Whitt2003.
book Page 89 Thursday, July 10, 2003 4:05 PM
PART II – CHAPTER 5: DESIGN PRACTICE 89
16-POINT
Form-A DO MODULE + -
Contacts
1
FU1
CR
2
2
FU3
CR
4
3
FU5
CR
6
4
FU7
CR
8
5
FU9
CR
10
6
FU11
CR
12
7
FU13
CR
14
8
FU15
CR
16
+ -
NOTE: Isolated Points, No Internal Bussing
Figure 35. Isolated digital output module
sources must be managed. In most cases, this is not a problem since extending PLC I/O
power to the field device is feasible. However, if a field device must source its own signal,
then an interposing relay must be added to the circuit to provide isolation.
Figures 36 and 37 show two different digital output modules. The first internally bus-
ses the DC(+) side of the circuit. The I/O point then provides a path to power, making it a
sourcing module.
If the module busses the DC-common side of the circuit as shown in Figure 37, then
the module is considered a sinking module. The I/O point completes the path to common.
This type of module is rarely used today, though several years ago it was common.
2. Analog Wiring
Unlike the discrete (on/off) circuit, analog signals vary across a range. Taking the same tank
described previously in Figure 29, how would the wiring change if we replaced the switch with a
level transmitter?
Notice in Figure 38 that we have the same circuit breaker panel as in Figure 29, but now it is
feeding a DC power supply. The power supply could be in its own cabinet, or it could be in the mar-
shalling panel. In any case, DC power is distributed in the marshalling panel. A single fuse could
power several circuits, or each circuit could be fused. The transmitter is fed +24 VDC at its positive
terminal. The 4–20 mA current signal is sourced from the (-) terminal to the PLC. Cabling is twisted
pair and shielded. The shield is terminated in the marshalling panel where all shields are gathered
and terminated to a ground lug that is isolated from the cabinet. Care should be used to ensure that
the shield is only grounded at one spot. Shields that are grounded in more than one spot inject large
noise spikes onto the signal. This condition is called a ground loop and can be a very difficult problem
to isolate, as the problem is intermittent. A “quiet” ground should be used to ground all the shields
at one point. The signal cable is numbered with the transmitter number, and the wires inside are
numbered to provide power source information.
Standard Analog Wiring
Whitt2003.book Page 90 Thursday, July 10, 2003 4:05 PM
90 SUCCESSFUL INSTRUMENTATION AND CONTROL SYSTEMS DESIGN
16-POINT
Form-A DO MODULE + -
Contacts FU1
CR
1
2
2
FU3
CR
3
4
4
FU5
CR
5
6
6
FU7
CR
7
8
8
FU9
CR
9
10
10
FU11
CR
11
12
12
FU13
CR
13
14
14
FU15
CR
15
16
16
NOTE: Internally-Bussed Power
Figure 36. Current-sourcing digital output module
16-POINT
Form-A DO MODULE - +
Contacts FU1
CR
1
2
2
FU3
CR
3
4
4
FU5
CR
5
6
6
FU7
CR
7
8
8
FU9
CR
9
10
10
FU11
CR
11
12
12
FU13
CR
13
14
14
FU15
CR
15
16
16
NOTE: Internally-Bussed Common
Figure 37. Current-sinking digital output module
That is the basic two-wire analog input circuit. Following is some specific information regard-
ing the various analog possibilities:
a. Circuit Protection (Fusing)
Analog circuits are always low voltage, usually 24 VDC, and thus below the danger thresh-
old. As a result, fusing individual analog circuits is generally not required for personnel
Whitt2003.book Page 91 Thursday, July 10, 2003 4:05 PM
PART II – CHAPTER 5: DESIGN PRACTICE 91
Twisted-Pair Unshielded Cable Power Supply Cabinet Breaker Panel
24-VDC Power Supply CB2
Marshalling Panel H 02H XXX H
03A 03FU DC(+) DC(+) 110 VAC
N 02N N XXN N
Twisted-Pair DCC
04FU 04A G
Shielded Cable G Level Transmitter
05FU 05A
06FU 06A
FJB LT-10
PLC 07FU
1 06A +
+ 06B 1 06B 2 06B -
PT
0
- 02N 2 DCC 3 SHLD
3 4
+
PT
4
-
+ Shield Ground
PT
2
Busbar
-
Twisted-Pair
+
PT
3
Shielded Cables
-
S
Figure 38. Analog circuit wiring technique
safety. Also, most analog I/O modules have current-limiting circuits onboard. So, fusing is
generally not required to protect the modules. If these two conditions are true—and the
designer should confirm this with the manufacturer—then per-point fusing can be avoided
if desired. If a designer wishes to save money by not fusing every point, then grouping the
circuits into damage-control zones should be considered. For example, if there is a pump
pair, a primary and a backup, instruments for the two should be in separate fuse groups to
prevent a single blown fuse from taking them both out.
b. Noise Immunity
Analog circuits are susceptible to electronic noise. If, for example, an analog cable lies adja-
cent to a motor’s high-voltage cable, then the analog signal cable will act as an antenna,
picking up the magnetically coupled noise generated by the motor. Other sources of noise
exist such as radio-frequency (RF) radiation from a walkie-talkie.
