Using RS-422 and RS-485 Devices
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Chapter 3: Using RS-422 and RS-485
Devices
The iNS can be used to communicate with RS-422 and RS-485 devices. This chapter describes
connecting these devices and configuring the iNS to communicate in this mode.
Connecting RS-422/485 Devices
RS-422 and RS-485 modes are very similar, except that in RS-422 mode the transmitter
remains enabled at all times; in RS-485 mode, the transmitter is disabled automatically when
no data is being transmitted. Consequently, RS-422 devices must be connected using a four-
wire cable (i.e. with separate transmit and receive pairs). RS-485 devices may be connected
with either two-wire or four-wire cables.
You can connect two RS-422/485 devices with a point-to-point connection, or more than two
RS-485 devices in a bus configuration.
The only legal RS-485 cabling topology is a bus topology (including point-to-point
connections)! All other topologies are expressly forbidden by the RS-485 specification. This
includes the following illegal configurations:
•
Connecting cables in any type of star topology (regardless of whether or not devices are
attached to the ends of the cables). This includes using star-based patch panels or any
other method that splits the physical cable off into multiple segments.
•
Connecting RS-485 devices to the bus using cable stubs of any length
Due to the resilience of the RS-485 signaling specification, some of these illegal topologies
may work in certain configurations. However, changing factors such as cable length, baud
rate, number of devices, bus loading, etc. may cause such configurations to fail sporadically
or to stop communicating entirely. The only topology guaranteed to work in all cable
configurations is a properly terminated bus topology.
RS-422/485 Point-to-point Configuration
Point-to-point connections can be established between two RS-422 devices, an RS-422 and an
RS-485 device, or two RS-485 devices. Figure 3-1 shows a four-wire, point-to-point
connection between two devices.
