Appendix B – Electrical Interface
RS-232
Quite possibly the most widely used communication standard is RS-232. This implementation has been
defined and revised several times and is often referred to as RS-232 or EIA/TIA-232. The IBM PC computer
defined the RS-232 port on a 9-pin D-sub connector, and subsequently, the EIA/TIA approved this
implementation as the EIA/TIA-574 standard. This standard is defined as the 9-Position Non-Synchronous
Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial
Binary Data Interchange. Both implementations are in widespread use and are referred to as RS-232 in this
document. The RS-232 standard defines operations at data rates up to 20K bps at distances less than 50 ft.
The absolute maximum data rate may vary due to line conditions and cable lengths. RS-232 is a single-
ended or unbalanced interface, meaning that a single electrical signal is compared to a common signal
(ground) to determine binary logic states. The RS-232 and the EIA/TIA-574 specification define two types of
interface circuits: Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE).
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for partyline or multi-drop
applications. The output of the RS-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, requiring 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.
©Sealevel Systems, Inc.
R4 Manual
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SL9260 06/2015