User manual - RATEMETER STI-N118
9.
OUTPUT VALUE CALCULATION
Lets assume that we have active current output and its parameters are:
“OUtmod”
=
“4-20”
,
“OUt LO”
= 100,
“OUt HI”
= 200,
“Lo r”
= 5.0,
“Hi r”
= 5.0
Parameters
“Lo r”
and
“Hi r”
define working range of current output to 3.8 ÷ 21 mA.
Output current will be calculated for three displayed values „D”:
a) D = „17.5”
According to formula from page
I
out
= (17.5-10.0) / (20.0-10.0) × 16 mA + 4 mA = 0.75 ·16 + 4 = 16 mA
Calculated I
out
does not exceed the output working range (3.8 - 21 mA).
b) D = „20.5”
According to formula from page
I
out
= (20.5-10.0) / (20.0-10.0) × 16 mA + 4 mA = 1.05 ·16 + 4 = 20.08 mA
Calculated I
out
does not exceed the output working range (3.8 - 21 mA).
c) D = „30.0”
According to formula from page
I
out
= (30.0-10.0) / (20.0-10.0) × 16 mA + 4 mA = 2 ·16 + 4 = 36 mA.
Calculated I
out
exceeds the output working range (3.8 - 21 mA), so current output will
generate current equal to the upper border of range defined by parameter
“Lo r”
i
“Hi r”
(it
means 21 mA).
10.
THE MODBUS PRO
TOCOL HANDLING
Transmission parameters: 1 start bit, 8 data bits, 1 or 2 stop bit (2 bits are send, 1 and 2 bits
are accepted when receive), no parity control
Baud rate:
selectable from: 1200 to 115200 bits/second
Transmission protocol:
MODBUS RTU compatible
The device parameters and display value are available via RS-485 interface, as HOLDING-
type registers (numeric values are given in U2 code) of Modbus RTU protocol. The registers
(or groups of the registers) can be read by 03h function, and wrote by 06h (single registers) or
10h (group of the registers) accordingly to Modbus RTU specification. Maximum group size for
03h and 10h functions does not exceed 16 registers (for single frame).
The device interprets the broadcast messages, but then do not sends the answers.
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