SP237AET-L/TR View Datasheet(PDF) - Signal Processing Technologies

Part Name

Description

Manufacturer

SP237AET-L/TR

+5V Powered Multi-Channel RS-232 Drivers/Receivers

Signal Processing Technologies 

of the charge pump. If the SP2XXB is powered

up with SD low, then only the 2ms delay time

will apply (refer to Figure 6). Under normal

operation, both the wait time and delay time

should be transparent to the user.

With only the receivers activated, the device

typically draws less than 5µA (10µA max) sup-

ply current. In the case of a modem interfaced to

a computer in power-down mode, the RI (ring

indicator) signal from the modem would be used

to “wake up” the computer, allowing it to accept

the data transmission.

After the ring indicator signal has propagated

through the SP2XXB receiver, it can be used to

trigger the power management circuitry of the

computer to power up the microprocessor and

bring the SD pin to the SP2XXB low, taking it

out of shutdown. The receiver propagation de-

lay is typically 1µs. The enable time for V+ and

V- is typically 2ms. After V+ and V- have

settled to their final values, a signal can be sent

back to the modem on the DTR (Data Terminal

Ready) pin signifying that the computer is ready

to accept and transmit data.

All receivers that are active during shutdown

maintain 500mV (typ.) of hysteresis.

Varying Capacitor Values

As stated earlier, the capacitor values are some-

what non-critical. Since they are an actual compo-

nent of the charge pump circuitry, their value will

affect its performance, which in turn affects the

VOH and VOL levels. There is no upper limit for the

value of any of the four capacitors; lower values

will impact performance. C1 and C2 are respon-

sible for the charge accumulation and can be

reduced to 1µF; this will increase the output im-

pedance of V+ and V–. Reducing these capacitor

values will limit the ability of the SP2XXA/B to

maintain the dc voltages needed to generate the

RS-232 output levels. Capacitors C3 and C4 can

also be reduced to 1µF; doing so will increase the

ripple on V+ and V–.

Typically each driver will require 1µF of capaci-

tance as a minimum to operate within all specified

parameters; if five drivers are active in the circuit,

then C3 and C4 should be 5µF. In order to operate

at these minimum values, the supply voltage must

be maintained at +5.0V ±5%. Also, the ambient

operating temperature must be less than 60°C.

The capacitor values must be chosen to suit the

particular application. The designer must bal-

ance board space, cost and performance to maxi-

mize the design. The capacitors can be polarized

or non–polarized, axial-leaded or surface-mount.

As the size and value decrease, so does the cost;

however, the value should be chosen to accom-

modate worst-case load conditions.

INTERFACE EXAMPLE – A MODEM

ON THE IBM PC SERIAL PORT

The RS-232 standard defines 22 serial interface

signals. These signals consist of ground lines,

timing, data, control and test signals, plus a set

of signals rarely used for a second data channel.

Many of these signal lines are not used in typical

RS-232 applications; in fact, the IBM® PC serial

port is implemented using only nine pins.

For example, consider the case of a PC using this

nine pin port to communicate with a peripheral

device such as a modem. We see the following

activity on each of the RS-232 lines as the

computer and modem are activated and commu-

nicate with each other as well as the remote

modem at the other end of the phone line.

Signal Ground (GND)

The Signal Ground pin acts as a reference for all

the other signals. This pin is simply maintained

at a 0V level to serve as a level to which all other

signals are referenced. Both the PC and the

modem will have this line connected to their

respective internal ground lines.

Ring Indicator

Data Terminal Ready

R

22

D

20

Data Carrier Detect

Signal Ground

Data Set Ready

R

8

7

R

6

Clear To Send

Ready To Send

Received Data

R

5

D

4

R

3

Transmitted Data

(To Be Printed)

D

2

SP239A

D

Ring Indicator

R

Data Terminal Ready

D

Data Carrier Detect

Signal Ground

D

Data Set Ready

D

Clear To Send

R

Ready To Send

D

Received Data

R

Received Data

(To Be Printed)

SP237A

Computer

Modem

IBM Modem Port Interconnections

Date: 8/3/04

+ 5V Powered Multi-Channel RS-232 Drivers/Receivers

10

© Copyright 2004 Sipex Corporation

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