Manual

Appendix C - Electrical Interface
Sealevel Systems ACB-104 Page 14
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. It is defined by the EIA as the Interface
between Data Terminal Equipment and Data Circuit- Terminating Equipment Employing Serial Binary Data
Interchange. The mechanical implementation of RS-232 is on a 25 pin D sub connector. 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. The voltage levels defined by RS-232 range from -12 to +12 volts. 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. A voltage of +12 volts (usually +3 to +10 volts) represents a binary 0
(space) and -12 volts (-3 to -10 volts) denotes a binary 1 (mark). The RS-232 and the EIA/TIA-574 specification
defines two type of interface circuits, Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment
(DCE). The Sealevel Systems adapter is a RS-232 Synchronous DTE interface.
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.
RS-530
RS-530 (a.k.a. EIA-530) compatibility means that RS-422 signal levels are met, and the pin-out for the DB-25
connector is specified. The Electronic Industry Association (EIA) created the RS-530 specification to detail the
pin-out, and define a full set of modem control signals that can be used for regulating flow control and line status.
The RS-530 specification defines two types of interface circuits, Data Terminal Equipment (DTE) and Data Circuit-
Terminating Equipment (DCE). The Sealevel Systems adapter is a DTE interface.
RS-449
RS-449 (a.k.a. EIA-449) compatibility means that RS-422 signal levels are met, and the pin-out for the DB-37
connector is specified. The EIA created the RS-449 specification to detail the pin-out, and define a full set of
modem control signals that can be used for regulating flow control and line status.