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The MAP Family OITs communicate with PLCs by using
point-to-point serial communications to read from and write to the internal
discrete and register memory of the PLC. Some of the PLC's discrete and
register memory is designated for special purposes: Message Request Register,
Current Message Register, Status Bit coils and Function Key
coils. 
Message Request Register
The Message Request Register (MRR) is a register
in the PLC that is continuously monitored by the OIT. When the PLC enters a
number into this register, the OIT displays the screen that corresponds to that
number. 
For example, the PLC programmer may want Message
#30 to be shown on the OIT whenever input coil X1 is turned on. Message #30
might say "Oven Door is Open!!!". The relay ladder logic could be the
following: 
In this case, D500 is the MRR that the OIT has
been configured to poll once every 200 milliseconds. When input coil X1 is
activated, the PLC puts the decimal number 30 into the MRR. The OIT then sees
the number 30 in the MRR and displays Message #30.
Current Message Register
The OIT can be programmed to send the number
representing the message currently displayed on the OIT to a register in the
PLC called the Current Message Register (CMR). The CMR can be used by the PLC
to determine which message is currently being displayed on the OIT. This might
be used to determine which message in a chained sequence the OIT operator is
currently seeing.
Status Bit Coils
The Status Bit coils are a block of discrete coils
in the PLC that are used by the OIT to communicate information to the
PLC.
- Message Received is set by the OIT when
a message has been successfully retrieved from the PLC.
- Invalid Display Message Number is set by
the OIT when the PLC requests a non-existent message.
- Message/Keytable Error is set by the OIT
when it detects invalid or corrupted data in the messages or
keytable.
- Reset is set by the OIT each time it
powers up or performs a re-initialization due to exiting the OIT's local setup
menu.
Function Key Coils
The Function Key coils are a block of discrete
coils in the PLC that are used to pass keypress data from the OIT to the PLC.
When a function key is pressed, the corresponding Function Key coil in the PLC
is activated. For example, the PLC programmer wants a motor to run whenever the
F1 function key is being pressed by the operator. The relay ladder logic could
be: 
In this case, internal control relay C389 has been
configured as the Function Key coil for the F1 function key. If the F1 function
key has been configured as momentary, then C389 will be activated as long as
the F1 function key is being pressed by the operator. Internal control relay
C389 is controlling output coil Y33 which is controlling a motor. Consequently,
the motor runs whenever the operator is pressing the F1 function
key.
Register Monitoring
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The PLC's discrete and register memory can
be monitored, displayed, and updated by the OIT. This can be done by
configuring the OIT's messages to display the PLC's discrete and register
memory as embedded data fields (register monitors). When the OIT displays a
message that contains a register monitor, the OIT reads the specified memory
address in the PLC and then displays the data. If the register monitor has been
configured as read/write, when the operator changes the data in the register
monitor on the OIT's display the OIT writes the change to the PLC's
memory.
Up to 10 read only or read/write register
monitors can be displayed in each OIT message. Each register monitor has an
adjustable field width and decimal location, high/low limits, left or right
justification, optional comma insertion and leading zeros, and optional 'hide
Data' format for secure data entry. |
| Register Monitor
Formats |
Signed
Decimal
Long
4-digit
BCD
8-digit BCD
Binary-1/0 coil
Binary-on/off coil
Binary-bank
8
Binary-bank 16
ASCII character
ASCII string (allows display of
ON/OFF or other text strings in place of 1/0 values) |
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Linear Scaling
Programmable Linear
scaling is available on the signed, decimal and long register monitor formats.
Linear scaling is the process of converting one unit of measure to another,
usually from a machine format to an operator-readable format, so the operator
has the data presented in readily understandable terms. A common application
involves converting analog or digital data into degrees, speed, voltage, or
temperature. If linear scaling is used on a read/write register, the operator
can modify the data in the operator-readable format and it will be converted
back into the machine format before being sent to the PLC. |
