Impulse 2113 - Manual (Page 21)

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 SeaLINK+16.VC 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 partyline 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.
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