DS8925 Datasheet, PDF (8/16 Pages) National Semiconductor (TI) – LocalTalk™ Dual Driver/Triple Receiver

Typical Application Information
(Continued)
DRIVER OUTPUT WAVEFORMS
The driver configuration on the DS8925 is unique among
TIA/EIA-422 devices in that it utilizes −5V V_EE supply. A typi-
cal TIA/EIA-422 driver uses +5V only and generates signal
swings of approximately 0V–5V.
By utilizing V_EE, the differential driver is able to generate a
much larger differential signal. The typical output voltage is
about |4| V, which gives |8| V differentially, thus providing a
much greater noise margin than +5V drivers. See Figure 12.
The receiver therefore has a range of +8V to −8V or V_SS of
16V (V_SS = V_OD–V_OD*).
Each side of the differential driver operates similar to a TIA/
EIA-423 driver. The output voltages are slightly different due
to the loading: the differential driver has differential termina-
tion, the single-ended driver is terminated with a resistor to
ground.
DS011895-15
Note 8: Star (*) represents the opposite input condition for a parameter.
FIGURE 12. Typical Driver Output Waveforms
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 DS8925 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 DS8925 is a single chip solution for a LocalTalk inter-
face. A typical application is shown in Figure 11.
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
13. The handshake function must then be accomplished in
software.
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