SDQLCAI-1 Circuit Description
May 2002 4-1
4 Circuit Description
The Critical Analog Input Module (CAI) receives signals from 2, 3 or 4 wire field devices connected to
32 channels. Input signals are digitized by the CAI and transferred to a QUADLOG control module
(CCM/ACM+) via the IOBUS.
The Critical Analog Input Module consists of the following circuit elements:
Dual Central Processing Units (CPUs)
Dual A/D Circuits
Memory (RAM, ROM, and EEPROM)
Redundant IOBUS Interface
32 Channels of Analog Input Circuits
Isolated and Non-Isolated Power Sources
LED Indicators
Figure 4–1 shows a simplified block diagram of the CAI. Figure 4-2 presents a schematic of the circuitry
used in one of the 32 electrically identical, but software-configurable I/O channels.
4.1 Isolated Power Supply
The on-board power converter incorporates a switching regulator to produce a non-isolated +5 V and
isolated 12 V. Power for the converters is supplied by dropping the +12Vdc unregulated and isolated
supply to +5 Vdc using linear regulator.
4.2 IOBUS Modem
The IOBUS elements consist of an IOBUS Modem and dual IOBUS Line Drivers/Receivers. The CPU
supervises IOBUS communications through handshaking operations with the IOBUS Modem. The
IOBUS Modem performs the following:
Converts received IOBUS data into a format for data-processing.
Converts digitized field data from a CPU into a format for transmission over the IOBUS to a control
module (CCM).
4.3 Multiplexers/Converters
Channel are read with redundant multiplexer (mux)/converter circuits. In each circuit, the outputs of the
mux circuits are buffered and read by an ADC (analog-to-digital converter). Data from each ADC is
transferred over a serial interface (SPI) bus to separate CPUs.
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