25
9.3
Data Access
MODBUS TCP supports three types of data access operations: Read, Write and Read/Write. Query
commands (registers) have Read access only.
NOTES
1. Register which don’t have Read access return 0.
2. Write of an out of range value results in an error. It is recommended to read register 935
(SYSTem:ERRor? query) to verify no errors.
9.4
Data Length (Number of Registers)
Each register length is 2 bytes. Read/Write of multiple registers results in Read/Write of a set of bytes
multiplied by two. For example, *IDN? query start from register address 3, its length is 50 registers, 100 bytes.
NOTE
Always start register access from the first register address according to commands mapping table (refer to
Section 9.7) for each command / query. Access to any other register won’t represent the actual data.
9.5
Parameters Range
Each command (register) has a specified data range. It is mandatory to preserve data range to avoid data
overrun. For example, voltage setting range is 0 up to 105% of rated (nominal) value, translated to 0x0000 up
to 0xDBED. An out of range parameter generates an error, which can be read by register 935
(SYSTem:ERRor? query).
Some other registers (commands) such as register 1 (*ESE command), are limited to a specific range, 0 up to
255. A value out of the specified range might result in an unexpected setting value.
NOTE
It is recommended to follow parameters range specification to avoid an unexpected behavior of a power
supply.
9.6
Read / Write Registers
MODBUS TCP supports three types of Read / Write operations. Read holding registers, Write
single register and Write multiple registers. Each operation is defined per command (per register or registers
range). Operation Function Code defines operation type.
Operation Name Operation Function Code Description
Read holding registers 0x03 Read a set of subsequent registers
Write single register 0x06 Write single (one) register
Write multiple registers 0x10 Write a set of subsequent registers
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