Appendix C - Electrical Interface
OMG-USB-SER-2
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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 OMG-USB-SER-2 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 s ingle 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.
