Retrofitting an old industrial CNC mill with EtherCAT components - MAHO MH400E
29 May 2026 17:20 #346741
by PedPEx
Replied by PedPEx on topic Retrofitting an old industrial CNC mill with EtherCAT components - MAHO MH400E
Hello guys,
Here is a little update. Since my last post, I had a detailed look at my picture gallery, specifically at the pictures of an EL5021 terminal I bought and disassembled back in early 2025. The original plan back then was to add an axial resistor in order to get it to read 11µA encoder signals. I quickly realized that wouldn't work, took some pictures, reassembled it, and returned it to the seller.
Last weekend, I took another detailed look at the PCB of the EL5021 and recognized the interpolator IC Beckhoff is using: an iC-Haus iC-NQI. For my own interpolator solution, I had actually copied the typical application circuit straight from that datasheet. That's why I immediately reordered an EL5021 terminal off eBay, opened it up, and properly reverse-engineered it today.
Beckhoff is actually following this required application circuit almost exactly ( Datasheet iC-NQI, Page 24, Figure 30 ). They are also using 4.7k Ohm resistors for RS1 and RS2. Those have to be swapped for 50k Ohm ones (I will test if these are too big; possibly 43k or 47k are better suited), and lastly the 120 Ohm resistors need to be removed. This way, the EL5021 terminal should be able to read 11µA sensors. By adding a 120 Ohm axial resistor across each input channel (A & /A, B & /B, and C & /C), the EL5021 can again be used for 1V signals. In theory, it would also support TTL signals, but that is outside the scope of this test.
Other than that, I vibe-coded a CiA402 dashboard tab for LinuxCNC to be able to see any problems with the Danfoss VFD directly in one place, without needing to decode the status word or the command word. The Danfoss VFD is now also able to read the frequency of an inductive sensor, internally calculates the motor speed, and reports that speed back via a PDO as feedback speed.
Additionally, the decision was made not to keep the original relay-based DC gearmotor solution, but to switch to two Beckhoff EL7332 terminals. Details about that coming soon™.
I'll test the functionality of the modified EL5021 terminal and keep you posted
Wishing you all a great weekend!
Here is a little update. Since my last post, I had a detailed look at my picture gallery, specifically at the pictures of an EL5021 terminal I bought and disassembled back in early 2025. The original plan back then was to add an axial resistor in order to get it to read 11µA encoder signals. I quickly realized that wouldn't work, took some pictures, reassembled it, and returned it to the seller.
Last weekend, I took another detailed look at the PCB of the EL5021 and recognized the interpolator IC Beckhoff is using: an iC-Haus iC-NQI. For my own interpolator solution, I had actually copied the typical application circuit straight from that datasheet. That's why I immediately reordered an EL5021 terminal off eBay, opened it up, and properly reverse-engineered it today.
Beckhoff is actually following this required application circuit almost exactly ( Datasheet iC-NQI, Page 24, Figure 30 ). They are also using 4.7k Ohm resistors for RS1 and RS2. Those have to be swapped for 50k Ohm ones (I will test if these are too big; possibly 43k or 47k are better suited), and lastly the 120 Ohm resistors need to be removed. This way, the EL5021 terminal should be able to read 11µA sensors. By adding a 120 Ohm axial resistor across each input channel (A & /A, B & /B, and C & /C), the EL5021 can again be used for 1V signals. In theory, it would also support TTL signals, but that is outside the scope of this test.
Other than that, I vibe-coded a CiA402 dashboard tab for LinuxCNC to be able to see any problems with the Danfoss VFD directly in one place, without needing to decode the status word or the command word. The Danfoss VFD is now also able to read the frequency of an inductive sensor, internally calculates the motor speed, and reports that speed back via a PDO as feedback speed.
Additionally, the decision was made not to keep the original relay-based DC gearmotor solution, but to switch to two Beckhoff EL7332 terminals. Details about that coming soon™.
I'll test the functionality of the modified EL5021 terminal and keep you posted
Wishing you all a great weekend! 
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