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Hydrastep 2468CB and 2468CD Manual 2468CB & 2468CD Dual Power Supply Version
24685034 2-5
2.3.1 INPUT BOARD (PCB 24680501 AND PCB 24680516)
The input board processes the electrode inputs to provide water level data for display
purposes and a current output representing the water level. Fault detection is also carried
out where the condition of the electrode inputs are examined and a FAULT is indicated
when:
1. An open circuit in either of the electrode conductor cores is present.
2. A short circuit to EARTH on either the electrode or conductor cores.
3. A water above steam condition exists.
4. An internal circuit fault condition exists.
A current output circuit is also provided on each input board. This gives an analogue
representation of the water level. The sense (forward or reverse) and type (0-20mA or
4-20mA) is selectable and described in Section 2.5 of this chapter. Fault indication on the
analogue output is an oscillating waveform (of approximately 0.5 Hz) superimposed on the
main analogue signal.
The electrode inputs to each input board are passed to the other input board so that each
analogue circuit can output the full range content of electrode input signals. However,
should either of the input boards fail, the remaining board recognises that data is missing
and doubles its own electrode input signal to remedy the data loss giving a maximum error of
± 1 electrode.
Provision is made on each input board to accept one or two output boards, either relay
outputs or opto-isolated outputs. Both types of output board are offered as options.
Each input board produces the power supplies for the whole instrument which are derived
either from the local mains voltage supply of 110V ac or 240V ac nominal for the 24680501
input board or 20V dc to 40V dc for the 24680516 input board.
Apart from the input voltage there are two other differences between the AC (24680501) and
DC (24680516) input boards. These are the analogue output drive capability and the
remote display drive capability.
2.3.1.1 Analogue Output Drive Capability
With the minimum DC supply voltage of 20V for the DC input board (24680516) the
maximum load that can be driven by the analogue output is 500.
At the minimum mains input voltage the AC input board (24680501) the maximum load that
can be driven by the analogue output is 600.
2.3.2 REMOTE DISPLAY DRIVE CAPABILITY
The remote display drive capability is only a consideration when the 2468C is providing
power for a remote display. With either input board the unit is capable of driving six remote
displays at up to 1000m (3280ft) from the unit but only powering one remote display (any
others must be locally powered). With the AC input board the maximum cable loop
resistance to the remote display is 27 whilst with the DC input the maximum cable loop
resistance to the remote display is 7. (See remote display section for more details).