Practical Notes On Cabling Of Rs-485 Bus; Choice Of Bus Rs-485 Cable Cross-Section - CAME PXC48 Manuel Installateur

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 overall 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 .
Greater than 12 V DC
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 .
YES
NO

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 data cross-section 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
LENGTH OF RS-485
0 .1 A
CABLE
0 .5 mm2 175 m
0 .75 mm2 262 m
1 mm2 350 m
1 .5 mm2 525 m
Where no cable of suitable cross-section is available, additional
auxiliary power supplies must be used .
Example:
Suppose we have:
• PXC48
• PXKWD (maximum power draw 60 mA)
• PX8IR (maximum power draw 40 mA)
• Sensors connected to remote inputs expansion (total power draw
150 mA)
• Connecting distance 200 m .
With a maximum power draw of 250 mA in this case use either a
cable with cross-section 1 .5 mm², or use a 0 .5 mm² cable and an
auxiliary power supply connected at the end of the line, as shown
below:
RS-485 CABLE
Screened 2 x 1.5 mm² + twisted 2 x 0.22 mm²
Control Unit
RS-485 CABLE
Screened 2 x 0.5 mm² + twisted 2 x 0.22 mm²
Auxiliary
Control Unit
power supply
POWER DRAW
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
Device on bus
Device
on bus