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GE Grid Solutions
Calculating the Maximum Distance of a D.20 Link
Following are formulae, and an example, for calculating the maximum distance of an RS-485
D.20 Link (+DC & -DC), taking into account the IR drop.
Variables
P
=
Total Power
Total
n
=
Number of units of type 'x'
x
X
=
Power Consumption of X module (see D. Power Consumption – D20 Modules), where X = M, S,
A, K, R, C, C A/I, C I/O, COM, UPC, SP.
I
=
Constant Current
V
=
Peripheral Supply Voltage (24, 48, 130 V)
p
V
=
Voltage drop/foot
d
Rs
=
Resistive Cable Loss:(#20 AWG [0.812mm] = 0.010 ohms/ft, #22 AWG [0.644mm] = 0.016
ohms/ft)
D
=
D.20 Link (+DC & -DC) Maximum Distance
max
Vt
=
Voltage Threshold (20 V)
Formulae
• P
= n
A + n
S + n
Total
A
S
• I = P
/ V
Total
p
• V
= I x Rs
d
• D
= (V
- V
) / (2 x V
max
p
t
GE Information
K + n
C + n
C A/I + n
K
C
C A/I
) or 1000 feet, whichever is less
d
D20/D200 Technical Overview
C I/O + n
COM + n
C I/O
COM
PRPI-019-3.00-10
SP + n
R
SP
R
95
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