User`s manual

Watlow Controls Communications Guide 12
Chapter Four -
Human Machine
Interfaces (HMI)
HMI stands for Human-Machine-
Interface, and is sometimes referred to as
Man-Machine-Interface (MMI). This is an
operator interface that allows you to
monitor and control your process. Many
software packages were written to allow a
PC to perform this function. Use care in
selecting software packages. Some will run
only under a specific disk operating system
such as Windows NT from Microsoft. The
serial interface and protocol must match
between the PC or PLC and the controller.
Many companies provide free working
demonstration copies. Obtain a
demonstration copy so you can see if the
program meets your needs.
Listed below are a few of the many
software packages that claim to support
the Modbus protocol. Watlow does not
recommend any one software package nor
supports the implementation of any
software package not sold by Watlow.
Contact the software manufacturer for
more information in applying their
software.
Write your own HMI
Writing an XON/XOFF
Application
The great thing about XON/XOFF
protocol is the simplicity of
communications. The basic structure is to
send information to the controller as
needed until you receive an XOFF (0x13).
You must wait until you receive an XON
(0x11) before continuing. The controller
follows this same scheme. There are no
confirmations of commands sent. This
protocol is highly efficient in that few
characters are sent to handle the flow of
information. This protocol is sometimes
referred to as “flow control”.
The disadvantage of this protocol is lower
reliability in ensuring that both parties
receive the proper information. In
addition, only two devices can be on a bus
because no means is provided to address
any controller.
Handling Error Codes (ER2)
All communications' related error codes
are ER2 error codes, which means that
they're not considered cause for a
shutdown of the unit itself. With
XON/XOFF flow control, error codes may
be generated, but there will be no standard
indication of this fact. Therefore, you
may want to query the status of ER2 after
each command sent to see if it was
successful.
User Responsibility
Users must refrain from altering prompts
that do not appear on the controller’s
front panel or are not included on the
specific model. For example, do not send
an A2LO command to a unit not equipped
with an alarm for output 2.
Most Watlow controllers contain a
register to disable saving of the current set
point to EEPROM. When using your PC
or PLC to control the set point in the
Watlow controller, the EEPROM may be
prematurely damaged. There is a limit to
the number of times you can store
information in the EPPROM. PCs can
quickly reach this limit if the set point is
continually changed, as in a ramping
controller. Disabling this feature will
prevent this damage.
Care must also be taken that the process
can not cause damage to property or
injury to personnel if the wrong
commands are sent due to operator error
or equipment malfunction. Be sure to use
limit devices on the equipment to prevent
system runaway.
Writing an ANSI x3.28 Application
Handling Error Codes (ER2)
All communications' related error codes
are ER2 error codes, which means that