PDP11 PeripheralsHbk 1972 - Trailing-Edge

PART I
Chapter 9
Data Acquisition
and Control Options
9.1 LOW LEVEL ANALOG INPUT SUBSYSTEM - AFCll
9.1.1 Introduction
The AFCII is a flexible, high performance, differential analog input subsystem for
IDACS-ll industrial data acquisition control systems.
The AFCII system multiplexes up to 1024 differentiClI input analog signals, selects
gain, and performs a 13-bit analog-to-digital conversion at a 200 channel
per second rate under program control. Three signal conditioning modules and
eight program-selectable gains allow the system to intermix and accept a wide
range of signals: low level (lOmv f.s.), high level (lOO.Ov f.s.), and current inputs
(1 to 50.rna f.s.).
Designed for accurate and reliable operation in demanding industrial environments,
the AFCll achieves high isolation and common mode noise rejection
through relay switched capacitor multiplexing. The subsystem also simplifies input
wiring, requiring only simple twisted pairs which connect to screw terminals.
Modularly constructed in eight-channel standard hardware units, the AFCll is
easy to configure to user applications, and simple to expand.
The analog input subsystem is particularly suited for data acquisition in the high
bnoise environments encountered in process monitoring and control, production
testing and laboratory applications. In such environments common and normal
mode noise, cabling and grounding problems can greatly affect the operation of
such transducers as thermocouples, strain gages, analytical bridges, and industrial
milliamp current transmitters. These problems can also affect the accuracy
and performance of the measuring system.
In typical applications, use of ungrounded sensors could cause common mode
voltages of up to 150 volts peak-to-peak (at power line frequency) to appear on
the input signal leads to the measuring system. For example, if thermocouples become
ungrounded during operation, large common mode voltages can appear in
coincidence with the signal. The design features of the AFCll allow either floating
or grounded signal sources thus insuring reliable, trouble-free operation. Due to
the flying capacitor design, the system tolerates common mode voltages in excess
of 200 volts. FET solid-state multiplexers, in contrast, can be seriously damaged
with common mode voltages over 25 volts.
9.1.2 System Organization
The AFCII system is completely modular for ease of system configuration and expansion.
For applications requiring 128 channels or less the system is available in
a single cabinet configuration. Systems requiring greater than 128 but less than
512 channels are housed in a dual cabinet configuration - one cabinet to mount
the electronics and one for the screw terminal connectors. Two dual cabinet con-
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