Specification sheet | FAQ

DataBus Extension Cables

ICT Proprietary 3-wire Databus

The ICT Proprietary Databus protocol operates on a three wire bus giving the customer maximum flexibility in the layout of their data logging system.  The three wires of the Databus provide power and communication for all Smart sensors connected to the SL5 data logger.  Due to the simplicity of this bus system, sensors and extension cables can be connected anywhere on the Databus.  The Databus system uses the following wiring for all connected Smart sensors and extension cables:

ICT DataBus Extension cables

ICT International offers a range of factory made and guaranteed extension cables for use with smart sensors connected to a Smart Logger.  These cables are compatible with any and all smart sensors.  The extension cables connect between Databus hubs but can also be used to connect between the end of a smart sensor cable (typically 5m long and terminated with a 3-Pin female connector, see Figure 1) and a Databus hub.

Standard sizes include the following part numbers. Custom sizes are available on request:

For the construction of extension cables, ICT uses Hartland HCG304 cable that is a 4-core cable with 16 strand 20 AWG conductors. Standard insulation colours of the conductors are Red, Yellow, Green, and Blue.

Figure 1 below shows a typical example of the connector on a standard extension cable and is identical at both ends of the cable.

Figure 1

Most ICT sensors can have cables extended up to 4km however some high peak current sensors such as those which have heaters incorporated into their design are limited in the length of extension cable due to voltage drops in the cable when the heater coil is energised.  If there is too much voltage drop caused by the resistance of the extension cable, the sensor interface may not function properly. 

The sensor current and cable resistance determines the voltage drop on the cable and if the resultant voltage drop is greater than 1 volt then the reduced operating voltage at the sensor interface may impact the operation of the sensor.   This is discussed in further detail in the Considerations for Customer Made Extension Cables section.

Considerations for Customer-Made Extension Cables

As a general rule, if a customer is to construct their own extension cables, the system should be designed to limit any voltage drops in the extension cable from the voltage supply to the sensor interface to no more than 2 volt from end to end.  For a measured 2V drop in an extension cable, there will be 1 Volt lost on the supply wire and 1 Volt lost on the ground wire.

Using Ohms Law of V = I R, the customer can calculate a maximum cable resistance from a known sensor current and the maximum allowed voltage drop of 1V with the equation:

R_max = V_{max drop} / I_{sensor current}

As an example, an HRM sensor interface draws 667mA of current when the heater is energised, this means that the maximum resistance of the extension cable should be:

R_max = 1V / 0.667 = 1.5 Ω.

Once the maximum cable resistance is known, the maximum cable length can be derived from the cable manufacturer's specified resistance relative to distance. Cable resistances are usually expressed as Ohms per Kilometre.  The maximum cable length is determined with the formula:

L _{max cable length} = R_max  / R per kilometre

For example, at 39 Ω/km, a 20AWG cable (cross-sectional area = 0.5 sq mm) as used in ICT extension cables can be calculated as:

L _{max cable length} = 1.5 Ω / 39 = 0.038 km or 38m

The above calculations are based on the current required for a single sensor.  If a system is designed to use a large number of any type of sensor, to minimise the consequences of too many high load sensors operating at the same time, ICT sensors have a staggered order of operation based on the serial number of the sensor.

The last two digits of the serial number determine the order of operation such that in any 20 sequential serial numbers of the one type of sensor, only one will operate at any one time.  The second last digit is ordered based on it being odd or even while the last digit is ordered based on its actual value.

As an example, if a series of sensors fill the serial numbers #500 to #540, the first sensors to operate at a given logging interval will be #500 & #520, then #501 & #521, then #502 & #522 and so on.

Because of this staggering of the sensor operation, the number of sensors operating at any one time is significantly reduced and the cable diameters required to minimise voltage drops is also consequently reduced.

In the above example of calculating a maximum cable length, if the system had 60 sequentially serial numbered HRM sensors, there would be a maximum of three sensors operating at any one time.  If all the sensors were situated at the end of the same extension cable, then the cable can only be one third of the calculated length of 38m.  If a longer cable run were required, a larger diameter extension cable would have to be installed.

If the customers’ system layout requires extension cables to be longer than can be achieved with the standard ICT extension cable, a simple solution is to use household electric wiring cable.

Using the method described above, a 4mm² electric wiring cable will have a resistance of 4.75Ω/km.  Therefore, a single HRM sensor can operate reliably on an extension cable with a length:

L _{max cable length} = 1.5 Ω / 4.75 = 0.315km or 315m

Figure 2 below shows a 4mm² cable stripped for insertion into the Databus connector. To fit into the screw terminal of the Databus connector, some of the individual copper strands of the cable need to be cut away first.  These strands need to be cut back as close to the insulation as possible so that there is no possibility of a short circuit between the wires when the cable is inserted into the screw terminals of the connector.

Figure 3 shows the cable inserted into the connector.  Note that the inner core insulation is only stripped back far enough that the conductors are not visible when inserted into the connector, thereby minimising any risk of a short circuit between the terminals.

As with the ICT Databus Extension cables, the connector wiring is identical at both ends of the cable.

Figure 2	Figure 3

High load ICT sensors

The typical loads of ICT Smart sensors are published on the Internet, but as a quick reference, the table below lists the load currents of some of the more frequently used sensors that, because of their high load demands, warrant careful consideration when extension cables are to be incorporated into a system.  Note also that the current drawn by a TDP sensor is continuous and as such does not benefit from the staggered switching of other sensors

Specifications

Note: Cables joined inline using the standard DataBus Conn-X-All connectors are not inherently weatherproof. When making these joins, use the ICT Cable Joining and Waterproofing Kit to make sure your connections are waterproof and reliable.

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Last Updated: 17:47 on Wednesday, October 24, 2007

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