18
SECTION IV
Modbus Registers
Modbus Function $03 can be used to read the
parameters described in this section. The read operation is
restricted to reading one register at a time. Requesting
more than one register invokes an error response with
exception code $03. Requesting a register greater than 24
(address 0017
hex
) also returns an exception code of $03.
Modbus Function $06 can be used to write
Registers 1 to 6 (addresses 0000 to 0005). Requesting a
register greater than 6 (address 0005) invokes an error
response with an exception code of $03.
OCM-101 PACS
®
OFF LINE TIMER VALUE
REGISTER ADDRESS (hex)
1 0000
The OCM-101 PACS
®
Master port communicates
to the DHC controller's PACS
®
Slave port through the
Option Module Connector. The OCM-101 maintains an
off line timer that allows the DHC controller a limited
amount of time to respond. If the OCM-101 detects an off
line time out, it will not provide a Modbus response.
Normally an off line time out occurs when power is lost to
the DHC controller. Since the OCM-101 is powered by
the DHC controller, it is not able to respond anyway.
Intermittent power drop outs can also cause an off
line time out condition. In this case, the OCM-101 detects
the condition and intentionally does not provide a Modbus
response. Once an off line time out is detected, an error
response with exception code $0B will be immediately
given to subsequent requests from the bus until the OCM-
101 corrects the detected PACS
®
error.
The most significant byte of the data is always
zero, and the least significant byte represents the PACS
®
Master off line time out period in 0.1 second increments.
So, a value of 26 ($1A) indicates a time out period of 2.6
seconds. On power up, the OCM-101 automatically sets
the Off Line Timer Value to 2 (0.2 seconds).
By design, the 0.2 second timeout period is
sufficient for a DHC controller. However, the Off Line
Timer Value can be changed to another value. Note that
the changed value is lost if the OCM-101 loses power, and
the value will need to be set again after power returns. If
the most significant byte is not zero when writing the
value, an error response with exception code $03 is
returned.
AUX CLOSED POSITION
REGISTER ADDRESS (hex)
2 0001
See page 13 for description of the data bytes.
AUX OPEN POSITION
REGISTER ADDRESS (hex)
3 0002
See page 13 for description of the data bytes.
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