Impulse SIO4-104.2+2 (3544) - Manual (Page 17)

© Sealevel Systems, Inc. - 15 - SIO4-104.2+2 User Manual
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-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 been 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).
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.