16
8-PORT USB HUB
Appendix B. Electrical Interface
B.1 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 defined the RS-232 port on a DB9
connector and subsequently the EIA/TIA approved this implementation as the
EIA/TIA-574 standard. This standard is defined as the Data Circuit-Terminating
Equipment Employing Serial Binary Data Interchange. Both implementations are
in widespread use. RS-232 is capable of operating at data rates up to 20 kbps at
distances less than 50 ft. (15.2 m). 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 8-Port USB Hub is a
DTE device.
B.2 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 allows for longer cable lengths. RS-422 is rated up to
10 Mbps and can have cabling 4000 ft. (1219.2 m) long. RS-422 also defines driver
and receiver electrical characteristics that will allow one 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.
