Datasheet

DS36C278
SNLS096C –JULY 1998–REVISED APRIL 2013 www.ti.com
Figure 16. Receiver Enable and Disable Waveforms (t
PLZ
, t
PZL
)
Figure 17. Receiver Enable and Disable Waveforms (t
PHZ
, t
PZH
)
Typical Application Information
Figure 18. Typical RS-485 Bus Interface
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 19). 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 guaranteed 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.
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