GE Grid Solutions
D20/D200 Technical Overview
GE Information
PRPI-019-3.00-10
95
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
=
Total Power
n
x
=
Number of units of type ‘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
p
=
Peripheral Supply Voltage (24, 48, 130 V)
V
d
=
Voltage drop/foot
Rs
=
Resistive Cable Loss:(#20 AWG [0.812mm] = 0.010 ohms/ft, #22 AWG [0.644mm] = 0.016
ohms/ft)
D
max
=
D.20 Link (+DC & -DC) Maximum Distance
Vt
=
Voltage Threshold (20 V)
Formulae
•
P
Total
= n
A
A + n
S
S + n
K
K + n
C
C + n
C A/I
C A/I + n
C I/O
C I/O + n
COM
COM + n
SP
SP + n
R
R
•
I = P
Total
/ V
p
•
V
d
= I x Rs
•
D
max
= (V
p
- V
t
) / (2 x V
d
) or 1000 feet, whichever is less
