SP503AN

SP503 Application Note

© Copyright 2000 Sipex Corporation

6

SP503 V.35 External Termination

Resistors

V.35 implementation requires external resistors

to be connected to the driver outputs and re-

ceiver inputs.

The resistors are needed for

reducing signal levels and maintaining input

and output impedance levels that conform to

CCITT Recommendation V.35. The drawings

below offer both receiver input termination and

driver output termination configurations that

will comply to V.35.

As you may know, the termination resistors are

for V.35 mode only and have to be disconnected

when another mode is programmed. There are

a few ways to connect and disconnect the net-

work. One method is to add a daughter-card

option slot onto the main printed circuit board

where the SP503 is mounted. This allows the

user to add a mini-PC card that contains the V.35

network to be connected to the driver outputs

and receiver inputs.

Another method is to

implant the V.35 network into a cable if the user

requires conversion cables to switch from one

protocol to another. For example, if the serial

port has a DB-25 connector and requires an

ISO-2593 connector for V.35; a DB-25 to

ISO-2593 conversion cable containing the V.35

network is connected to the port.

Of course this method will not allow program-

mability to the serial port if the user wants to run

various protocols to that port. To actively switch

protocols by software control, you can use ana-

log switches or relays. Sipex recommends us-

ing low-ON resistance analog switches, such as

outputs, and solid-state relays such as the AT&T

page for an example of the SP503 V.35 configu-

ration. For other termination networks, please

consult the factory.

50

Ω

50

Ω

120

Ω

A

B

Ω

Ω

[90

Ω to 110Ω]

[135

Ω to 165Ω]

232

Ω

232

Ω

200

Ω

200

Ω

107

Ω

A

B

100

Ω

LOAD

-5V

±5%

Ω

Ω

[50

Ω to 150Ω]

[135

Ω to 165Ω]

[+0.44V to +0.66V]

[-0.6V to +0.6V]

Typical values

V.35 Receiver Input Termination Resistor Network

V.35 Driver Output Termination Resistor Network