer the data

between the cable and the COM20051. The

nPULSE1 signal transmits the data, provided the

nTXEN signal is active. The

nPULSE1 signal

issues a 200nS negative pulse to transmit a logic

"1".

The RXIN pin receives

the data.

negative pulse on this input indicates a logic "1".

Lack of pulse indicates a logic "0".

The

transmitter portion of the COM20051 is disabled

during reset and the nPULSE1, nPULSE2 and

nTXEN pins are inactive.

COM20051 数据表(PDF) 21 Page - SMSC Corporation

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