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

cal TIA/EIA-422 driver uses +5V only and generates signal

swings of approximately 0V–5V.

By utilizing V

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

16V (V

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.

UNUSED PINS

Unused driver outputs should be left open. If tied to either

ground or supply, the driver may enter an I

sume excessive power. Unused driver inputs should not be

left floating as this may lead to unwanted switching which

may affect I

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

Noise induced on the supply lines can affect the signal qual-

ity of the output; V

V

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 100

Ω termination resistor is commonly specified by TIA/

EIA-422 for differential signals. The DS8925 is also specified

using 140

Ω 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.

DS011895-15

FIGURE 12. Typical Driver Output Waveforms

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