Technical / Programming Manual
FT128 Rev 2.2
164
23 FT128 cable length calculation
23.1 COM Loop Cable Length
On the COM loop, up to 255 COM loop units can be connected (i.e. address 1-255). The
cable length and maximum COM loop current are dependent on the number and type of
loop units and cable type, see Figure 57 page 165. Brooks recommends 1.50 mm
2
, twisted
pair cable or equivalent for the COM loop. Brooks BAC0898 is available in the following
size reels:
The cable specifications for these 1.5mm
2
twisted cables are:
DC Cable Resistance: 11.9 Ω/Km per conductor at 20C.
Cable capacitance: 50pF/m at 1MHz.
Generally, the maximum allowed loop current is 350mA while the maximum allowable
loop resistance is 42.3Ω.
This maximum loop current should be taken into account when calculating the maximum
loop cable length. Either use the graph in Figure 57 “COM Loop Current Consumption vs.
Cable Length” page 165 or use this equation that represents the graph to calculate the
cable length.
Calculations of the maximum allowable distance from the control unit to the furthest end or
the total COM loop distance are based on the above specifications. Note that these
calculations do not take into account device loading along the loop.
Excel sheet
An Excel spreadsheet is also available for an easy check of the current consumption, cable
length, etc.
EBLWin
In the COM loop pop-up menu select "Properties..." to open a window showing the
quiescent and maximum current consumption for the COM loop units.
Example:
A fire alarm manufacturer (Panasonic) specifies a maximum allowable loop resistance of
42.3Ω. What is the maximum allowable distance from the control unit to the last device if
Brooks BAC0898 stranded, un-coated copper wire was used? How does this change if the
temperature is increased to 80C?
Given that,
Therefore, the maximum allowable distance
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