SP504AN

SP504 Application Note

© Copyright 2000 Sipex Corporation

4

SP504 - DTE/DCE Configuration

Many systems may require the serial port to be

configured as either a DTE or DCE. The SP504

is an ideal candidate for a space saving DTE/

DCE solution.

To perform this with the SP504, the driver out-

puts of the first SP504 are connected back into

the receiver inputs of the second SP504, and

vice versa. The common input/output lines can

be routed to the connector and be used as ei-

ther driver outputs or receiver inputs. Refer to

the following page for configuration details.

When the two SP504 devices are connected as

such, one device must be disabled while the

other is transmitting and receiving data. Dis-

abling one of the SP504 devices allows the

other to communicate over the serial bus. One

is dedicated to a DTE configuration to the se-

rial port and the other is a dedicated DCE de-

vice. Disabling the DTE device implies that

the serial port will be configured as a DCE, and

vice versa.

Disabling the SP504 drivers are important be-

cause a bus contention problem can occur if

two drivers are active at the same time. The

drivers of the nonactive SP504 must be tri-

stated by writing "0000" into the driver decoder

(TDEC

inputs into the receivers. The receivers do not

have to be tri-stated but should be configured

so that the receiver input impedance is rela-

tively high (

≥12kΩ) such as RS-422 or RS-485.

The receiver inputs will not affect the signals

on the driver outputs if the input impedance is

at least 12k

Ω. If the RDEC

"0000", the input impedance defaults to at least

12k

Ω.

Please note that most of the V.35 termination

resistors are internal to the SP504. As such, the

driver output impedance during tri-state is

approximately 20k

Ω. This should not affect or

degrade the incoming driver signal from the

other end. Refer to the waveform graphs in the

next few pages. The "DTE" waveforms cap-

ture the driver and receiver signals at the serial

port containing the two SP504 devices. The

"DCE" waveforms are measured in the SP504

at the other end. For example, the TxD driver

output on the DCE side corresponds to the RxD

receiver on the DTE side (side containing two

SP504 devices). Similarly, the TxC driver out-

put on the DTE side should be the same as the

RxC receiver input on the DCE side.

Even though the signal is not degraded, the

lower impedance basically adds the driver out-

put impedance (20k

Ω) and the receiver input

impedance in parallel. When the active SP504

is configured to RS-232 mode, the typical re-

ceiver input impedance is 5k

Ω which will yield

4k

Ω. When in the other modes except V.35,

the typical input impedance is 15k

Ω which

yields 7.5k

Ω when the receivers are configured

in differential mode (i.e. RS-422, RS-449, V.36,

etc.) which is still greater than the 4k

Ω mini-

mum requirement for RS-422 and RS-423. The

input impedance for the V.35 receiver is typi-

cally 100

Ω. The driver tri-state impedance will

decrease the input approximately 0.5

Ω to 1Ω

given that the external 150

Ω termination resis-

tor is switched off or disconnected from the non-

inverting driver output of the disabled SP504.

This configuration will allow the proper data

communications between DTE and DCE. How-

ever for NET1/NET2 certification testing, the

driver outputs must be physically disconnected

from the receiver inputs. The common I/O paths

will interfere with various impedance and cur-

rent testing for the driver or receiver in V.28,

V.11 and V.35 modes.