NM485D Murata Power Solutions, NM485D Datasheet - Page 3

• Manufacturer Part Number: NM485D
• Description: RS-485 Interface IC TX-RX EIA-485
• Manufacturer: Murata Power Solutions

Specifications of NM485D

• Product Category: RS-485 Interface IC
• Data Rate: 1.25 Mbps
• Operating Supply Voltage: 5 V
• Operating Temperature Range: 0 C to + 60 C
• Mounting Style: Through Hole
• Package / Case: DIL-24
• Function: Driver/Receiver
• Maximum Operating Temperature: + 60 C
• Minimum Operating Temperature: 0 C
• Number Of Drivers: 2
• Number Of Receivers: 2
• Shutdown: Yes

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APPLICATION NOTES
The EIA-485 standard is an upgrade version of the EIA RS-422 standard. The use of balanced data transmission lines in distributing data to several systems components
and peripherals over relatively long lines, requires the use of multiple driver/receiver combinations on a single twisted pair line. This is referred to as a party-line (see
figure 1).
The NM485DC is an isolated interface device providing EIA-485 compatibility. A single supply provides all necessary power for the device, either side of the isolation bound-
ary. The isolation feature allows the logic and differential grounds to be isolated from each other, eliminating ground loop current and inherently long noise paths.
The maximum number of drivers and receivers that may be placed on a single communication bus depends upon their loading characteristics, relative to the definition of
a Unit Load (U.L.), transmission speed does not affect the unit load capability. As the NM485DC complies with the EIA-485 standard a maximum of 32 unit loads per line is
recommended. A unit load usually consists of a driver-receiver pair, but not the line termination resistors.
With only a 5V supply connected, the NM485DC receiver and differential outputs give levels greater than +2.7V and ±1.5V respectively. This known output state may be
used as a reference for use in environments with considerable levels of noise. An unused driver should be disabled by grounding it’s enable pin to reduce power dissipation.
Figure 2 illustrates the way one receiver and one driver of the NM485DC may be connected to implement an isolated EIA-485 interface with a typical UART and processor
bus. Figure 3 demonstrates how the NM485DC may be used as a transceiver. This is possible by using the driver enable to determine the data direction. For example, the
device could receive when the driver enable is low and transmit when it is high.
Precise circuitry will depend on the application and in particular the use of the control lines shown will need to be altered to suit the situation. For instance the maximum
sychronous baud rate of the 8251A is 64kbps, whereas the NM485DC had a maximum baud rate of 1.25Mbps.
Implementing an isolated LONWORKS
NM485DC
The Echleon LONWORKS (Local Operating Network) network
is designed to be used in industrial applications in which other
eletrical equipment is operated. Often the LON
method of controlling machinery or sensing machine activity.
The environment is therefore likely to be electrically noisy and
to reduce the possibility of data corruption, an isolated network
communications system is a preferred method of data transfer.
The EIA-485 standard provides a method of achieving multi-
point (multi-drop) data transmission over balanced twisted
pair transmission lines. The standard is a differential scheme
offering a large degree of common mode immunity compared
to single ended schemes. The isolated differential method offers
the highest common mode and line noise immunity for wire
based systems.
The NM485DC is a fully isolated EIA-485 standard dual driver
and receiver, which requires only a single 5V supply. The
device offers full data direction programming and can hence be
configured as a transceiver. The NM485DC can be operated at
transmitting or receiving data rates of up to 1.25Mbps, hence
is fully compatible with the LONTALK
standards.
Configuring the NM485D as a transceiver
The NM485D is configured as a transceiver simply by con-
necting the inverting R1
the non-inverting R1
similarly R2
(see figure 3), in this way the NM485D is configured as a dual
transceiver. The data direction is determined by the driver en-
able pins (D
when the enable pin is high and a receiver when the enable pin
is low.
If only a single transceiver is required it is recommended that
the second transceiver driver is disabled, this reduces the power
consumption to around 0.6W. The second driver is disabled by
taking the driver enable pin low, this should be via a pull-down
resistor (recommend value is 1KΩ).
B
X
to D2
ENABLE), the transceiver acting as a transmitter
Z
and R2
A
receive to the non-inverting D1
B
receive to the inverting D1
A
to D2
Y
for the second transceiver
(
™
)
(
™
transmission rate
)
network using the
(R)
will be the
Z
drive and
Y
drive,
Figure 1
Figure 3
Figure 2
Isolated Dual EIA-485 Driver and Receiver
NM485DC Series
KII_NM485DC.A05 Page 3 of 6