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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
530.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 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.
RS-530
RS-530 (a.k.a. EIA-530) compatibility means that RS-422 signal levels are met, and the pin-out for
the DB-25 connector is specified. The EIA (Electronic Industry Association) created the RS-530
specification to detail the pin-out, and define a full set of modem control signals that can be used for
regulating flow control and line status. The RS-530 specification defines two types of interface
circuits, Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The
Sealevel Systems adapter is a DTE interface.
© Sealevel Systems, Inc.
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ULTRA 530.LPCI User Manual
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