TABLE 1. Device Pin Descriptions (Continued)

Pin

#

Name

Description

5

GND

Ground Connection

6

DO/RI

Driver Output/Receiver Input, 485 Bus Pin.

7

DO*/RI*

Driver Output/Receiver Input, 485 Bus Pin.

8V

CC

Positive Power Supply Connection: Recommended operating range for V

Unit Load

A unit load for a RS-485 receiver is defined by the input cur-

rent versus the input voltage curve. The gray shaded region

is the defined operating range from −7V to +12V. The top

border extending from −3V at 0 mA to +12V at +1 mA is de-

fined as one unit load. Likewise, the bottom border extending

from +5V at 0 mA to −7V at −0.8 mA is also defined as one

unit load (see

Figure 18 ). A RS-485 driver is capable of driv-

ing up to 32 unit loads. This allows upto 32 nodes on a single

bus. Although sufficient for many applications, it is sometime

desirable to have even more nodes. For example an aircraft

that has 32 rows with 4 seats per row could benefit from hav-

ing 128 nodes on one bus. This would allow signals to be

transferred to and from each individual seat to 1 main sta-

tion. Usually there is one or two less seats in the last row of

the aircraft near the restrooms and food storage area. This

frees the node for the main station.

The DS36C278, the DS36C279, and the DS36C280 all have

1

devices will allow upto 64 nodes or 128 nodes guaranteed

over temperature depending upon which option is selected.

Figure 18 are scaled. Both 0 mA reference points at +5V and

−3V stay the same. The other reference points are +12V at

+0.5 mA for the top border and −7V at −0.4 mA for the bot-

tom border (see

top and bottom borders shown in

Figure 18 are scaled also.

Again, both 0 mA reference points at +5V and −3V stay the

same. The other reference points are +12V at +0.25 mA for

the top border and −7V at −0.2 mA for the bottom border

(see

Figure 18 ).

creased number of nodes on one bus. In a single master

multi-slave type of application were the number of slaves ex-

ceeds 32, the DS36C278/279/280 may save in the cost of

extra devices like repeaters, extra media like cable, and/or

extra components like resistors.

The DS36C279 and DS36C280 have addition feature which

offer more advantages. The DS36C279 has an automatic

sleep mode function for power conscious applications. The

DS36C280 has a slew rate control for EMI conscious appli-

cations. Refer to the sleep mode and slew rate control por-

tion of the application information section in the correspond-

ing datasheet for more information on these features.

Slew Rate Control

The DS36C280 features an adjustable slew rate control.

This feature allows more control over EMl levels than tradi-

tion fixed edge rate devices. The slew rate control may be

adjusted with or without any external components. The

DS36C280 offers both low power (I

EMI for an RS-485 interface.

The slew rate control is located at pin two of the device and

only controls the driver output edges. The slew rate control

pin (SR) may be left open or shorted to ground, with or with-

out a resistor. When the SR pin is shorted to ground without

a resistor, the driver output edges will transition typically

350 ns. When the SR pin is left open, the driver output edges

will transition typically 3 µs. When the SR pin is shorted to

ground with a resistor, the driver output edges will transition

between 350 ns and 3 µs depending on the resistor value.

Refer to the slew rate versus resistor value curve in this

datasheet for determining resistor values and expected typi-

cal slew rate value. Please note, when slowing the edge

rates of the device (see

Figure 19 ) will decrease the maxi-

mum data rate also.

DS012052-19

FIGURE 18. Input Current vs Input Voltage

Operating Range

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