differential driver and receiver designed for bal-
anced mulipoint bus transmission at rates up to
1.25Mbits per second. The device provides two
receive channels and two driver channels. The
two driver channels have an active high enable.
No external components are needed and a single
5V input supply powers all functions either side of
the isolation boundary. The driver outputs provide
limiting for positive and negative currents and
thermal shutdown protection from line fault condi-
tions on the transmission bus line. The receiver
outputs will always be high if the receiver inputs
are open. The isolation voltage between input and
output is 1000Vrms, the devices are supplied in a
low profile 24 pin DIL plastic package.
RECEIVER SWITCHING CHARACTERISTICS
Parameter
Propagation delay time L to H, T
PLH
Propagation delay time H to L, T
PHL
Test conditions
V
ID
=-1.5V to 1.5V, C
L
=15pF
Min.
Typ.
70
72
Max.
155
135
Units
ns
ns
RECEIVER FUNCTION TABLE
Differential inputs A-B
V
ID
≥0.2V
-0.2V<V
ID
<+0.2V
V
ID
≤-0.2V
RX
OUT
High level
Undefined
Low level
ISOLATION CHARACTERISTICS
Parameter
Isolation test voltage
Conditions
Flash tested for 1 second
Min.
1000
Typ.
Max.
Units
Vrms
ABSOLUTE MAXIMUM RATINGS
Supply voltage V
CC
with respect to pin 11
Input voltage D
X
ENABLE and D inputs
Input voltage range, receiver A or B inputs
Receiver differential input voltage range
Output voltage range, driver
Power dissipation
Isolation capacitance
Isolation voltage (flash tested for 1 second)
Data transmission rate
Lead temperature 1.5mm from case for 10 seconds
7V
7V
-25V to +25V
-25V to +25V
-10V to +15V
2000mW
40pF
1000Vrms
1.25Mbps
300ºC
All data taken at T
A
=25°C, V
CC
=5V.
For full details go to
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KII_NM485DC.A02
Page 1 of 7
NM485DC
Isolated Dual EIA-485 Driver and Receiver
EACH DRIVER ELECTRICAL CHARACTERISTICS
Parameter
High level output voltage, V
OH
Low level output voltage, V
OL
Differential output voltage, V
OD1
Differential output voltage, V
OD2
Change in magnitude of differential output voltage, ΔV
OD
Common mode output voltage, ΔV
OC
Change in magnitude of common mode output voltage, ΔV
OC
Output current power off, I
O
High level input current, I
IH
Low level input current, I
IL
Short circuit output current, I
OS
Test conditions
I
OH
=-33mA
I
OH
=33mA
I
O
=0
R
L
=100Ω
R
L
=54Ω
R
L
=54Ω or 100Ω
V
CC
=0, V
O
=-7.0V to 12V
V
IH
=4.0V
V
IL
=0.8V
V
O
=-7.0V
1
V
O
=12V
1
Min.
Typ.
3.7
1.1
Max.
Units
V
V
V
V
V
V
V
V
μA
μA
mA
mA
mA
1.5
2.0
1.5
-1.0
6.0
5.0
±0.2
3.0
±0.2
±100
20
-15
-250
250
2.0
DRIVER SWITCHING CHARACTERISTICS
Parameter
Differential output delay time, T
DD
Output disable time from high level, T
PHZ
Output disable time from low level, T
PLZ
Propagation delay time L to H, T
PLH
Propagation delay time H to L, T
PHL
Differential output transition time, T
TD
Output enable time to high level, T
PZH
Output enable time to low level, T
PZL
Test conditions
R
L
=54Ω, CL=50pF
R
L
=110Ω, CL=50pF
R
L
=110Ω, CL=50pF
R
L
=27Ω, CL=50pF
R
L
=54Ω, CL=50pF
R
L
=110Ω, CL=50pF
R
L
=110Ω, CL=50pF
Min.
Typ.
20
90
30
70
70
150
80
40
Max.
25
120
45
145
145
300
120
60
Units
ns
ns
ns
ns
ns
ns
ns
ns
DRIVER FUNCTION TABLE
DX
IN
High level
Low level
Irrelevant
D
X
ENABLE
High level
High level
Low level
DX
Y
Output
High level
Low level
High impedance
DX
Z
Output
Low level
High level
High impedance
TEMPERATURE CHARACTERISTICS
Parameter
Operating free-air temperature range
Storage temperature range
1. Duration of short circuit should not exceed 1 second.
Min.
0
-40
Typ.
Max.
60
125
Units
ºC
ºC
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KII_NM485DC.A02
Page 2 of 7
NM485DC
Isolated Dual EIA-485 Driver and Receiver
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
(
™
)
network using the
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
(R)
will be the
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
(
™
)
transmission rate
standards.
Figure 1
Figure 2
Configuring the NM485D as a transceiver
The NM485D is configured as a transceiver simply by con-
necting the inverting R1
B
receive to the inverting D1
Z
drive and
the non-inverting R1
A
receive to the non-inverting D1
Y
drive,
similarly R2
B
to D2
Z
and R2
A
to D2
Y
for the second transceiver
(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
X
ENABLE), the transceiver acting as a transmitter
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).
