2

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SP505_00_08308

mode selection is done via a 4–bit control

word, which is the same as the driver's 4-bit

control word.

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 requirements of clock, data,

and control line receivers. Table 2 shows

the mode of each receiver in the different

interface modes that can be selected.

There are three basic types of receiver cir-

cuits — V.28, V.0, and V..

V.28 Receivers

The V.28 receiver is single–ended and ac-

cepts V.28 signals from the V.28 driver. The

V.28receiverhasanoperatingvoltagerange

of+5Vandcanreceivesignalsdownto +3V.

The input sensitivity complies with RS-232

and V.28 specifications at +3V. The input

impedance is 3kΩ to 7kΩ in accordance to

RS-232 and V.28 over a +5V input range.

The receiver output produces a TTL/CMOS

signal with a +2.4V minimum for a logic ""

and a +0.8V maximum for a logic "0". V.28

receivers are used in RS-232 mode for all

data,clockandcontrolsignals. Theyarealso

used in V.35 mode for control line signals:

CTS, DSR, LL, and RL. The V.28 receivers

can operate to at least 20kbps.

V.10 Receivers

The V.0 receivers are also single–ended

as with the V.28 receivers but have an in-

put threshold as low as +200mV. The input

impedance is guaranteed to be greater than

4KΩ, with an operating voltage range of +7V.

The V.0 receivers can operate to at least

20kbps. V.0 receivers are used in RS-

449, EIA-530, EIA-530A and V.36 modes

as Category II signals as indicated by their

corresponding specifications.

V.11 Receivers

The third type of receiver is a differential

which supports V. and RS-485 signals.

This receiver has a typical input impedance

of 0kΩ and a typical differential threshold

of +200mV, which complies with the V.

specification.Sincethecharacteristicsofthe

V. receivers are actually subsets of RS-

485, the V. receivers can accept RS-485

signals. However, these receivers cannot

support32-transceiversonthesignalbusdue

to the lower input impedance as specified in

the RS-485 specification. Three receivers

(RxD, RxC, and SCT) include a typical 20Ω

cable termination resistor across theAand B

inputs. The resistor for the three receivers is

switched on when the SP505 is configured

in a mode which uses V. receivers. The

V.cableterminationresistorisswitchedoff

when the receiver is disabled or in another

operating mode not using V. receivers.

TheV.receiversareusedinX.2,RS-449,

EIA-530, EIA-530A and V.36 as Category I

signals for receiving clock, data, and some

control line signals not covered by Category

II V.0 circuits. The differential receivers can

receive signals over 0Mbps.

V.35 Receiver

The V. receivers are also used for the

V.35 mode. Unlike the older implementa-

tions of differential receivers used for V.35,

the SP505 contains an internal resistor

termination network that ensures a V.35

input impedance of 00Ω (+0Ω) and a

short-circuitimpedanceof50Ω(+5Ω). The

traditional V.35 implementations required

external termination resistors to achieve

the proper V.35 impedances. The internal

network is connected via low on-resistance

FET switches when the decoder is changed

to V.35 mode. These FET switches can ac-

cept input signals of up to +5V without any

forward biasing and other parasitic affects.

The V.35 termination resistor network is

switched off when the receiver is disabled

either by the decoder or receiver enable

pin. The termination network is transparent

when all other modes are selected. The V.35

receivers can operate over 0Mbps.

V.11 TERMINA TION

MODE [0100]

V.35 MODE

To Non-Inverting

Input of R eceiver

To Inverting Input

of R eceiver

51Ω

51Ω

124Ω

Figure 51. Simplified R