How can I stop the interference of the A4988 on MAX31855?
See figures.
The moment I enable the stepper (A4988), the temperature measurement drops.
Formerly I had everything on one board. So I decided to separate the temperature controlling and motor by using two boards (and using I2C-bus). But the problem is still not solved.
Hi, @JohanGoris
Welcome to the forum.
Can you please post some images of your project so we can see your component layout?
Is there a reason you are using two Arduino controllers?
How far apart are they?
What is your power supply for the stepper?
Can you please post link to specs/data of the stepper motor?
Thanks.. Tom.. 
How close are the thermocouples to the motor?
How long are the thermocouple wires?
Images ? : did not I? (The image is a bit simplified)
Reason using two controllers : I thought that I could separate the stepper from the thermocouples.
The two controllers are about 10cm from each other.
The power supply is 22V DC.
Specs stepper : model
42A02 from szruitech.com S/N : S3 10060268
http://www.rtelligent.net/upload/wenjian/motor/42A02（42HS40-D0524-001）.PDF
Regards
The thermocouples are more than 10cm from the motor away. There is even no metallic contact.
Thermocouple 1: wire is about 30 cm long till MAX31855 with shielding. No shielding from MAX31855 till arduino (about 40cm).
Thermocouple 2: wire is about 50 cm long till MAX31855 without shielding. No shielding from MAX31855 till arduino (about 40cm).
Sorry, actual photographs of your project.
So we can see your component layout.
With low level signals and noisy stepper motor environment, component and wiring layout are very important.
Tom.
Its interesting that
1: the thermocouples have different shielding and
2: the effect is more significant on the blue trace than the purple.
I cant see from the key which is which. Nor is there a vertical axis scale - ie is it temperature, microvolts or adc reading (0 - 16383)?
Are your thermocouples physically attached to something?
It appears to be an EMI problem.
Use shielded thermocouples and make sure the shield is grounded.
Make the thermocouple wires as short as possible and move as far away from the motor as you can.
Grounding the motor case may also help
This is still a test setup. That is why it still seems messy. The aim is to make a coating machine for laboratory purposes. A container of wax is heated. This is kept at temperature by controller 2 (Mega : slave). Via the PC I can give commands to regulate the temperature, speed of the motors, etc... The controllers return their data to the PC, such as the temperature.
I guess you gave me the solution. And so it's probably not an electronic influence from A4988 to MAX31855. It is most likely the influence of the stepper on the wires of the thermocouple. (EMI as you said). I will therefore do another test in a few days : I will wrap some aluminum paper around the wires, maybe also a shield of the MAX31855. And also taking care of the grounding. I'll let you know. I thank you in advance.
Hi,
If you tidy up the other wires, I know its the prototype/proof of concept, you should see some improvement in performance.
Tom...
What current rating?
Thanks.. Tom...
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