User`s manual

Watlow Controls Communications Guide 9
Chapter Three -
Watlow Controls
Approach to Data
Communications
Interface Standards
An interface is a means for electronic
systems to interact. It’s a specific kind of
electrical wiring configuration. Four
interfaces are commonly used:
EIA-232 (Full Duplex)
An EIA-232 (formerly RS-232) interface
uses three wires: a single transmit wire; a
single receive wire; and a common line.
Only two devices can use an EIA-232
interface. A -3 to -24 volt signal indicates
a 1 and a +3 to +24 volt signal indicates a
0. The EIA-232 signal is referenced to
the common line rather than to a separate
wire, as in EIA-485 and EIA-422. An
EIA-232 cable is limited to 50 feet, due to
noise susceptibility.
EIA-422 (Full Duplex)
The EIA-422 interface uses five wires: a
“talk” pair; a “listen” pair; and a common
line. It can handle one master and up to
ten remote devices in a multidrop (more
than one controller shares the same wires)
network up to 4,000 feet long. EIA-422
uses the difference in voltage between the
two wires to indicate a 1 or a 0 bit. A 1 is
a difference of -5 volts, while a 0 is a
difference of +5 volts.
EIA-423 (Full Duplex)
An EIA-423 interface is compatible with
EIA-232. It is a newer standard designed
for more speed and distance. It uses three
wires: a single transmit wire; a single
receive wire; and a common line. Only
two devices can use an EIA-423 interface.
A -3 to -6 volt signal indicates a 1 and a
+3 to +6 volt signal indicates a 0. The
EIA-423 signal is referenced to the
common line rather than to a separate
wire, as in EIA-485 and EIA-422. An
EIA-423 cable is limited up to 4,000 feet,
due to noise susceptibility.
EIA-485 (Half Duplex)
An EIA-485 interface uses three wires: a
T+/R+, a T-/R- and a common line. A -5-
volt signal is interpreted as a 1, a +5-volt
signal as a 0. As many as 32 remote
devices can be connected to a master on a
multi-drop network up to 4,000 feet long.
Wiring
Most PCs and some PLCs have a standard
EIA-232 port (usually referred to as RS-
232). In these instances, you must use an
interface converter to connect to EIA-
422 or EIA-485. These interface standards
are required to have a multi-drop system
(more than one controller on the bus). See
the list below for some vendors who sell
these converters.
Should your PC or PLC have the
appropriate interface, just connect using
the wiring diagram supplied with your
controller.
For EIA-422, the T+ connects to the R+,
sometimes labeled “B” while the T-
connects to the R- , sometimes labeled
“A”. For EIA-485, the terminal marked
“A” usually connects to the T-/R- while
the “B” terminal connects to the T+/R+
of the controller.
The standards do not specify the wire size
and type. Use of AWG 24 twisted pair
provides excellent results. If shielded cable
is used, terminate the shield at one end
only.
Always follow the manufacturer’s
instructions supplied with the interface
converter. See Biasing of Buses next.
Biasing of Buses
The EIA-485 standard requires the bus to
be biased for reliable communication. This
means to provide termination resistors