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pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 15 / 67
Attention:
When assigning measurement values to 4-20 mA analog output by using register 4378 / 4506, you
need to consider the following:
The PMC you have mapped to AO1 / AO2 (register 4364 / 4492)
The physical unit currently in use for the selected PMC (register 2090 for PMC1 (pH) and
register 2410 for PMC6 (temperature).
Therefore, when the operator redefines one of the register 4364 / 4492, 2090 / 2410, the definitions of
the register 4378 / 4506 should be reviewed. If not, the current output at the 4-20 mA interfaces may be
wrong.
Note:
The physical unit of the analog output corresponds always to the unit that is set for the selected PMC
(register 2090 for PMC1 or register 2410 for PMC6). Accordingly, not only the pH value is selectable at
the 4-20 mA interface, but also mV values, degrees centigrade or Kelvin.
Example:
Register 4364 is set to 1 (PMC1 is mapped to AO1).
Register 2090 is set to 0x1000 (the unit “pH”is assigned to PMC1).
Register 4378 is set to 2 and 10 (4 mA = pH 2, 20 mA = pH 10).
The sensor reads currently pH 4, the output at the 4-20 mA is accordingly 8 mA.
The operator now re-assigns register 2090 to the value of 0x200000 (unit = mV), but does not modify
all other registers. The sensor is still at pH 4 and reads now +170 mV. At the analog output, as 20 mA
is programmed to a value of 10 by register 4378, the current will go to the maximum value of 20 mA.
This will generate an interface warning “4-20 mA current set point not met”.
2.4.5.7 Defining a Constant Current Output for Testing
Note:
For constant current output, the AO1 / AO2 must be set to analog interface mode 0x01:
Start
register
Number of
registers
Reg1 / Reg2 Modbus
function code
Read
access
Write
access
4384 2 Constant current output value for
AO1 [mA]
3, 4, 16 U/A/S S
4512 2 Constant current output value for
AO2 [mA]
3, 4, 16 U/A/S S
Figure 2.4.5.7.1: Definition of register 4384 / 4512.
Command: Fixed value AO1
Modbus address: 4384 Length: 2 Type: 3
Read
Parameter: Fixed value [mA]
Format: Float
Value:
10
Figure 2.4.5.7.2: Example to read the constant current output in mode 0x01 for AO1. It is set to 10 mA.