SP504AN

SP504 Application Note

© Copyright 2000 Sipex Corporation

9

SP504 - Switching the 150

The SP504 requires one 150

Ω resistor to ground

on each non-inverting (b) driver output. The

150

Ω resistor to ground is necessary to comply

with the V.35 short circuit impedance of

150

Ω±15Ω. The resistors will need to be

switched out when V.35 mode is not active. To

perform this, the designer can use one of two

methods: add a relay or switch in series with the

resistor or implant the resistors inside the V.35

cable.

Adding the resistors to the cable is relatively

easy. In DTE mode, two drivers are usually ac-

tive, therefore you will only need two resistors.

The 150

Ω resistors can be connected individu-

ally from pin S (data) to signal ground (pin B)

and pin W (clock) to signal ground (pin B) of

the V.35 ISO-2593 connector.

For DCE applications, an additional driver may

be used for a secondary clock signal. If a cable

is used on the DCE end, a conversion or "mini"

cable is necessary to insert the resistors and route

the proper signals to the DTE cable. The resis-

tors can be connected from pin S (data), pin W

(clock) and an unassigned pin (2nd clock) to sig-

nal ground, pin B. Of course the pins on the

"mini" cable will have to be routed to the appro-

priate end on the DTE V.35 cable. (ie. pin S, pin

W and the unassigned pin will be routed to pin

T, pin X and pin AA, respectively, on the DTE

end.)

Another method is to include the 150

Ω resistors

on the printed circuit board with the SP504 add-

ing switches or relays. This method may be pre-

ferred if a common connector/cable (ie.

DB-25) is used for V.35 and some other mode.

The switches must have low on-resistance, pref-

erably less than 1

Ω. NMOS FETs can be used

as configured on the opposite page. The gate of

the NMOS device is connected to pin 18 of the

SP504. This pin is HIGH when the SP504 is in

V.35 mode and goes LOW in all other modes.

Pin 18 is not described in the data sheet for the

SP504 but can be used to control the switching

of the FETs or relays.

Sipex recommends Siliconix's LITTLE FOOT™

MOSFET devices such as the Si9959DY™ Dual

N-Channel Enhancement-Mode MOSFET. The

r

are in 8-pin SOIC packages.

™ - LITTLE FOOT is a trademark of Siliconix, member of TEMIC Group.

Termination Resistor Implementation in the Cable

W

150

Ω

150

Ω

150

Ω

ISO-2593 male connector

ISO-2593 female connector

S – Transmit Data (b)

W – Terminal Timing (b)

B – Signal Ground

B – Signal Ground

T – Receive Data (b)

X –Receive Timing (b)

S

EE

B

T

X

AA

B

AA – Transmit Timing (b)