Brooks Firetracker FT128 CIE - Cable Length Calculations for 1728 and 1736

Technical / Programming Manual
FT128 Rev 2.2
166
23.2 Cable Length Calculations for 1728 and 1736
Up to 8 display units type 1728 and/or 1736 can be connected (if 4552 module is fitted)
depending on the cable (type and length) being used.
Brooks recommendations:
Cable type Brooks BAC0898 1.5 mm
2
twisted pair for the 24V supply and for RS485.
However other twisted pair cables with minimum 0.5 mm
2
can be used for RS485 e.g.
category 5 (CAT5) cables, normal CAT5 cable resistance is 188Ω/Km (ensure cable
resistance < 500 Ω / Km). RS485 cable length must not exceed 1200 metres.
In order to reduce noise immunity due to noisy environment, it is recommended to use
CAT5 with shield and terminate the shield in the CIE
Note: End of line resistor on the last unit is 120 Ω, this must be set in the last unit using
jumper link in the unit, refer to the Technical Manual for 1728 and 1736.
For ease of calculation, it is recommended to use 2 pairs of Brooks BAC0898 twisted pair
cable for 24VDC supply and RS485.
See Table 16 page 167, this should be the benchmark against other alternative cables if
other cables are used.
Wire resistance for this cable (R
u
), is approx. 11.9 Ω / Km at 20C.
The current consumption of these units are given in Table 16 page 167. Typically,
calculations are based on their “Active State”. For the External Presentation Unit 1728 to
continue working under worst case scenario, i.e. mains failure and system is relying only
on the back-up battery. The battery voltage should not drop below 12VDC and should still
be able to deliver the active current consumption (88mA). Here is an example to calculate
the cable distance from the CIE to the last 1728 unit under these conditions (assuming that
only 1728 units are connected and Brooks cable BAC0898 was used
Current consumption of each 1728, I
unit
= 0.088 A
Voltage of 1728 in "Active State", V
act
= 12 V
Number of units required = 8
Nominal battery voltage, V
batt
= 24 V
Maximum allowable battery voltage drop, V
drop
= V
batt
- V
act
= 24-12 V
= 12 V
Total current consumption for 8 units, I
total
= 8x0.088 A
= 0.704 A
Allowed cable resistance, R
allowed
= V
drop
/I
total
Ω
= 12/0.704 Ω
= 17.05 Ω
= 11.9 Ω/Km
The maximum cable length for 8 units, L =
R
allowed
(Ω) / R
u
(Ω/Km) Km
= 17.05/11.9 Km
= 1.432 Km
R
u
, for BAC0898 1.5mm
2
twisted pair wire
resistance was given as
Note: The total length of the cable must be limited to 1200 metres although the calculation
result is 1432 metres.