Figure 3
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Page 3 of 7
NM485DC
Isolated Dual EIA-485 Driver and Receiver
APPLICATION NOTES (continued)
System Performance
The EIA-485 standard allows a maximum of 32 unit loads to be connected to the network, this is less than the LONWORKS standard of 64 nodes. A unit load is any single
driver, receiver or transceiver in the EIA-485 standard, or any single node under the LONWORKS scheme. Similarly the EIA-485 standard specifies a maximum data rate
standard of 10Mbps, whereas the maximum LONWORKS data rate standard is 1.25Mbps. The resultant maximum system performance for the LONWORKS EIA-485 con-
figuration is therefore 32 nodes at 1.25Mbps. The NM485D isolated serial interface devices supports this configuration, as well as any lower specified system.
The EIA-485 standard defines the maximum line length as a function of data rate (in Mbps). This implies that the user must choose between the line length of the network
and its maximum data transmission rate.
The isolated interface has been used in previous configurations (e.g. NM232D) to increase the available line length as isolated data lines are much less susceptible to
ground currents and variations in local supplies. The feature of isolation in a LON environment is intended to be used primarily to improve noise susceptibility, therefore,
unless the line length improvements can be reliably demonstrated by the user, the EIA-485 recommendations on maximum cable length are assumed to apply.
The complete hardware implementation for the LONWORKS EIA-485 network is relatively simple and straight forward (see figure 5). There is a minimum of components
required, only 1 interface part and one resistor, and the complete LONTALK transmission protocols are supported. The isolation barrier of 1000Vrms offers improved noise
immunity compared to a non-isolated system and eliminates node-to-node supply voltage mismatch and possible ground current loops.
If protection from voltage transients is required, then a bi-directional TransZorb from each line side to ground should be connected (see figure 6). A TransZorb with a
breakdown voltage higher than the common-mode voltage required should be used. However the added parasitic capacitance will load down the bus, and should therefore
be taken into consideration.
Figure 4
Figure 5
Figure 6
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Page 4 of 7
NM485DC
Isolated Dual EIA-485 Driver and Receiver
APPLICATION NOTES (continued)
NM485D receiver output status
The receiver output will be high (>2.7V) when the differential input lines are left open (open circuit). However, when a line termination resistor is attached, the inputs are
effectively shorted together, not left floating. Since the receiver has typically 70mV of hysteresis, the output will remain in its last active state, high or low.
To force the receiver output to a known state, the configuration shown in figure 7 should be used. It should be noted that this arrangement will use typically 100mW of
power, for each receiver connected in this manner.
The termination resistors are used to generate a DC bias, which forces the receiver into a high state, when no signal is applied. This arrangement will still allow the output
to switch due to a change at the input, whilst maintaining line termination characteristics.
TECHNICAL NOTES
ISOLATION VOLTAGE
‘Hi Pot Test’, ‘Flash Tested’, ‘Withstand Voltage’, ‘Proof Voltage’, ‘Dielectric Withstand Voltage’ & ‘Isolation Test Voltage’ are all terms that relate to the same thing, a test voltage,
applied for a specified time, across a component designed to provide electrical isolation, to verify the integrity of that isolation.
Murata Power Solutions NM485DC series of DC/DC converters are all 100% production tested at their stated isolation voltage. This is 1000Vrms for 1 second.
A question commonly asked is, “What is the continuous voltage that can be applied across the part in normal operation?”
For a part holding no specific agency approvals, such as the NM485DC series, both input and output should normally be maintained within SELV limits i.e. less than 42.4V peak, or
60VDC. The isolation test voltage represents a measure of immunity to transient voltages and the part should never be used as an element of a safety isolation system. The part
could be expected to function correctly with several hundred volts offset applied continuously across the isolation barrier; but then the circuitry on both sides of the barrier must
be regarded as operating at an unsafe voltage and further isolation/insulation systems must form a barrier between these circuits and any user-accessible circuitry according to
safety standard requirements.
REPEATED HIGH-VOLTAGE ISOLATION TESTING
It is well known that repeated high-voltage isolation testing of a barrier component can actually degrade isolation capability, to a lesser or greater degree depending on materials,
construction and environment. The NM485DC series has toroidal isolation transformers, with no additional insulation between primary and secondary windings of enameled wire.
While parts can be expected to withstand several times the stated test voltage, the isolation capability does depend on the wire insulation. Any material, including this enamel
(typically polyurethane) is susceptible to eventual chemical degradation when subject to very high applied voltages thus implying that the number of tests should be strictly limited.
We therefore strongly advise against repeated high voltage isolation testing, but if it is absolutely required, that the voltage be reduced by 20% from specified test voltage.
This consideration equally applies to agency recognized parts rated for better than functional isolation where the wire enamel insulation is always supplemented by a further
insulation system of physical spacing or barriers.
RoHS COMPLIANCE INFORMATION
This series is compatible with RoHS soldering systems with a peak wave solder temperature of 300ºC for 10 seconds. The pin termination
finish on this product series is Matte Tin over Nickel Preplate. The series is backward compatible with Sn/Pb soldering systems.
For further information, please visit www.murata-ps.com/rohs
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