Ametek Drexelbrook Universal IV CM, Installation And Operating Instructions Manual

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Installation
16
2.5 Modbus Validation
Legacy Modbus Mode
While Legacy Modbus Mode is ENABLED, the DCM will automatically use the address offset that
corresponds to the function code that is used in the message. See the Latest Modbus Addressing
section for additional details.
While Legacy Modbus Mode is DISABLED, the DCM will use the fixed address offset of 1 for all
function code that is used in messages. See the Legacy Modbus Addressing section for additional
details.
NOTE: Register address should be entered into the DCM to take into account the offset
that will be applied based on the Modbus mode selected.
Note: There are legacy and current Modbus
addressing formats described below.
Legacy Modbus Addressing
Legacy Modbus register addressing is organized based on the type of data to be accessed (coil,
contact, input register, holding register). Each type is segmented into specific address ranges as
shown below.
Valid address ranges as originally defined for Modbus were 0 to 9999 for each of the above register
types
For example, the first Holding Register, number 40001, has the Data Address 0000.
The difference between these two values is the offset.
Each table has a different offset. 1, 10001, 30001 and 40001.
Latest Modbus Addressing
Valid ranges allowed in the latest Modbus specification are 0 to 65,535 and is referred to as
extended addressing. For the current Modbus specification, the offset is the same for all tables
and is 1. Table addressing is determined by the function code used in the request as shown below.