CSE Kingfisher PLUS+, Справочник по аппаратуре

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Superseded Hardware Manual 7.12 Page 60
1.21 AI-4: An a lo g Cu rre n t In p u t, 8 Ch a n n e l (Flyin g
Ca p a c ito r)
The AI-4 module provides eight individually isolated input channels, each capable of converting an
analog input signal to a digital signal for use as required by your application. This module provides two
input ranges. The default range is 4 to 20 mA with user data scaled so that 4 mA corresponds to a
count of 0 and 20 mA corresponds to a count of 32760 with each 1000 counts representing 0.5
µ
A. A
link on the rear of the module selects 4-20 mA or 0-20 mA for all channels.
Conversion speed for each of the eight channels is 250 ms. By default the update rate is 8 seconds
but can be configured in the range of 1-10 seconds. Resolution of the converted signal is 12 bits
binary (1 part in 4096) over the range. User data in the #AI registers is in 16-bit 2'x complement
format. The placement of the 12 bits from the A/D converter in the #AI data word is shown below.
MSB
LSB
0 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 0 0 0
If the current source is reversed into the input, or is less than the low end of the current range, then
the module will output a data word corresponding to the low end of the current range (0000H in #AI). If
an input that is out of range is entered (i.e. greater than 20 mA), the A/D converter will output up to full
scale (corresponding to 7FF8H in #AI).
Scaling of the input is shown below.
32760
0
0 4 20
Current (mA)
(4 to 20 mA range)
Raw
Value
(decimal)
32760
0
0
20
Current (mA)
(0 to 20 mA range)
Raw
Value
(decimal)
Figure: AI-4 Scaling for Analog Current 4 to 20 mA
Input protection for the module is sufficient to guarantee operation with reduced performance with up
to 1500 V common-mode. The module provides electrical isolation of externally generated noise
between field wiring and the backplane through the use of flying capacitor relay scanning system.
To minimise the capacitive loading and noise, all field connections to the module should be wired
using a good grade of twisted, shielded instrumentation cable. The shields can be connected to GND.
The GND connection provides access to the backplane (frame ground).