Impulse 2123 - Manual (Page 21)

Appendix C – Electrical Interfaces
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).
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 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.
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
pinout or a set of modem control signals. RS-485 does not define a physical connector.
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