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© Sealevel Systems, Inc.
SeaI/OUSBSERIAL | SL9177 04/2022
Appendix C
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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 20 Kbps 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). The 4.VC is a DTE device.
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.
