10
4 PROTOCOL IMPLEMENTATION
4.1 Modbus Register Addresses
The PM130 Modbus registers are referred to by using addresses in the range of 0 to 65535.
From within the Modbus applications, the PM130 Modbus registers can be accessed by
simulating holding registers of the Modicon 584, 884 or 984 Programmable Controller, using a
5-digit “4XXXX” or 6-digit “4XXXXX” addressing scheme. To map the PM130 register address
to the range of the Modbus holding registers, add a value of 40001 to the PM130 register
address. When a register address exceeds 9999, use a 6-digit addressing scheme by adding
400001 to the PM130 register address.
4.2 Data Formats
The PM130 uses three data formats to pass data between a master application and the
instrument: a 16-bit integer format, a 32-bit modulo 10000 format, and a 32-bit long integer
format.
4.2.1 16-bit Integer Format
A 16-bit data is transmitted in a single 16-bit Modbus register as unsigned or signed integer
(whole) numbers without conversion or using pre-scaling to accommodate large-scale and
fractional numbers to a 16-bit register format. Scaling can be made using either the LIN3 linear
conversion, or decimal pre-scaling to pass fractional numbers in integer format.
Non-scaled data
The data will be presented exactly as retrieved by the communications program from the
instrument. The value range for unsigned data is 0 to 65535; for signed data the range is -
32768 to 32767.
LIN3 (Linear) Scaling
This conversion maps the raw data received by the communications program in the range of 0 -
9999 onto the user-defined LO scale/HI scale range. The conversion is carried out according to
the formula:
Y = (X / 9999)
×
(HI - LO) + LO
where:
Y
- the true value in engineering units
X
- the raw input data in the range of 0 - 9999
LO, HI - the data low and high scales in engineering units
When data conversion is necessary, the HI and LO scales, and data conversion method are
indicated for the corresponding registers.
EXAMPLE
Suppose you have read a value of 5000 from register 256 that contains a voltage reading (see
Table 5-1
). If your instrument has the 144V input option, and you use potential transformers
with the ratings of 22,000V : 110V = 200, then the voltage high scale is HI = 144
×
200 = 28,800,
and in accordance with the above formula, the voltage reading in engineering units will be as
follows:
5000
×
(28800 - 0)/9999 + 0 = 14401V
When a value is written to the instrument, the conversion is carried out in reverse to produce
the written value in the range of 0 - 9999:
X = 9999
×
(Y - LO) / (HI - LO)
