SP504AN Datasheet, PDF (9/11 Pages) Sipex Corporation – Application Note

SP504 - Switching the 150Ω V.35 Termination Resistor
The SP504 requires one 150Ω resistor to ground
on each non-inverting (b) driver output. The
150Ω resistor to ground is necessary to comply
with the V.35 short circuit impedance of
150Ω±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 150Ω 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 150Ω 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 1Ω. 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_DS(on) is typically less than 0.3Ω and the devices
are in 8-pin SOIC packages.
W – Terminal Timing (b)
S – Transmit Data (b)
SW
EE
B
150Ω
150Ω
XT
AA
150Ω ^B
AA – Transmit Timing (b)
B – Signal Ground
ISO-2593 male connector
Termination Resistor Implementation in the Cable
™ - LITTLE FOOT is a trademark of Siliconix, member of TEMIC Group.
B – Signal Ground
T – Receive Data (b)
X –Receive Timing (b)
ISO-2593 female connector
SP504AN
SP504 Application Note
9
© Copyright 2000 Sipex Corporation