Impulse 7804e - Manual (Page 24)

©Sealevel Systems, Inc.
SL9219 - 06/2011
7804e Manual 24
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-C/D/E or
EIA/TIA-232-C/D/E. It is defined as “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 has 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 20K bps / 50 ft. The absolute maximum data rate may vary due to line conditions
and cable lengths. RS-232 often operates at 38.4K bps 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) denote a binary 1 (mark). 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 Sealevel Systems Adapter is a DTE interface.
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 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