could it be a modbus or instrument wiring issue

Good day
I am wondering if the problem i have is the hard wiring or the modbus comms. I have a couple of temperature sensors pt100 reporting to a temperature polling station. Then the polling station reports to a siemens s7 400 plc via RS485. The problem is 6 out of the 18 temperature sensors are not showing on the scada hmi. could it be an electrical wiring problem on the instrumentation or a modbus coms issue
 
Have the missing six temperature values ever appeared on the HMI? Or is the issue getting them to appear on the HMI?

If the temperature values have never appeared, then presumably Modbus configuration of the polling station is likely the culprit. The communication between the HMI, presumably a Modbus master, is over a couple wires to the polling station. If some values come through and others don't, it is not likely the wirng, because it works as it's been programmed to work. It's more likely the HMI configuration not polling the slave polling station or not displaying the values it receives.
 
Have the missing six temperature values ever appeared on the HMI? Or is the issue getting them to appear on the HMI?

If the temperature values have never appeared, then presumably Modbus configuration of the polling station is likely the culprit. The communication between the HMI, presumably a Modbus master, is over a couple wires to the polling station. If some values come through and others don't, it is not likely the wirng, because it works as it's been programmed to work. It's more likely the HMI configuration not polling the slave polling station or not displaying the values it receives.
Thanks David its indeed a Modbus issue. We tested all the 6 temperature sensors electrically, all ok. and they are able to display on the polling station. From the polling station an rs485 cable runs to a moxa switch which has inputs for other modbus devices. The switch then terminates to an S7 400 plc. apparently even some of the other modbus devices are having some erratic comms challenges.
suspecting the switch now. Question is how to interrogate this switch? The problem of the measurants is just a month old.
 
Normally you make a Modbus RTU interface with RS-485 a multi drop network.
Does the communication work ok with only a few sensors (other ones disconnected) and start when adding more sensors 1 by 1 to the network. Maybe the interface cannot drive so many slaves and you need to add a RS-485 signal booster like this one: https://www.proconel.com/product/pm485rep-rs232-rs485-to-rs485-isolated-repeater/
Maybe the cabling is not ok.
I always use Lapp Unitronic Bus LD for Modbus RTU. Is has been specifically made for 120ohm terminated RS-485 signals.
You can buy per meter at Conrad: https://www.conrad.nl/p/lapp-2170204-1-buskabel-unitronic-bus-2-x-2-x-022-mm-violet-per-meter-600618
Then you make a so called multidrop network.
Starting with the PLC with its RS-485 modbus interface output signals (A/B and SG signal ground, only A/B is also possible). Or is this your Moxa unit?
Then you start wiring from the PLC to 1st sensor, then from there to the next and so on to the last one, number 18.
At the end, where the cable ends, you connect a 120ohm resistor (normal one, 1/4 or 1/8W metalfilm) between A and B.

The benefit of this 120ohm impedance corrected cable, is that the signals remain the same from beginning to the end.

A mistake often made, is that somebody by accident puts a 120ohm termination resistor on on one of the Modbus slaves (sometimes they have a switch for this) and than you don't see the modules after this one.

Remember that 1 modbus slave can draw 20mA of current. So when you have 18 modbus slaves the modbus master must be able to drive 18x20mA=360mA. Then the repeater comes into the picture again.
I had problems in the past using Radysys embedded PCs that could only drive up to 5 modbus slaves this way.

I had machines with up to 80 modbus slaves, so then I had to use a good repeater and thicker cabling of 0.75qmm iso 0.22qmm....

But I see it as a hardware problem here (often it is).
But like what said in the previous answers: if the configuration in the software is not right, you also don't see it.
Start by connecting 1 and check the HMI (disconnect the other ones)
And add more slaves step by step.

Remember the 120ohm resistor that must be at the end of the network (not somewhere inbetween or on the master).
 
No one on the forum is at your facility, so we don't know
- what devices you have
- what your network topology is
- exactly what works and what doesn't work and when it worked or didn't work

Everyone has paper, pencil and a cell phone that takes photos.

Please sketch the network with lots of identification labels, because boxes with no identification are meaningless, and post the photo (insert image) here (it does not have to be super high resolution).

Doing so will help answer the questions below:

1. There are 18 RTDs. Where do the 12 working RTD's connect? Where do the 6 non-working RTD's connect? How many polling stations are there? What is polling station (brand/model)?

2. What is a Moxa switch? The term switch is frequently used for Ethernet communications, I doubt I've ever seen the term 'switch' used for Modbus RTU over RS-485.

Is the Moxa box a protocol converter gateway that converts from Modbus RTU (RS-485) to Modbus/TCP (ethernet)?

If not, what switching function does it provide?

3. >The problem of the measurants is just a month old.

Two years ago all 18 temperature signals worked OK and then one month ago 6 signals did not. Correct? If not, then what is the situation? Have these 6 temperature signals ever appeared properly on the HMI?
 
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