Appendix C – Electrical Interface
Sealevel Systems
ULTRA
COMM+I.LPCI
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
ULTRA COMM+I.LPCI
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.
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for party line or multi-drop applications.
The output of the RS-422/485 driver is capable of being
Active
(enabled) or
Tri-State
(disabled). This capability
allows multiple ports to be connected in a multi-drop bus and selectively polled. RS-485 allows cable lengths up to
4000 feet and data rates up to 10 Megabits per second. The signal levels for RS-485 are the same as those defined by
RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one line.
This interface is ideal for multi-drop or network environments. RS-485 tri-state driver (not dual-state) will allow the
electrical presence of the driver to be removed from the line. Only one driver may be active at a time and the other
driver(s) must be tri-stated. RS-485 can be cabled in two ways, two wire and four wire mode. Two-wire mode does
not allow for full duplex communication, and requires that data be transferred in only one direction at a time. For
half-duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx- to
Rx-). Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin-out or a set of
modem control signals. RS-485 does not define a physical connector.
