Typical Application Information

TABLE 1. Device Pin Descriptions

Pin

#

Name

Description

1

RO

Receiver Output: When DE/RE* (Receiver Enable) is LOW, the receiver is enabled (ON), if DO/RI

≥

DO*/RI* by 200 mV, RO will be HIGH. If DO/RI

≤ DO*/RI* by 200 mV, RO will be LOW. Additionally RO will

be HIGH for OPEN (Non-terminated) inputs.

2

SR

Slew Rate Control: A resistor connected to Ground controls the Driver Output rising and falling edge rates.

3

DE/RE*

Combined Driver and Receiver Output Enable: When signal is LOW the receiver output is enabled and the

driver outputs are in TRI-STATE (OFF). When signaI is HlGH, the receiver output is in TRI-STATE (OFF)

and the driver outputs are enabled.

4

DI

Driver Input: When DE/RE* is HlGH, the driver is enabled, if DI is LOW, then DO/RI will be LOW and

DO*/RI* will be HIGH. If DI is HIGH, then DO/RI is HIGH and DO*/RI* is LOW.

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

current 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

defined as one unit load. Likewise, the bottom border ex-

tending 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 driving 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 having 128 nodes on one bus. This would allow

signals to be transferred to and from each individual seat to

1 main station. 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

the 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 ).

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

multi-slave type of application were the number of slaves

exceeds 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

portion of the application information section in the corre-

sponding datasheet for more information on these features.

01205217

FIGURE 17. Typical Pin Connection

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