INM831B Rev 4
5
4.1.3
3-wire RTDs (see Figure 8)
NOTE
3-wire RTDs cannot be mixed with 2 or 4-wire RTDs in the
same terminal bank - see start of Section 4.1 for more details.
1)
Connect the +ve and –ve leads of up to 15 3-wire RTDs to the
terminal pairs
I
1
to
I
15
, making sure polarity is correct.
2)
Connect the third (–ve) lead of each RTD, except the first, to the
+ve terminal of the previous RTD.
3)
Connect the third (–ve) lead of the first RTD to the –ve end of the
current loop (terminal 7).
Note: If the first RTD does not occupy terminal pair
I
1
, the +ve
terminal of the preceding terminal pair can be used for linking –
as shown in Figure 8.
4)
Add a link from the +ve terminal of the last RTD to the +ve end
of the current loop (terminal 38).
5)
To monitor the RTD excitation current, connect terminal 39 to 41
and connect terminal 40 to 42. The unit can then calculate the
current from the voltage developed across the built-in 100 ohm
resistor.
4.2
THCs and mV inputs
Connect up to 16 THC or mV inputs directly to the input terminal pairs
I1
to
I16
, making sure the polarity is correct (see figure 9).
Do not ground the THC or mV sensors unless it can be guaranteed
that they all are, and will remain, effectively at the same earth
potential.
4.3
Mixed inputs
The ability to mix different input sensor types depends on the
configurations available with the MTL838B receiver selected. See the
instructions supplied wtih the MTL838B receiver for details.
Connect the different sensors appropriately, as described in sections
4.1 and 4.2 above.
4.4
Highway connections
The data highway connections H1 (1,2) and H2 (4,5) are also the
source of d.c. power for the MTL831B. The user may choose either,
or both to provide redundancy.
4.4.1
Transmission distances
The highway length will depend upon two key factors: the type and
quality of the cable used, and the level of electrical interference
present in the environment.
Data highway
Data highway
Figure 8
- 3-wire RTD connection
21
20
19
I
6
18
17
I
5
16
15
I
4
14
13
I
3
12
11
10
9
8
7
Spare
6
5
H2
39
4
Spare
3
2
40
H1
1
1OOR
42
I
1
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
22
23
22
I
9
24
25
26
27
I
10
I
5
I
11
28
29
I
12
30
31
32
33
I
14
34
35
I
15
36
37
38
41
I
16
M
TL
8
31
B
An
al
og
ue
m
ul
tip
le
xe
r t
ra
ns
m
itt
er
Data highway
Data highway
Figure 9
- Typical THC and mV input installation
21
20
19
I
6
18
17
I
5
16
15
I
4
14
13
I
3
12
11
10
9
8
7
Spare
6
5
H2
39
4
Spare
3
2
40
H1
1
1OOR
42
I
1
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
22
23
22
I
9
24
25
26
27
I
10
I
5
I
11
28
29
I
12
30
31
32
33
I
14
34
35
I
15
36
37
38
41
I
16
M
TL
8
31
B
A
na
lo
gu
e
m
ul
tip
le
xe
r t
ra
ns
m
itt
er
mV
mV
mV
