SP508E Datasheet, PDF(21/29 Page) Exar Corporation – Rugged 20Mbps, 8 Channel Multi-Protocol Transceiver with Programmable DCE/DTE and Termination Resistors

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SP508E Datasheet, PDF (21/29 Pages) Exar Corporation – Rugged 20Mbps, 8 Channel Multi-Protocol Transceiver with Programmable DCE/DTE and Termination Resistors

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The fourth type of drivers are V.35 differential

drivers. There are only three available on the

SP508E for data and clock (TxD, TxCE, and

TxC in DCE mode). These drivers are current

sources that drive loop current through

a differential pair resulting in a 550mV

differential voltage at the receiver. These

drivers also incorporate fixed termination

networks for each driver in order to set the

VOH and VOL depending on load conditions.

This termination network is basically a âYâ

configuration consisting of two 51â¦ resistors

connected in series and a 124â¦ resistor

connected between the two 50â¦ resistors

and a V35TGND output. Each of the three

drivers and its associated termination will

have its own V35TGND output for grounding

convenience. Filtering can be done on

these pins to reduce common mode noise

transmitted over the transmission line by

connecting a capacitor to ground.

The drivers also have separate enable pins

which simplifies half-duplex configurations

for some applications, especially program-

mable DTE/DCE. The enable pins will either

enable or disable the output of the drivers

according to the appropriate active logic

illustrated on Figure 45. The enable pins

have internal pull-up and pull-down devices,

depending on the active polarity of the re-

ceiver, that enable the driver upon power-on

if the enable lines are left floating. During

disabled conditions, the driver outputs will

be at a high impedance 3-state.

The driver inputs are both TTL and CMOS

compatible.All driver inputs have an internal

pull-up resistor so that the output will be at

a defined state at logic LOW (â0â). Unused

driver inputs can be left floating. The inter-

nal pull-up resistor value is approximately

500kâ¦.

Receivers

The SP508E has eight enhanced inde-

pendent receivers. Control for the mode

selection is done via a three-bit con-

trol word that is the same as the driver

control word. Therefore, the modes for

the drivers and receivers are identical in the

application.

Like the drivers, the receivers are prear-

ranged for the specific requirements of

the synchronous serial interface. As the

operating mode of the receivers is changed,

the electrical characteristics will change

to support the required serial interface

protocols of the receivers. Table 2 shows

the mode of each receiver in the different

interface modes that can be selected.

There are two basic types of receiver cir-

cuitsâITU-T-V .28 (RS-232) and ITU-T-V.11,

(RS-422).

The RS-232 (V.28) receiver is single-ended

and accepts RS-232 signals from the RS-

232 driver. The RS-232 receiver has an

operating input voltage range of +15V and

can receive signals downs to +3V. The input

sensitivity complieswith RS-232 and V .28

at +3V. The input impedance is 3kâ¦ to 7kâ¦

in accordance to RS-232 and V .28.The

receiver output produces a TTL/CMOS signal

with a +2.4V minimumfor a logic â1â and a

+0.4V maximum for a logic â0â. The RS-232

(V.28) protocol uses these receivers for all

data, clock and control signals. They are also

used in V.35 mode for control line signals:

CTS, DSR, LL, andRL.The RS-232 receiv-

ers can operate over 120kbps.

The second type of receiver is a differential

type that can be configured internally to

support ITU-T-V.10 and CCITT-V.35

depending on its input conditions. This

receiver has a typical input impedance

of 10kâ¦ and a differential threshold of

less than +200mV, which complies with

the ITU-T-V.11 (RS-422) specifications.

V.11receivers are used in RS-449/V.36,

EIA-530, EIA-530A and X.21 as Category I

signals for receiving clock, data, and some

control line signals not covered byCategory

II V.10 circuits.The differential V.11 trans-

ceiver has improved architecture that allows

over 20Mbps transmission rates.

Receivers dedicated for data and clock (RxD,

RxC, TxC) incorporate internal termination

for V.11. The termination resistor is typically

120â¦ connected between the A and B inputs.

The termination is essential for minimizing

crosstalk and signal reflection over the

transmission line . The minimum value is

guaranteed to exceed 100â¦, thus complying

with the V.11 and RS-422 specifications.

Exar Corporation 48720 Kato Road, Fremont CA, 94538 â¢ 510-668-7017 â¢ www.exar.comSP508E_100_072412

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