DS481

www.ti.com

SNLS131B – JULY 2000 – REVISED MARCH 2013

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 16 ). 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.

The DS481 has ½ unit load and will allow up to 64 nodes guaranteed over temperature.

For a ½ UL device the top and bottom borders shown in Figure 16 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 16). 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 16).

The advantage of the ½ UL device is the increased number of nodes on one bus. In a single master multi-slave

type of application where the number of slaves exceeds 32, the DS481 may save in the cost of extra devices like

repeaters, extra media like cable, and/or extra components like resistors.

Figure 16. Input Current vs Input Voltage Operating Range

Sleep Mode

The DS481 features an automatic sleep mode that allows the device to save power when not transmitting data.

Since the sleep mode is automatic, no external components are required. It may be used as little or as much as

the application requires. The more the feature is utilized, the more power it saves.

The sleep mode is automatically entered when both the driver and receiver are disabled. This occurs when both

the device will enter sleep mode after 50 ns. The DS481 is guaranteed to go into sleep mode within 600 ns after

both pins are asserted. The device wakes up (comes out of sleep mode) when either the DE pin is asserted to a

the device to wake up than it does for the device to enable from TRI-STATE. Refer to datasheet specifications

(7)

Non Terminated, Open Input only

Copyright © 2000–2013, Texas Instruments Incorporated

Submit Documentation Feedback

9

Product Folder Links: DS481