Datasheet

DS36C279

SNLS098B –JULY 2000–REVISED APRIL 2013

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TYPICAL APPLICATION INFORMATION

Figure 19. Typical RS-485 Bus Interface

Table 1. DEVICE PIN DESCRIPTIONS

Pin No. Name Description

1 RO Receiver Output: When 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 RE* Receiver Output Enable: When RE* is LOW the receiver output is enabled. When RE* is HIGH, the receiver output

is in TRI-STATE (OFF). When RE* is HIGH and DE is LOW, the device will enter a low-current sleep mode after

600 ns.

3 DE Driver Output Enable: When DE is HIGH, the driver outputs are enabled. When DE is LOW, the driver outputs are

in TRI-STATE (OFF). When RE* is HIGH and DE is LOW, the device will enter a low-current sleep mode after 600

ns.

4 DI Driver Input: When DE (Driver Enable) is HIGH, 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.

8 V

Positive Power Supply Connection: Recommended operating range for V

is +4.75V to +5.25V.

UNIT LOAD

A unit load for an 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 extending from +5V at 0 mA to −7V at

−0.8 mA is also defined as one unit load (see Figure 20). An RS-485 driver is capable of driving up to 32 unit

loads. This allows up to 32 nodes on a single bus. Although sufficient for many applications, it is sometimes

desirable to have even more nodes. For example, an aircraft that has 32 rows with 4 seats per row would 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 ½ unit load and ¼ unit load (UL) options available.

These devices will allow up to 64 nodes or 128 nodes specified over temperature depending upon which option

is selected. The ½ UL option is available in industrial temperature and the ¼ UL is available in commercial

temperature.

First, for a ½ UL device the top and bottom borders shown in Figure 20 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 bottom border (see Figure 20). Second, for a ¼ UL device the top and bottom borders shown in

Figure 20 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 20).

Product Folder Links: DS36C279