Regarding the issue of not being able to read the value, if there is always no change in 32767, the value already exists, it is just an overrange. If the value is 0, then attention should be paid to whether there is a problem with the analog quantity. Using a multimeter to measure the on-site signal does not exceed the limit. Why do these two phenomena occur? This is because the selected reference potential is different. For example, if the signal coming from the site is 5V, the first thing to ask is what voltage is the reference point? 10-15 is 5V, and -10~-5 is also 5V. If the reference point of the measuring end is 0V, there will be a problem with the measurement, so it is necessary to ensure that both ends are equipotential. The reference potential point of the analog module is MANA, and all wiring is related to it.

02
Isolation and Non Isolation Issues Series

The isolation here refers to the isolation between the reference potential point MANA of the analog module and the ground (also the data ground of the PLC). The isolation module MANA can be disconnected from the ground M and used as the reference potential for the measurement end; The non isolated module MANA must be connected to ground M, so that ground M becomes MANA as the reference potential for the measurement end. The advantage of isolation modules is that they can avoid common mode interference. How to determine if a module is an isolated module, such as the SM331 module, can be found in the template specification. Only one SM334 module (excluding SM355) in S7-300 is non isolated, and the analog signals integrated in CP31XC are also non isolated. The common feature is that the output and input of the module share the same M-terminal.

Similarly, sensors also have issues with isolation and non isolation. Usually, the negative terminal of a non isolated sensor power supply shares a common terminal with the negative terminal of the signal, for example, a sensor has three terminals L, M and S+, through L, The M terminal supplies power to the sensor, S+, M is the output of the signal, and the common M terminal. It is best to refer to the manual to determine whether the sensor is isolated. The negative terminal of the isolation sensor signal can be disconnected from the ground M, and the signal negative terminal can be used as the reference potential for the signal source terminal. The negative end of the non isolated sensor signal must be grounded at the source end (device end), with the ground at the source end as the reference potential for the signal.

The following is how to ensure equipotential wiring between the measuring end and the signal source end. The abbreviations and mnemonics used in the suggested connection diagram below have the following meanings:

M +：  Measuring wire (positive)

M -：   Measuring wire (negative)

MANA：  Reference potential point of analog module

Attention should be paid to MANA here, as different wiring methods are based on MANA as the reference potential.

M：     ground terminal

L+: 24 VDC power terminal

UCM: The potential difference between MANA and analog input channels or between analog input channels

UCM common mode voltage has two types:

1) The potential difference at the negative end of different input signals, for example, if one input signal is 3V and the other input signal is also 3V, but their reference point potentials may be different, possibly 1~4V or 3~6V, then the common mode voltage between them is 2V.

2) The potential difference between the negative terminal of the input signal and MANA.

The UCM of the module is the main cause of analog values exceeding the upper limit. The maximum value of UCM varies among different modules.

UISO: The potential difference between the MANA and CPU's M terminals

03
Connect isolated analog modules to isolated sensors

The connection diagram between the isolation sensor and the isolation analog signal is shown in Figure 1:

Figure 1: Connecting isolated sensors to isolated analog input modules

This method is the simplest, both are isolated from ground and do not require grounding. However, if the input signal (sensor) negative terminal and MANA voltage exceed the maximum limit of UCM, for example, if SM331 (6ES7331-7KF02-0AB0) is 2.5 VDC, it is necessary to short-circuit the signal negative terminal and MANA, otherwise there will be an over limit problem. On site inspection shows that almost all over limit issues are due to the lack of connection between the signal negative terminal and MANA. If the UISO exceeds the limit, such as 75V DC, it is necessary to connect the signal negative terminal, MANA terminal, and ground terminal M. At this time, the module uses the ground terminal M as the reference potential and actually becomes non isolated, which is rare.

Some module channel groups are isolated without MANA, such as module 6ES7331-7NF10-0AB0, with wiring as shown in Figure 2:

At this point, the M - of each channel group (2 channels per group) is MANA, and the maximum UCM between input channel groups is to achieve 75VDC.

Connect the negative terminal of the signal to the MANA terminal when all are isolated (except for 2-wire system and resistance measurement). The MANA in each module wiring diagram of the manual is recommended to be grounded, which I believe is in the case of good grounding and no common mode voltage (such as single ended grounding).

04
Connect isolated sensors using non isolated analog modules

This time I will talk about the case of using non isolated analog modules to connect isolated sensors. The MANA of the module is not isolated from ground M, so it must be connected to ground M. The reference point potential of the analog signal becomes ground M. The typical wiring is shown in Figure 3:

Non isolated modules require a connection between MANA and ground M, such as module SM334 (6ES7334-0CE01-0AA0). It is emphasized in the prompt that it must be connected. The following is the prompt section of the reference manual.

05
Connect non isolated sensors using isolated analog modules

If the sensor is not isolated, the signal source end will use the local ground of the sensor as the reference point potential. The module is isolated, with the MANA point as the measurement reference potential. The typical wiring is shown in Figure 4:

From Figure 4, it can be seen that the negative terminal of the non isolated sensor signal is grounded at the source end. However, if multiple non isolated sensors are connected and distributed in different places (different grounding points), this situation can be quite troublesome. The negative terminals of each sensor signal will have a common mode voltage UCM. In order to eliminate UCM, short and thick wires are used to equipotentially connect the negative terminals of each signal at the source end. Due to the potential difference between the module's MANA and the ground at the signal source end, the MANA must also be equipotentially connected to the ground at the source end. Short circuiting cannot be performed at the module here, otherwise UCM cannot be eliminated.
If the factory is poorly grounded, it is best to use isolated sensors.

06
Connect non isolated analog modules to non isolated sensors
If non isolated analog signals are used to connect non isolated sensors, all points must be grounded and subjected to equipotential treatment. The typical wiring is shown in Figure 5:

From Figure 5, it can be seen that according to the requirements of isolation and non isolation, if the module is not isolated, it must be connected to MANA and ground M. If the sensor is not isolated, it needs to be connected to the negative terminal of the signal to the local ground, so that the ground of the signal source is used as the reference point while the ground M of the module is used as the reference point. In order to eliminate the potential difference (common mode voltage UCM) between the two, a sufficiently thick wire needs to be used for equipotential connection.

If the entire factory has an equipotential grounding grid, it is relatively simple to use non isolated instruments and modules. Simply connect the MANA to the local ground M, as each point is equipotential. In the past, things went against our wishes. Due to the cheap price of non isolated instruments, the more places where such instruments are used, the ground is usually not well grounded, let alone the grounding grid and equipotential connection. Not taking measures is definitely a problem, and it is necessary to ensure equipotential. A multimeter can be used for measurement because it is isolated from ground, and the maximum common mode voltage UCM may also be different and not under the same conditions as the module. Suggest using isolated sensors and modules.

After discussing a series of wiring methods, the final conclusion is that the various methods of analog wiring all converge on one point, which is that the signal source and measurement terminals must be equipotential.

At this point, I think we need to further expand on this principle as it can also solve the problem of digital wiring. The following is a problem encountered on site, as shown in Figure 6. The power supply for the CPU and I/O is separated. I/O is a non isolated module, and when a signal is given on site, the input light of the I/O module does not light up and cannot be read in the CPU. A multimeter is used to measure, and there is a 24V voltage on the terminal. There is no problem with the module. By short circuiting the M terminals of the two power supplies PS, the input signal can be detected, which is also caused by the different reference point potentials. I hope a small tip can help everyone solve the problem of on-site analog wiring.