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
V
CC
Positive Power Supply Connection: Recommended operating range for V
CC
is
a
4.75V to
a
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 re-
gion is the defined operating range from
b
7V to
a
12V. The
top border extending from
b
3V at 0 mA to
a
12V at
a
1 mA
is defined as one unit load. Likewise, the bottom border
extending from
a
5V at 0 mA to
b
7V at
b
0.8 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
a
5V
and
b
3V stay the same. The other reference points are
a
12V at
a
0.5 mA for the top border and
b
7V at
b
0.4 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
a
5V and
b
3V stay the same. The other reference points are
a
12V
at
a
0.25 mA for the top border and
b
7V at
b
0.2 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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