Filter
Top Products
IP258
44
Technical Notes:
1. At no time can the SMART communicator be attached across A1-A2, since the DC supply effectively short
circuits the transmitted and returned digital communications signals.
2. The minimum DC Voltage V1 required for satisfactory 20mA loop operation can be calculated from the
formula –
V1 > 2.5 + V2 + [20 x 10
-3
]x (R1 + R2 + R4 +R5)
D.3.6.3. An alternative way of looking at this voltage requirement is in terms of the maximum loop resistance
that can be tolerated, which has to be less than (V1 – 2.5) x 50 ohms.
The maximum allowable value for V1 is 30 Volts.
D.3.6.4. The HART protocol itself sets the maximum values that can apply to the loop resistances, labelled R1
to R5.
The total load on the loop (R1 + R2 + R4) must not exceed 1100 ohms.
In addition the maximum length of cable in a loop working on the HART protocol is specified as 3000
metres on a single screened loop cable, or 1500 metres if multicore multi-loop cabling is used.
D.4.0. How to connect the SMART Communicator
Assemble the Psion based SMART Communicator unit as shown in Fig. DI fitting the battery, 100 SMART
Datapak, SMART Interface unit and lead. The push on crocodile clips are optional.
Power the MSM400 Control unit from a DC supply as shown in Figure DII. A 250 ohm (or higher value) load
resistance
must be incorporated in the loop.
The SMART Communicator is self powered and draws no current from the loop.
The two wires from the SMART Communicator are interchangeable – it does not matter which way round they
are connected.
a) These two wires can be connected to the HART test pins inside the lid of the MSM400 control unit, using
the crocodile clips provided on the CK1 or CK3.
b) Alternatively connection can be made via the crocodile clips or hook on probes to each wire of the 2
wire current loop at a convenient terminal box or strip.
c) At the control room the two wires can be connected either across the “SMART” Load resistor or between
a terminal on the chart recorder/indicator and the other side of the loop.