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IP258
14
SENSOR CONNECTIONS
The sensor connections are on the left side of the terminal enclosure.
Each sensor has two screened twisted wire pairs, either as one dual pair cable, or two separate pairs
One pair is connected to the TX (transmit) group and the other to the RX (receive) group. The sensors are
symmetrical so either of the pairs can be chosen as TX or RX. The two cores in each group are connected to A and
B, the polarity is not important. Each screen connection, normally coloured green, is connected to the group’s SCN
terminal. Cable screens must not be earthed at any other point.
The un-screened length of the cores should be as short as possible, to prevent crosstalk, but in any case no longer
than 30mm.
RELAYS
The relay NC-C-NO labels represent the relay terminals in the de-energised state.
HART CONNECTIONS AND JUMPER SETTINGS
There are two clearly labelled HART Test Points, labelled A and B. These test points are for connection of a HART
Hand Held Communicator, to provide a local interface to the MSM400 if required. Above terminal blocks 7-12 there
is a plug selector labelled PL1.
The normal position is with the plug shorting out the left hand pins. In this case, the external 20 mA loop must have
at least a 250 ohm impedance to enable HART communictions. With the plug in the central position, the MSM400
itself provides this load in the 20 mA loop. The right hand position enables HART communication when there is no
external loop connected by connecting a 270 ohm resistor across the current output. (See Appendix D).
NOTE:
The 20 mA output will not function correctly when the link is in this right hand position, so replace it in position 1 or
2 after use!
CURRENT OUTPUT
The current output may be connected in loop-powered mode or internally powered mode.
See connections in Fig 10 below.
In Loop-powered mode an external power source is required. A minimum of 2.5v is required across terminals 16 and
17 for correct operation. The external voltage should not be more than 30v.
Note that the current output must not be routed through hazardous areas unless protected by an additional I.S.
barrier.
Terminal Number
Figure 10 : Alternative current output configurations