Issue with LOGO! memory bits setting and Modbus TCP/IP

Hello everyone,

I have programmed a LOGO! and the program works perfectly locally. However, I am experiencing an issue with the integration of LOGO! logic and Modbus TCP/IP in the SCADA system. I've tried many solutions, but let's start with what works: when I connect contact M1 (a memory bit) directly to coil M1, it works. When I set the M1 bit from 0 to 1 via Modbus, the online test shows that M1 is set to 1.

The problem arises when I add something else in that line. For example, if I connect another contact in series with coil M1, the program does not work and M1 cannot be set from 0 to 1. I am providing photos 1 and 2 for reference. The logic in photo 1 works correctly; I can set M1 from 0 to 1 and from 1 to 0. However, in photo 2, where I have an RS contact connected to coil M1, I cannot set M1.

Interestingly, coil M1 doesn't need to be connected to a contact to work; it can be left unconnected and I can still set the M1 bit. Additionally; if I connect contact M1 to coil M2, and contact M2 to coil M1, and if i set M1 or M2 to 1, both M1 and M2 start blinking rapidly.

Could someone advise if I am missing something or misusing memory bits? Note that I use a converter from ladder to FBD to import the diagram into the network project.

Thank you, is it possible to reset an M flag within the program if I cannot connect anything to the left of the M flag (since the M flag is previously set by Modbus)? If it is not possible, would it be better to use network input/output or another type of block for this?

I have a question about retentivity as well. I understand that retentivity in LOGO! is only possible with certain blocks like the pulse relay or latch. However, I encountered an issue:

• I set M4 to 1, Q4 was consequently on 1 - then I powered off my LOGO!.
• After 10 seconds, I powered it back on, but Q4 was not set.

Therefore, it seems that retentivity is not working as I expected. Is it possible to make M flag or some other type directly retentive? 

I want to set the remote fault signal Q1 with M1 if remote control is enabled (I5) ; this is line 1 in the photo.

Then I want to reset the remote fault Q1 when I acknowledge the fault locally; this is line 2.

After that, I would like to reset the M1 bit; otherwise, line 1 will become true again whenever the remote control is enabled.