RC Systems ST-72, Instruction Manual

Page: 62 / 142

ST-72 User's Manual
Document: UM-1007 Revision Level A
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3.2.1 OPTIONAL ANALOG INPUT BOARD # 10-0334
In compliance with CSA C22.2 No. 152 certification, the 4-20 ma
input does not include or imply approval of the LEL gas detection apparatus
such as sensors, transmitters, or devices connected to the system. In order to
maintain CSA Certification of the system, all 4-20 ma gas detection
instruments connected to the input must also be CSA Certified.
Many transmitters or sensors have analog output signals and the 12 bit Analog Input
Board, shown in Figure 3-2, is available to accept these. TB1 and TB2 with 24 positions
each, offers 3 terminals per channel for distributing power and receiving analog inputs.
These are
EXC and HI / LO inputs. TB3 and TB4 with only two positions each, is for daisy
chaining the power supply from one option board to another. When the transmitters
are hi power three wire units it is recommended this power be supplied to each option
board with its own pair of wires connected directly to the DC power supply. Precision
150 ohm 5 watt resistors (R1 – R16) between each channel’s
IN LO and IN HI terminals
are termination resistors for 4-20mA inputs.
EXC and IN LO terminals are bussed together internally. EXC terminals are tied directly
to TB3 and TB4 +24 VDC and IN LO terminals are tied to TB3 and TB4 power supply
common. Bussing allows transmitter power to be brought into the system at a single
point (TB3 or TB4) and distributed back out at each channel’s
EXC / IN LO terminals to
simplify field wiring. Figure 3-2 includes typical wiring to 2 & 3 wire 4-20mA
transmitters.
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