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

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 DS8935 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 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.

SINGLE +5V SUPPLY

The DS8935 is derived from the DS3691/92 which could be

configured using a single +5V supply (V

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

state will be near 0V instead of V

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

the threshold may shift slightly. The inputs can still vary over

DS012066-17

FIGURE 14. Differential Communication, Transformer-Coupled to a Twisted-Pair Line

www.national.com

9