DS36C280M National Semiconductor, DS36C280M Datasheet - Page 7

• Manufacturer Part Number: DS36C280M
• Description: Slew Rate Controlled CMOS EIA-RS-485 Transceiver
• Manufacturer: National Semiconductor

Typical Application Information
Pin
Unit Load
A unit load for a RS-485 receiver is defined by the input cur-
rent 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 de-
fined 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 18 ). A RS-485 driver is capable of driv-
ing 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 hav-
ing 128 nodes on one bus. This would allow signals to be
transferred to and from each individual seat to 1 main sta-
tion. 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.
The
the
First, for a
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 bot-
tom border (see Figure 18 ). Second, for a
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 ).
The advantage of the
creased number of nodes on one bus. In a single master
multi-slave type of application were the number of slaves ex-
ceeds 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
#
5
6
7
8
⁄
2
unit load and
1
1
⁄
4
⁄
2
UL is available in commercial temperature.
UL option is available in industrial temperature and
GND
DO/RI
DO*/RI*
V
Name
CC
1
⁄
2
UL device the top and bottom borders shown in
1
⁄
4
unit load (UL) options available. These
Ground Connection
Driver Output/Receiver Input, 485 Bus Pin.
Driver Output/Receiver Input, 485 Bus Pin.
Positive Power Supply Connection: Recommended operating range for V
1
⁄
2
UL and
1
⁄
4
UL devices is the in-
TABLE 1. Device Pin Descriptions (Continued)
1
⁄
4
UL device the
(Continued)
7
DS36C280 has a slew rate control for EMI conscious appli-
cations. Refer to the sleep mode and slew rate control por-
tion of the application information section in the correspond-
ing datasheet for more information on these features.
Slew Rate Control
The DS36C280 features an adjustable slew rate control.
This feature allows more control over EMl levels than tradi-
tion fixed edge rate devices. The slew rate control may be
adjusted with or without any external components. The
DS36C280 offers both low power (I
EMI for an RS-485 interface.
The slew rate control is located at pin two of the device and
only controls the driver output edges. The slew rate control
pin (SR) may be left open or shorted to ground, with or with-
out a resistor. When the SR pin is shorted to ground without
a resistor, the driver output edges will transition typically
350 ns. When the SR pin is left open, the driver output edges
will transition typically 3 µs. When the SR pin is shorted to
ground with a resistor, the driver output edges will transition
between 350 ns and 3 µs depending on the resistor value.
Refer to the slew rate versus resistor value curve in this
datasheet for determining resistor values and expected typi-
cal slew rate value. Please note, when slowing the edge
rates of the device (see Figure 19 ) will decrease the maxi-
mum data rate also.
Description
FIGURE 18. Input Current vs Input Voltage
Operating Range
CC
is +4.75V to +5.25V.
CC
500 µA max) and low
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