Impulse 2807 - Manual (Page 13)

© Sealevel Systems, Inc. - 11 - SeaLINK+8.VC User Manual
Appendix C – 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 wide spread 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 SeaLINK+8.VC is a DTE device.
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-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.