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SP504

WAN Multi-Mode Serial Transceiver

Signal Processing Technologies 

Receivers

The SP504 has seven (7) independent receivers

which can be programmed for the different

interface modes. Control for the mode selection

is done via a 4–bit control word that is indepen-

dent from the driver control word. The coding

for the drivers and receivers is identical. There-

fore, if the modes for the drivers and receivers

are supposed to be identical in the application,

the control lines can be tied together.

Like the drivers, the receivers are pre-arranged

for the specific requirements of the interface. As

the operating mode of the receivers is changed,

the electrical characteristics will change to sup-

port 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

— RS-232, RS-423, and RS-485.

The RS-232 receiver is a single–ended input

with a threshold of 0.8V to 2.4V. The RS-232

receiver has an operating voltage range of ±15V

and can receive signals up to 120kbps. The

input sensitivity complies with EIA-RS-232 and

V.28 at +3V to -3V. The input impedance is

3kΩ to 7kΩ. RS-232 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 such as CTS and DSR.

The RS-423 receivers are also single–ended but

have an input threshold as low as ±200mV. The

input impedance is guaranteed to be greater than

4kΩ, with an operating voltage range of ±7V.

The RS-423 receivers can operate up to 120kbps.

RS-423 receivers are used in RS-449, EIA-530,

EIA-530A and V.36 modes as Category II sig-

nals as indicated by their corresponding specifi-

cations.

The third type of receiver is a differential which

supports RS-485. The RS-485 receiver has an

input impedance of 15kΩ and a differential

threshold of ±200mV. Since the characteristics

of an RS-422 (V.11) receiver are actually

subsets of RS-485, the receivers for RS-422

requirements are covered bytheRS-485receivers.

RS-422 receivers are used in RS-449,

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

signals for receiving clock, data, and some con-

trol line signals. The differential receivers can

receive data up to 10Mbps.

The RS-485 receivers are also used for the V.35

mode. Unlike the older implementations of

differential or V.35 receivers, the SP504 con-

tains an internal resistor termination network

that ensures a V.35 input impedance of 100Ω

(±10Ω) and a short-circuit impedance of 150Ω

(±15Ω). The traditional V.35 implementations

required external termination resistors to acheive

the proper V.35 impedances. The internal net-

work is connected via low on-resistance FET

switches when the decoder is changed to V.35

mode. The termination network is transparent

when all other modes are selected. The V.35

receivers can operate up to 10Mbps.

All receivers include a fail-safe feature that

outputs a logic HIGH when the receiver inputs

are open. For single-ended RS-232 receivers,

there are internal 5kΩ pull-down resistors on the

inputs which produces a logic HIGH ("1") at the

receiver outputs. The single-ended RS-423 re-

ceivers produce a logic LOW ("0") on the output

when the inputs are open. This is due to a pull-

up device connected to the input. The differen-

tial receivers have the same internal pull-up

device on the non-inverting input which pro-

duces a logic HIGH ("1") at the receiver output.

The three differential receivers when config-

ured in V.35 mode (RxD, RxC & SCT) do not

have fail-safe because the internal termination

resistor network is connected.

Decoder

The SP504 has the ability to change the inter-

face mode of the drivers or receivers via an 8–bit

switch. The decoder for the drivers and receiv-

ers is not latched; it is merely a combinational

logic switch.

The control word can be externally latched

either HIGH or LOW to write the appropriate

code into the SP504. The codes shown in Tables

1 and 2 are the only specified, valid modes for

the SP504. Undefined codes may represent other

interface modes not specified (consult the fac-

Rev: A Date:1/27/04

SP504 Multi–Mode Serial Transceivers

16

© Copyright 2004 Sipex Corporation

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