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參數(shù)資料
| 型號： | NM485SLC |
| 廠商： | Murata Power Solutions Inc |
| 文件頁數(shù)： | 3/5頁 |
| 文件大?。?/td> | 0K |
| 描述： | ISOLATED INTERFACE LP EIA-485 |
| 標準包裝： | 15 |
| 系列： | IsoLoop® |
| 類型： | 收發(fā)器，隔離式 |
| 驅(qū)動器/接收器數(shù)： | 1/1 |
| 規(guī)程： | RS485 |
| 電源電壓： | 4.75 V ~ 5.25 V |
| 安裝類型： | 通孔 |
| 封裝/外殼： | 24-DIP（0.667"，16.95mm） |
| 供應商設備封裝： | 24-DIL |
| 包裝： | 管件 |
| 產(chǎn)品目錄頁面： | 2721 (CN2011-ZH PDF) |
| 其它名稱： | 811-1446-5 |
NM485SLC
Low Power Isolated EIA-485 Driver and Receiver
KDC_NM485SLC.A04 Page 3 of 5
www.murata-ps.com/support
APPLICATION NOTES
The increased use of balanced data transmission lines, (distributing data to several system components and peripherals over relatively long lines) has brought about the need for
multiple driver/receiver combinations on a single twisted pair line. This resulted in an upgraded version of EIA RS-422, named EIA-485. EIA-485 takes into account EIA RS-422
requirements for balanced line data transmission, and allows for multiple drivers and receivers.
The NM485SLC is a low power isolated differential interface providing EIA-485 compatibility. The use of a differential communications interface such as the NM485SLC allows
data transmission at high rates and over long distances to be accomplished. This is because effects of external noise sources and cross talk are much less pronounced on
the data signal. Any external noise source coupling onto the differential lines will appear as an extra common mode voltage which the receiver is insensitive to. The difference
between the signal levels on the two lines will therefore remain the same. Similarly a change in the local ground potential at one end of the line will appear as just another change
in the common mode voltage level of the signals. Twisted pair cable is commonly used for differential communications since its twisted nature tends to cause cancellation of the
magnetic elds generated by the current owing through each wire, thus reducing the effective inductance of the pair.
Computer and industrial serial interfacing are areas where noise can seriously affect the integrity of data transfer, and a proven route to improve noise performance for any
interface system is galvanic isolation. Galvanic isolation removes the ground loop currents from data lines and hence the impressed noise voltage which affects the signal is also
eliminated. The isolation feature of the NM485SLC also means that common mode noise effects are removed and many forms of radiated noise are reduced to negligible limits.
The NM485SLC has driver thermal shutdown protection which protects the device from line fault conditions. If the outputs of the driver are accidently shorted to a power supply
or low impedance source, up to 250mA can ow through the part. The thermal shutdown circuit disables the driver output when the internal temperature of the I.C. reaches 150C
and turns it back on when the temperature cools to 130C. If two or more NM485SLCs are used and drivers are shorted directly, the driver outputs can not supply enough current
to activate the thermal shutdown. Thus the internal shutdown circuit will not prevent contention faults when two drivers are active on the same bus at the same time.
Figure 1 demonstrates how the differential lines of the NM485SLC can be connected to form a transceiver. Data direction is controlled by the driver enable and receiver enable
pins. This means the device can receive when the receiver enable is low and transmit when the driver enable is high. As the driver is active high, to reduce the power dissipation
even further, it is advisable to disable the driver when not transmitting data.
Some data encoding schemes require the output of the receiver to maintain a known state, usually a logic 1, when the data transmission is complete and all drivers are forced
into three-state, high impedance.
The NM485SLC receiver has a fail safe feature which guarantees the output to be in a logic 1 state when the receiver inputs are left oating (open circuit). However, when the
cable is terminated with 120Ω, the differential inputs to the receiver are shorted together, not left oating. Since the receiver has about 70mV hysteresis, the output will maintain
the last bit received.
Implementing an isolated LONWORKS () network using the NM485SLC
The Echleon LONWORKS (Local Operating Network) network is designed to be used in industrial applications in which other electrical 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 NM485SLC is a fully isolated EIA-485 standard driver and receiver, which requires only a single 5V supply. The device offers full data direction programming and
can hence be congured as a transceiver. The NM485SLC can be operated at transmitting or receiving data rates of up to 2.5Mbps, hence is fully compatible with the
LONTALK () transmission rate standards.
Conguring the NM485SLC as a transceiver
The NM485SLC is congured as a transceiver simply by connecting the inverting RB receive to the inverting DZ drive and the non-inverting RA receive to the non-inverting
DY drive. The data direction is determined by the driver enable pins (D ENABLE and R ENABLE), the transceiver acting as a transmitter when the enable pin is high and a
receiver when the enable pin is low.
Figure 1
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