Twisted-Pair Cables
Electronic noise may be greatly reduced by the use of twisted-pair cabling. Most instru-
ments use two wires to transmit their signals. Current flows out to the device in one wire
and back from the device in the other. If these wires are twisted, then the noise induced
will be very nearly the same in each wire. Since the current flow is identical in each except
that it travels in opposite directions, the noise is mostly cancelled out.
Shielding
A further refinement in noise rejection is shielding. Most of the noise is rejected through
the use of twisted-pair cabling. Additional noise may be filtered out by use of a grounded
braid or foil shield. The shield should never be grounded in more than one place to avoid
ground loops. Most instrument manufacturers recommend grounding the shield at the
field instrument. However, a better place to do it is in the marshalling panel. It is easier to
verify and manage the grounds if they are in one place. Also, it is possible to ensure a good
ground at that point.
Whitt2003.book Page 92 Thursday, July 10, 2003 4:05 PM
92 SUCCESSFUL INSTRUMENTATION AND CONTROL SYSTEMS DESIGN
Conduit
A final refinement in noise rejection is grounded metallic conduit. This is rarely required
except for data communications cables, but is an option for particularly critical circuits.
c. Resistance Temperature Detector
A resistance temperature detector (RTD) is made of a special piece of wire whose electrical
properties of resistance change in a predictable way when exposed to varying tempera-
tures. The material of choice today is 100-ohm platinum, though other types such as 10-
ohm copper are sometimes used. For the platinum RTD, the rating is for 100-ohms at 0ºC.
Changing the temperature causes the resistance to change in the wire. These changes are
very minute, causing voltage variations in the millivolt range.
RTDs are connected to a Wheatstone bridge circuit that is tuned to the RTD. But, tun-
ing occurs on the bench. What about the field environment? We have already discussed the
line attenuation difficulties inherent in millivolt signals (Part II, Chapter 4). This problem is
overcome in the RTD circuit by the use of one or two “sense” inputs. These inputs help
negate the effects of copper losses due to long lines and temperature variations and are
additional wires that must be included in the RTD cable, hence the terms three-wire and
four-wire RTDs.
d. Thermocouple
A thermocouple device employs the property of physics that when two dissimilar metals
are joined, an electromotive force (EMF) is developed that changes with temperature. This
EMF manifests itself as a millivolt signal. When certain combinations of these dissimilar
metals are used, a predictable curve results as temperature at the junction is changed. The
signal is measured at the open end of the two wires, and a millivolt-per-degree scale is
used to convert the voltage to engineering units.
The thermocouple is thus a two-wire device. It is susceptible to radiated and induced
noise and so is usually housed in a shielded cable if extended for a very long distance. The
thermocouple signal is also susceptible to degradation due to line loss, so minimizing the
cable length is desirable.
Also, it is important to use the proper extension wire. A thermocouple usually comes
with a short “pigtail” connection to which extension wire must be attached. If a different
wire material, such as copper, is used to extend the signal to the PLC, a cold junction is cre-
ated that causes a reverse EMF that partially cancels out the signal. Therefore, the proper
extension wire should be used, else a device called a cold-junction compensator, or ice-point
reference, needs to be installed between the copper wiring and the thermocouple wiring.
Thermocouple I/O modules already have the cold-junction compensation onboard, so
using the proper thermocouple extension wire is required.
Specific types of thermocouples exhibit different temperature characteristics. A type J
thermocouple is formed by joining an iron wire with a constantan wire. This configuration
provides a curve that is relatively linear between 0 and 750ºC.11 A type K thermocouple
has a nickel-chromium wire mated to a nickel-aluminum wire, sometimes called chromel/
alumel. The type K thermocouple spans a temperature range of -200 to 1250ºC. Other com-
binations yield different response curves.
e. 0–10 Millivolt (mV) Analog
Analog signals were first generated by voltage modulation. In the old days, a transmitter
would generate a weak signal that had to be captured and then filtered and amplified so it
