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PRACTICAL NOTES ON CABLING OF RS-485 BUS
Cabling of the bus connecting the control unit to the remote modules, readers and keypads must follow the rules dictated by RS-485 .
Only use shielded and twisted pair cable with a cross-section of at least:
•
2 x 0 .5 mm² for power supply and twisted 2 x 0 .22 mm² for data .
•
2 x 0 .5 mm² for power supply and twisted 2 x 2 x 0 .22 mm² for data
(used to implement branching) .
The sheathing must be continuous and connected to the control unit's negative
(-) terminal .
The maximum length of the cable (maximum sections of 800 m) and the
cross-section of the conductors depend on the number of modules and the over-
all power draw on the different bus sections . As the power consumed increases
the voltage drop on the cables increases and therefore the power supplied to the
devices on the bus is reduced .
The voltage at all the + and – terminals of the devices on the bus (keypads,
readers, modules, etc .) must not be less than 12 V DC .
If less, add an auxiliary power unit, disconnecting the positive and keeping the
negative in common .
Where there are long bus sections always install 150 ohm resistors between
terminals A and B at both ends of the bus .
Always do the bus cabling in "entry-exit" mode . Do not cable the bus in a star
configuration .
If this configuration is required follow the instructions given in this section .
CHOICE OF BUS RS-485 CABLE CROSS-SECTION
The recommended cable for the RS-485 bus is:
• Twisted cable shielded to level 4 (600/1000 V) .
• A twisted pair for section data 2 x 0 .22 mm² .
• A pair for the power cable 2 x 0 .5 mm² or greater in cross-section depending on distances and voltages (check table below) .
Below is the table for calculating the cross-section of power cables for different distances and power draws .
Any drops in power voltage must not exceed 1 .4 V (voltage measured on power supply terminals and those of the most remote
device on the bus) .
MAXIMUM CABLE LENGTH RS-485 BUS
POWER DRAW
0 .1 A
0 .5 mm2
175 m
0 .75 mm2
262 m
1 mm2
350 m
1 .5 mm2
525 m
0 .25 A
0 .5 A
1 A
70 m
35 m
17 m
105 m
52 m
26 m
140 m
70 m
35 m
210 m
105 m
52 m
Greater than 12 V DC
YES
NO
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