24-3
Chapter 24: Thermocouple Input Module
Figure 2: Module powered from the main unit
It is extremely important that the EG pin be connected to a good earth ground. This will pre-
vent communication errors due to differences in ground potential between modules in addition
to possible damage due to voltage transients and surges.
The two LED lights in the center of the module, marked Power and Pulse, indicate that the
module is receiving power and operating properly. The Pulse LED will blink at a regular rate,
like a heartbeat, once it establishes communication with the main unit.
Communications Wiring
The Thermocouple Input Module communicates with the SCADA 3000 using a high-speed
serial communications bus. This 4-wire bus is used to connect up to 15 modules to the
main unit to provide additional inputs and/or outputs. The communications cable must be
4-Conductor Twisted Pair
. You may use unshielded cable in electrically quiet environments,
but be sure to use shielded cable if your cabling must navigate around electrical machinery or
equipment.
Modules may be located a maximum of 2000' away from the main unit and should be connect-
ed in a daisy-chain fashion from one module to the next. Each module connects to the next via
a 4-wire communications cable connected to the terminals labeled
A,B, Y
&
Z
. The cable must
be
4-Conductor Twisted Pair
(shielded or unshielded) with a nominal impedance of 120W
(for example, Belden #8132 or 9842 cable). Use one pair for A & B and the other pair for Y
& Z. Note how the wiring is reversed between the first module and the main unit but between
modules the wiring is straight-thru. The proper wiring from the main unit to the modules, and
from module to module, is shown in Figure 3:
Figure 3: Incorrect daisy chain setup: Main unit in the middle of the chain.
The Main Unit should always be on one end of the chain, never in the middle.
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