SA380TX Hardware

The basic TX unit includes 10 digital inputs, RS485 and a 4G LTE modem.

Up to two expansion cards can be added; these cards may either add an additional 4 analogue or 8 digital inputs.

Block diagram and isolation policy

All of the green lines in the diagram below denote an isolation barrier.  Every block of the TX shown is isolated from every other block and from the case.

All of the individual analogue inputs are isolated from each other and from the auxiliary power supplies.  Isolation barriers are to a minimum of 10M Ohms at 1,000V DC.

The thick green lines denote the two analogue cards which may be fitted.

Power supply

The “universal input” power supply accepts any voltage from 85 to 264V ac.  A suggested connection is shown below.

The power supply is internally isolated from earth and the rest of the SA380TX.

Power can therefore be taken directly from the signalling 110V supply and no additional isolating transformer is required.

The earth pin of the IEC C6 socket is not connected internally.  The unit must be earthed through its connection to the equipment racking.

The unit requires earthing for functional purposes (EMC ground). The unit does not require a protective earth connection in a rail environment.

An internal 2A slow-blow fuse is provided.  The external fuse must also be a minimum of 2A slow-blow (often marked T2A).

Slow blow is required because of the inrush current of the power supply at startup.

Power consumption and external equipment

Power consumption of the unit is between 2-10W.  If the unit is having to supply external equipment, for instance current clamps or network routers, then power consumption will rise accordingly.

Maximum rated consumption is 40W.  At 110V, this is 0.36A.

Internal UPS

An internal hot standby UPS is fitted with 7.2Wh of backup. The backup time will vary from a few minutes to an hour or so depending on external equipment.

Digital inputs

Ten digital inputs are provided to monitor spare contacts of signalling relays.  These inputs are connected as follows:

All of the terminals marked “C” are connected together internally.  This allows easy wiring to signalling relays using twisted pair cable, as specified in railway standards.

Internal resistors limit the sense current to a few milliamps at 12V.

The digital inputs are fully isolated to a minimum of 10M Ohm at 1,000V DC.  This isolation ensures that the inputs are fully separate from earth, the logger’s internal logic and the analogue inputs.

Digital inputs must never be connected into a live circuit (eg across a contact that is already in use by the signalling system).

They must only ever be connected to spare relay contacts.

Analogue cards

Two types of analogue card are available.

4-20mA inputs

This is the card shown in the drawing of the back panel, which can be seen at the top of this page.  It has four isolated channels which are capable of powering 4-20mA current clamps.

The terminals are:

24:  24V out

S:   Signal input

0:    0V

The input impedance between S and 0 terminals is 200W.

The maximum output power of the 24V sensor power feed is 2W, or 83mA, per analogue input.

30V dc inputs

This type of card simply has four inputs that are capable of measuring 0-30V.  The input impedance between S and 0 terminals is 172kΩ.

Auxiliary power out

Each card can be factory fitted with an auxiliary power output on request.  For instance a +12V output could be used to power an Ethernet router or a fibre-ethernet converter.

As another example, adding a power output to the 30V card allows it to be used with sensors that require a ±15V power supply.

4-20mA analogue input card

This is the most common type of card.  Many types of external sensor are available with a 4-20mA output, including current clamps, temperature sensors, pressure sensors and voltage transducers.

The simplest 4-20mA sensors only have two connections and take their power entirely from the loop.  Others have three or four wires.

The four wire types use a separate signal and power ground to avoid interference between the power supply and measurement currents.

Example wiring to the different types is shown below:

30V dc input card

Where an external voltage is being measured, the voltage is simply applied between the S and 0 terminal.

Use with ±15V sensors

Where ±15 sensors are required, an auxiliary power supply can be factory fitted to the card on request.

External looping is used to connect the –15V from the supply to the 0V terminal of each analogue input. 

LED’s

The TX front panel features 5 LEDs.

The meaning of these LEDs is detailed in the table below: