Flowserve Logix 1200e LGAIM0044 - Manual (Page 7)

Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611 44-7
Compliance Voltage (Figure 6)
Output compliance voltage refers to the voltage limit
that can be provided by the current source. A current
loop system consists of the current source, wiring
resistance, barrier resistance (if present), and the
Logix 1200e positioner impedance. The Logix 1200e
digital positioner requires that the current loop system
allows for a 12.0 VDC drop across the positioner at
maximum loop current. The 12.0 VDC drop across the
Logix 1200e positioner terminals is generated by the
positioner from the 4-20 mA loop current input.
CAUTION: Never connect a voltage source directly
across the positioner terminals. This could cause
permanent circuit board damage.
Determine if the loop will support the Logix 1200e dig-
ital positioner by performing the following calculation.
Voltage = Compliance Voltage ( @ Current
MAX
)
- Current
MAX
*(R
barrier
+R
wire
)
The calculated voltage must be greater than 12 VDC
in order to support the Logix 1200e digital positioner.
Example: DCS Compliance Voltage = 19 V
R
barrier
= 300
Ω
R
wire
= 25
Ω
CURRENT
MAX
= 20 mA
Voltage = 19 V - 0.020 A*(300
Ω
+ 25
Ω
)
= 12.5 V
The voltage 12.5 V is greater than the required 12.0 V;
therefore, this system will support the Logix 1200e dig-
ital positioner. The Logix 1200e positioner has an
input resistance equivalent to 625
Ω
at a 20 mA input
current.
Cable Requirements
The Logix 1200e digital positioner utilizes the HART
Communication protocol. This communication signal
is superimposed on the DC 4-20 mA current signal.
The two frequencies used by the HART protocol are
1200e Hz and 2200 Hz. In order to prevent distortion
of the HART communication, cable capacitance and
cable length restrictions must be calculated. The cable
length must be limited if the capacitance is too high.
Selecting a cable with lower capacitance/foot rating
will allow longer cable runs. In addition to the cable
capacitance, the network resistance also affects the
allowable cable length.
In order to calculate the maximum network capaci-
tance, use the following formula.
Example: R
barrier
= 300
Ω
R
wire
= 50
Ω
To control cable resistance, No. 24 AWG cable should
be used for runs less than 5000 feet. For cable runs
longer than 5000 feet, No. 20 AWG cable should be
used.
Figure 6: Compliance Voltage
C
network
µ
F
( ) 65
R
barrier
R
wire
390 + +
( )
-------------------------------------------------------------- 0.0032 –
≤
C
cable
22
ρ
F
foot
--------------
0.000022
µ
F
foot
-------------------------------- = =
65
300 50 390 + +
--------------------------------------- 0.0032 – 0.08
µ
F C
network
µ
F
( )
Max
( )
= =
Max Cable Length
C
network
µ
F
( )
C
network
----------------------------------- =
Max Cable Length
0.08
µ
F
0.000022
µ
F foot
⁄
----------------------------------------------- =