User`s manual

Watlow Controls Communications Guide 10
across the T+/R+ and T-/R- wires. One
resistor is placed at the PC or PLC where
it connects to the EIA-485 bus. The
second resistor is placed at the last
controller on the network. Do not place
resistors at each controller. The
impedance of the wires used for the bus
determines the resistor value. For twisted
pair, the value is typically 120 ohms. In
addition, it may be necessary to have a
pull-up and pull-down resistor between the
power supply and ground of the interface
adapter. Check the documentation that
came with your interface adapter.
Biasing the bus reduces reflection of signals
sent down the bus. These reflections are
sometimes referred to as a standing wave.
This condition is most notable when
communicating at high baud rates.
Interface Converters
The purpose of an interface converter is
to allow two different buses to be
connected. Interface converters are
required when connecting an EIA-232 port
to an EIA-422 or EIA-485 bus.
The EIA-422 bus is a full duplex bus. This
means that it can send and receive data at
the same time. The EIA-485 bus is a half
duplex bus. This means that it can only
send or receive data at any given time.
Some interface converters on the market
have provided the ability to have full
duplex with the EIA-485 bus. This is
accomplished by using two receivers and
transmitters tied in tandem. This type of
converter will not work with the Watlow
controllers. Be sure that the model you
purchase is designed for half duplex.
Another consideration when using
interface converters is how the converter
handles switching between transmit and
receive. When connecting between an
EIA-232 and an EIA-485, the converter
must convert two items. First it must
convert the voltage level. Second it must
convert from half duplex to full duplex.
This is not an easy task. Typically it is
accomplished via a handshake line from
the PC or PLC. When data flows into the
converter from the PC, a handshake line is
placed high. When data flows out of the
converter to the PC, the handshake line is
placed low. In this way, the handshake line
controls the direction of information.
Another method of achieving this is to use
a built-in timer. The converter switches to
transmit when a character is sent to it
from the PC. After a period of time when
the PC has not transmitted, the converter
switches to a receive mode.
It is important that you understand how
your converter accomplishes this task.
You are required to wire this feature or
make settings on the converter to enable
this function. The PC will not talk to the
controllers correctly with out properly
setting this.
Your converter may also require settings
through dip switches, to set up
communications parameters like baud rate,
data bits, start bits, stop bits and
handshaking. See the documentation that
comes with your converter for more
detail.
The converter may require a separate
power supply. Some converters get their
power from the handshake lines of the PC.
If you rely on this method, you will need
to wire these additional lines. In addition,
your software must set these lines high. A
more reliable method is to use the external
power supply. This is especially necessary
when using a laptop computer.
Protocols
Protocol describes how to initiate an
exchange. It also prevents two machines
from attempting to send data at the same
time. There are a number of different data
communications protocols, just as there
are different human cultural protocols that
vary according to the situation.
The protocol part of Watlow
communications is very important,
because it gives us a quality of
communication that others often don’t
have. Protocol-driven communications