Appendix C - Electrical Interface - SeaLevel COMM+8.422.PCIe User Manual

Appendix C – Electrical Interface
RS-422
The RS-422 specification defines the electrical characteristics of balanced voltage digital interface circuits.
RS-422 is a differential interface that defines voltage levels and driver/receiver electrical specifications. On
a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or
inputs. In contrast, a single ended interface, for example RS-232, defines the logic levels as the difference
in voltage between a single signal and a common ground connection. Differential interfaces are typically
more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces
also have greater drive capabilities that allow for longer cable lengths. RS-422 is rated up to 10 Megabits
per second and can have cabling 4000 feet long. RS-422 also defines driver and receiver electrical
characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels range
from 0 to +5 volts. RS-422 does not define a physical connector.
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for party-line or multi drop
applications. The output of the RS-422/485 driver is capable of being Active (enabled) or Tri State (disabled).
This capability allows multiple ports to be connected in a multi drop bus and selectively polled. RS-485
allows cable lengths up to 4000 feet and data rates up to 10 Megabits per second. The signal levels for RS-
485 are the same as those defined by RS-422. RS-485 has electrical characteristics that allow for 32 drivers
and 32 receivers to be connected to one line. This interface is ideal for multi drop or network environments.
RS-485 tri state driver (not dual state) will allow the electrical presence of the driver to be removed from
the line. Only one driver may be active at a time and the other driver(s) must be tri stated. RS-485 can be
cabled in two ways, two wire and four wire mode. Two wire mode does not allow for full duplex
communication and requires that data be transferred in only one direction at a time. For half duplex
operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to Rx- ).
Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin out or a set of
modem control signals. RS 485 does not define a physical connector.
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© Sealevel Systems, Inc. 7802e Manual | SL9249 11/2022