Appendixc - Electrical Interface - SeaLevel Route 56 User Manual

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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. 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. 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 often operates at 38.4 Kbps over very short distances. 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 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.

Sealevel Systems ROUTE 56

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