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COMHP 101 CyberResearch
®
Serial Communication
12 ©Copyright 2003 CyberResearch, Inc.
Appendix B - 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. It is defined by the EIA as the
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. 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 often operates at 38.4 Kbps 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 (u3 to +10 volts) represents a binary 0 (space) and -12 volts (-3 to -10 volts) denotes a binary 1 (mark).
The RS-232 and the EIA/TIA-574 specification defines two type of interface circuits, Data Terminal Equipment
(DTE) and Data Circuit-Terminating Equipment (DCE). The adapter is a DTE interface.
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 / 530A
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 major difference between RS-530 and RS-530A lies in the modem control interface signals. In RS-530 the
signals are differential, in RS-530A the signals are single ended. The RS-530 specification defines two types of
interface circuits, Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). The adapter is
a DTE interface.