Systech Corporation RCS/3000, Hardware Manual

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14 Connecting RS-422/485 Devices
Chapter 3: Physical Layer Interface
Settings
The RCS/3000 can be used for communication with RS-232, RS-422, or RS-485 devices. By
default, all the RCS/3000 ports are configured to communcate using an RS-232 electrical
interface, as described in the preceding chapters. This chapter describes connecting RS-422
and RS-485 devices and configuring the RCS/3000 to communicate in RS-422 or RS-485
mode.
Connecting RS-422/485 Devices
RS-422 and RS-485 mode are very similar, except that in RS-422 mode, the transmitter
remains enabled at all times; while in RS-485 mode, the transmitter is disabled automatically
when no data is being transmitted. Consequently, to transmit and receive, RS-422 devices
must be connected using four-wire communication. RS-485 devices may be connected on
either two-wire or four-wire systems.
You can connect two RS-422/485 devices in a point-to-point connection or more than two
RS-422/485 devices in a bus configuration.
The only legal RS-422/485 cabling topology is a bus topology (including point-to-point
connections)! All other topologies are expressly forbidden by the RS-422/485 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-422/485 devices to the bus using cable stubs of any length.
Due to the resilience of the RS-422/485 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 stop
communicating or to communicate sporadically. The only topology guaranteed to work in
all cable configurations is a properly terminated bus topology.