Owner's manual

Appendix C - Electrical Interface
Sealevel Systems SeaPORT+4 Page 12
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 SeaPORT+4 is a
DTE device.
RS-422
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.