Logix
®
420 Digital Positioner FCD LGENIM0106-07-AQ – 10/15
12
7 ELECTRICAL CONNECTIONS
7.1 Electrical Terminals
.
7.2 Command Input (4-20 mA) Connection
The Logix 420 is reverse polarity protected, however, verify polarity
when making field termination connection. Wire 4-20mA current source
to the input terminal labeled “HART 4- 20mA INPUT”. See Figure 6:
Terminal Diagram. Depending on the current source, a HART filter may
be required. See 14.1 Troubleshooting Guide.
7.2.1 Compliance Voltage
Output compliance voltage refers to the voltage limit the current source
can provide. A current loop system consists of the current source,
wiring resistance, barrier resistance (if present), and the Logix 420
impedance.
The Logix 420 requires that the current loop system allow for a 10 VDC
drop across the positioner at maximum loop current. The operating
current range is from 3.8 to 24 mA.
In order to determine if the loop will support the Logix 420, perform
the calculation in Equation 1. The Available Voltage must be greater
than 10VDC in order to support the Logix 420. Also, see Table 1: Input
Signal.
Equation 1
Available Voltage = Controller Voltage (@Current
max)
−
Current
max
× (R
barrier
+ R
wire
)
Example:
DCS Controller Voltage = 19 V
Current
max
= 20 mA
R
barrier
= 300 Ω
R
wire
= 25 Ω
Available Voltage = 19 V − 0.020 A × (300Ω + 25 Ω)
Available Voltage = 12.5 V
The available voltage (12.5 V) is greater than the required voltage
(10.0 V) therefore; this system will support the Logix 420. The Logix
420 has an input resistance equivalent to 500 Ω at a 20 mA input
current.
1
CAUTION: The current must always be limited for 4-20mA
operation. Never connect a voltage source directly across the
Logix 420 terminals. This could cause permanent circuit board
damage.
Housing EARTH Terminal
Internal
External
HART 4-20 mA
Input-
Input+
Figure 6: Terminal Diagram
