SP505ACF datasheet(21/36 Pages) SIPEX

21 / 36 page

Rev. 7/9/03

SP505 Multi–Mode Serial Transceiver

serial interface. As the operating mode of the

receivers is changed, the electrical characteris-

tics 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 circuits

— V.28, V.10, and V.11.

V.28 Receivers

The V.28 receiver is single–ended and accepts

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

receiver has an operating voltage range of +15V

and can receive signals down to +3V. The input

sensitivity complies with RS-232 and V.28 speci-

fications at +3V. The input impedance is 3k

Ω to

Ω in accordance to RS-232 and V.28 over a

+15V input range. The receiver output pro-

duces a TTL/CMOS signal with a +2.4V mini-

mum for a logic "1" and a +0.8V maximum for

a logic "0". V.28 receivers are used in RS-232

mode 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 V.28

receivers can operate to at least 120kbps.

V.10 Receivers

The V.10 receivers are also single–ended as

with the V.28 receivers but have an input thresh-

old as low as +200mV. The input impedance is

guaranteed to be greater than 4K

Ω, with an

operating voltage range of +7V. The V.10 re-

ceivers can operate to at least 120kbps. V.10

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.11 and RS-485 signals. This re-

ceiver has a typical input impedance of 10k

Ω

and a typical differential threshold of +200mV,

which complies with the V.11 specification.

Since the characteristics of the V.11 receivers

are actually subsets of RS-485, the V.11 receiv-

ers can accept RS-485 signals. However, these

receivers cannot support 32-transceivers on the

signal bus due to the lower input impedance as

specified in the RS-485 specification. Three

receivers (RxD, RxC, and SCT) include a typi-

cal 120

Ω cable termination resistor across the A

and B inputs. The resistor for the three receivers

is switched on when the SP505 is configured in

a mode which uses V.11 receivers. The V.11

cable termination resistor is switched off when

the receiver is disabled or in another operating

mode not using V.11 receivers. The V.11 re-

ceivers are used in X.21, 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.10 circuits.

The differential receivers can receive signals

over 10Mbps.

V.35 Receiver

The V.11 receivers are also used for the V.35

mode. Unlike the older implementations of

differential receivers used for V.35, the SP505

contains an internal resistor termination net-

work that ensures a V.35 input impedance of

100

Ω (+10Ω) and a short-circuit impedance of

150

Ω (+15Ω). The traditional V.35 implemen-

tations required external termination resistors to

achieve the proper V.35 impedances. The inter-

nal network is connected via low on-resistance

FET switches when the decoder is changed to

V.35 mode. These FET switches can accept

input signals of up to +15V 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 de-

coder or receiver enable pin. The termination

network is transparent when all other modes are

selected. The V.35 receivers can operate over

10Mbps.

Receiver Enable and Output

Only one receiver includes an enable line. The

SCTEN input for the SCT receiver can enable or

tri-state the output of the receiver. When the pin

is at a logic "0", the receiver output is high

impedance and any input termination internal

connected is switched off. The inputs will be at

approximately 10k

Ω during tri-state.

V.11 TERMINATION

MODE [0100]

V.35 MODE

IN [a]

IN [b]

To Non-Inverting

Input of Receiver

To Inverting Input

of Receiver

ON = 20Ω

ON = 1Ω

51Ω

124Ω

ON = 1Ω

Figure 51. Simplified R

IN Termination Circuit

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