DS8935 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – LocalTalk™ Dual Driver/Triple Receiver

Typical Application Information
(Continued)
UNUSED PINS
Unused driver outputs should be left open. If tied to either
ground or supply, the driver may enter an I_OS state and con-
sume excessive power. Unused driver inputs should not be
left floating as this may lead to unwanted switching which
may affect I_CC, particularly the frequency component. Un-
used driver inputs should be tied to ground.
Receiver outputs will be in a HIGH state when inputs are
open; therefore, outputs should not be tied to ground. It is
best to leave unused receiver outputs floating.
RECEIVER FAILSAFE
All three receivers on this device incorporate open input fail-
safe protection. The differential receiver output will be in a
HIGH state when inputs are open, but will be indetermined if
inputs are shorted together. Unused differential inputs
should be left floating.
Both single-ended receivers (inverting and non-inverting) are
biased internally so that an open input will result in a HIGH
output. Therefore, these inputs should not be shorted to
ground when unused.
BYPASS CAPACITORS
Bypass capacitors are recommended for both V_CC and V_EE.
Noise induced on the supply lines can affect the signal qual-
ity of the output; V_CC affects the V_OH and V_EE affects the
V_OL. Capacitors help reduce the effect on signal quality. A
value of 0.1 µF is typically used.
Since this is a power device, it is recommended to use a by-
pass capacitor for each supply and for each device. Sharing
a bypass capacitor between other devices may not be suffi-
cient.
TERMINATION
On a multi-point transmission line which is electrically long, it
is advisable to terminate the line at both ends with its charac-
teristic impedance to prevent signal reflection and its associ-
ated noise/crosstalk.
A 100Ω termination resistor is commonly specified by TIA/
EIA-422 for differential signals. The DS8935 is also specified
using 140Ω termination which will result in less power asso-
ciated with the driver output. The additional resistance is
typical of applications requiring EMI filtering on the driver
outputs.
TWO-WIRE LocalTalk
The DS8935 is a single chip solution for a LocalTalk inter-
face. A typical application is shown in Figure 12.
An alternative implementation of LocalTalk is to only use two
wires to communicate. The differential data lines can be
transformer-coupled on to a twisted pair medium. See Figure
14 . The handshake function must then be accomplished in
software.
DS012066-17
FIGURE 14. Differential Communication, Transformer-Coupled to a Twisted-Pair Line
SINGLE +5V SUPPLY
The DS8935 is derived from the DS3691/92 which could be
configured using a single +5V supply (V_EE = 0V). This device
is not specified for this type of operation. However, the de-
vice will not be damaged if operated using a single +5V sup-
ply.
Both drivers require the −5V supply in order to meet the out-
put voltage levels specified. When the device switches from
a positive voltage to the complimentary state, it is pulled to-
ward the V_EElevel. If that level is 0V, then the complimentary
state will be near 0V instead of V_EE. Thus, the output would
switch from about 4V to 0V, instead of 4V to −4V. The differ-
ential driver will meet TIA/EIA-422, but with a reduced noise
margin. The single-ended driver will not meet TIA/EIA-423
without the −5V supply.
The receivers will be functional but may suffer parametri-
cally. The inverting receiver is referenced to V_EE therefore,
the threshold may shift slightly. The inputs can still vary over
the ±10V common mode range.
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