SP508CF datasheet(22/30 Pages) SIPEX

22 / 30 page

Date: 8/19/04

SP508 Enhanced WAN Multi–Protocol Serial Transceiver

The fourth type of drivers are V.35 differential

drivers. There are only three available on the

SP508 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 V

OH and VOL depending on load conditions.

This termination network is basically a “Y”

configuration consisting of two 51

Ω resistors

connected in series and a 124

Ω resistor connected

between the two 50

Ω 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 programmable

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 receiver, 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 or CMOS

compatible. All driver inputs have an internal

pull-up resistor so that the output will be at a

defined state at logic LOW (“0”). Unused driver

inputs can be left floating. The internal pull-up

resistor value is approximately 500k

Ω.

Receivers

The SP508 has eight enhanced independent

receivers. Control for the mode selection is done

via a three-bit control 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 prearranged

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 circuits—ITU-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 complies

with RS-232 and V .28 at +3V. The input

impedance is 3k

Ω to 7kΩ in accordance to RS-

232 and V .28. The receiver output produces a

TTL/CMOS signal with a +2.4V minimum for

a logic “1” and a +0.4V maximum for a logic “0”.

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, and RL. The RS-232 receivers

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 10k

Ω and a differential

threshold of less than +200mV, which complies

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

V.11 receivers 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 by Category II V.10

circuits. The differential V.11 transceiver 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 120

Ω

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

100

Ω, thus complying with the V.11 and RS-422

specifications. This resistor is invoked when the

receiver is operating as a V.11 receiver, in modes

EIA-530, EIA-530A, RS-449/V.36, and X.21.

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