Maple Systems OIT Family - Manual (Page 21)

Message Request Sequence
To display a screen, the following activities occur within the PLC and the OIT:
• The PLC checks the message received status coil to ensure that the previous screen has
already been processed by the OIT (optional).
• The PLC clears the message received status coil in preparation for writing a new screen
number into the MRR (optional).
• The PLC writes the screen number into the MRR.
• The OIT polls the MRR for a new screen number once every 200 milliseconds.
• The OIT displays the screen, performing any special functions required by the screen
contents (example: if screen is an alarm, sound buzzer).
• The OIT sets the message received status coil to inform the PLC that the screen was
properly displayed. If the status coils are disabled, the OIT does not set the message
received status coil.
NOTE:The status coils can only be disabled on the OIT3100 and OIT4100 Series.
Once the OIT displays the screen requested in the MRR, the OIT must see a change in
the number in the MRR in order to display another screen request from the PLC. For
example, the PLC requests that Screen #5 be displayed. The OIT operator then presses
Function Key #1 which displays Screen #30. The number 5 is still in the MRR, but since
no change has occurred it is ignored by the OIT. The PLC can get the OIT to display the
same screen twice without an intervening PLC screen request, by simply putting the
number 0 into the MRR between the two identical screen requests. The OIT does not
perform any action as a result of seeing the number 0 in the MRR, but it does note that a
change occurred.
To use the MRR, the MRR must be enabled and have a PLC address assigned to it. For
instructions on configuring the MRR, refer to the Configuring the Message Request &
Current Message Registers section in CHAPTER 1: Getting Started. During OIT power
up, the MRR is set to 0.
Operational Overview 21
1010-0099, Rev. 07
PLC
Program
Message
Received
Coil
Message
Request
Register
OIT
Message
Displays
Message
Received
Coil
PLC Reads Coil
to Check Message
Displayed
OIT Sets "1"
OIT Reads
Message # PLC Writes
Message #
PLC Clears "0"