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TGoddard/SP332/9617R0

SP332 RS-232/RS-485 Multi-Mode Serial Transceiver

© Copyright 2000 Sipex Corporation

THEORY OF OPERATION...

The SP332 is a single chip device that can be

configured via software for either RS-232, RS-

485 or both interface modes at any time. The

SP332 is made up of three basic circuit ele-

ments, single-ended drivers and receivers, dif-

ferential drivers and receivers and charge pump.

DIFFERENTIAL DRIVER/RECEIVER...

RS-485, RS-422 Drivers...

The differential drivers and receivers comply

with the RS-485 and RS-422 standards. The

driver circuits are able to drive a minimum of

1.5V when terminated with a 54

Ω resistor

across the two outputs. The typical propagation

delay from the driver input to output is 60ns. The

driver outputs are current limited to less than

250mA, and can tolerate short circuits to ground,

or to any voltage within a +10V to -7V range

with no damage.

RS-485, RS-422 Receivers...

The differential receivers of the SP332 comply

with the RS-485, RS-422 and V.11 standards.

The input to the receiver is equipped with a

common mode range of +12V to -7V. The input

threshold over this range is a minimum of

±200mV. The differential receivers can receive

data up to 10Mbps. The typical propagation

delay from the receiver input to output is 90ns.

SINGLE ENDED DRIVER/RECEIVER...

RS-232 (V.28) Drivers...

The single-ended drivers and receivers comply

the with the RS-232E and V.28 standards. The

drivers are inverting transmitters which accept

either TTL or CMOS inputs and output the RS-

232 signals with an inverted sense relative to the

input logic levels. Typically, the RS-232 driver

output voltage swing is

±9V with no load and is

guaranteed to be greater than

±5V under full

load. The drivers rely on the V+ and V- voltages

generated by the on-chip charge pump to main-

tain proper RS-232 output levels. With worst

case load conditions of 3k

Ω and 2500pF, the

four RS-232 drivers can still maintain

±5V

output levels. The drivers can operate up to

120kbps; the propagation delay from input to

output is typically 2

µs.

RS-232 (V.28) Receivers...

The RS-232 receivers convert RS-232 input

signals to inverted TTL signals. Each of the four

receivers features 500mV of hysteresis margin

to minimize the affects of noisy tranmission

lines. The inputs also have a 5k

Ω resistor to

ground, in an open circuit situation the input of

the receiver will be forced low, committing the

output to a logic high state. The input resistance

will maintain 3k

Ω-7kΩ over a ±15V range. The

maximum operating voltage range for the re-

ceiver is

±30V, under these conditions the input

current to the receiver must be limited to less

than 100mA. Due to the on-chip ESD protection

circuitry, the receiver inputs will be clamped to

±15V levels. The RS-232 receivers can operate

up to 120kbps.

CHARGE PUMP...

The charge pump is a Sipex-patented design

(5,306,954) and uses a unique approach com-

pared to older less-efficient designs. The charge

pump still requires four external capacitors, but

uses a four-phase voltage shifting technique to

attain symmetrical 10V power supplies. Figure

8a shows the waveform found on the positive

side of capacitor C2, and Figure 8b shows the

negative side of capacitor C2. There is a free-

running oscillator that controls the four phases

of the voltage shifting. A description of each

phase follows.

Phase 1

-Vss charge storage- During this phase of the

clock cycle, the positive side of capacitors C1

and C2 are initially charged to +5V. C1+ is then

switched to ground and charge in C1- is trans-

ferred to C2-. Since C2+ is connected to +5V,

the voltage potential across capacitor C2 is

now 10V.

Phase 2

-Vss transfer- Phase two of the clock connects

the negative terminal of C2 to the Vss storage

capacitor and the positive terminal of C2 to

ground, and transfers the generated -10V to C3.

Simultaneously, the positive side of capacitor

C1 is switched to +5V and the negative side is

connected to ground.