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© Sealevel Systems, Inc.
RELIO R2 SYNC Manual | SL9255 10/2021
Appendix B
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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 will be referred to as RS-232 in this
document. RS-232 is capable of operating 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 party line 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 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.
