National DS36C278 manual Typical Application Information, Unit Load

Typical Application Information

		TL/F/12040 – 17
		FIGURE 17. Typical RS-485 Bus Interface
		TABLE I. 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 t DO*/RI* by
		200 mV, RO will be HIGH. If DO/RI s 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).
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).
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	VCC	Positive Power Supply Connection: Recommended operating range for VCC is a4.75V to a5.25V.

Unit Load

A unit load for an RS-485 receiver is defined by the input current versus the input voltage curve. The gray shaded re- gion is the defined operating range from b7V to a12V. The top border extending from b3V at 0 mA to a12V at a1 mA is defined as one unit load. Likewise, the bottom border extending from a5V at 0 mA to b7V at b0.8 mA is also defined as one unit load (see Figure 18 ). 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 applica- tions, 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 indi- vidual 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 (/2 unit load and (/4 unit load (UL) options available. These devices will allow up to 64 nodes or 128 nodes guaranteed over temperature depending upon which option is selected. The (/2 UL option is available in industrial temperature and the (/4 UL is available in commercial temperature.

First, for a (/2 UL device the top and bottom borders shown in Figure 18 are scaled. Both 0 mA reference points at a5V and b3V stay the same. The other reference points are a12V at a0.5 mA for the top border and b7V at b0.4 mA for the bottom border (see Figure 18 ). Second, for a (/4 UL device the top and bottom borders shown in Figure 18 are scaled also. Again, both 0 mA reference points at a5V and b3V stay the same. The other reference points are a12V

at a0.25 mA for the top border and b7V at b0.2 mA for the bottom border (see Figure 18 ).

The advantage of the (/2 UL and (/4 UL devices is the in- creased number of nodes on one bus. In a single master multi-slave type of application where 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 an additional feature which offers more advantages. The DS36C279 has an auto- matic sleep mode function for power conscious applica- tions. The DS36C280 has a slew rate control for EMI con- scious applications. Refer to the sleep mode and slew rate control portion of the application information section in the corresponding datasheet for more information on these fea- tures.

TL/F/12040 – 19

FIGURE 18. Input Current vs Input Voltage

Operating Range

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