Chapter 2
Hardware Installation and Configuration
©
National Instruments Corporation
2-9
Figure 2-10.
Host Computer Connected to Three FP-1001 Network Modules
When the FP-1001 is cabled directly to an RS-485 port on a host computer,
as shown in Figure 2-10, you can wire it in either 4-wire full duplex mode
or 2-wire half duplex mode.
The multidrop topology used by the RS-485 network in these two
networking configurations protects against power failures at individual
FieldPoint banks. It eliminates the possibility that a power failure at one
FP-1001 bank might affect the communication between other banks in your
network. Because the FP-1000 provides an RS-485 repeater, a power
failure at that bank affects communications.
Caution
Do not use the isolation barrier of the RS-485 interface as a way to protect human
contact from common-mode voltages of more than 250 Vrms. The RS-485 interface on
both the FP-1000 and the FP-1001 modules is isolated from the rest of the FieldPoint
system, including the power supply, backplane, and the RS-232 interface on the FP-1000.
It is isolated with a galvanic and optical isolation barrier, which has been designed and
tested to withstand fault voltages of up to 2500 Vrms. However, the safety rating of this
barrier is only for working voltages of up to 250 Vrms. Also, notice that while the RS-232
interface on the FP-1000 is isolated from the RS-485 interface, it is NOT isolated from the
power supply inputs (the V and C terminals).
RS-485 Interface Specifications
RS-485 specifies a maximum cabling distance of 4,000 ft, but
improvements in line drivers and cabling technology often allow you to
design your network beyond the recommendations of the specification.
If even longer distances are desirable, you can use RS-485 repeaters.
Host Computer with
RS-485 Board Installed
FP-1001
FP-1001
FP-1001
RS-485
