©Sealevel Systems, Inc.
SL9249 – 12/2013
7802e Manual
20
Appendix C - Electrical Interface
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 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.
