NM485D Murata Power Solutions, NM485D Datasheet - Page 4

• 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

NM485DC Series
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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