our problem is that if we connect the device directly to our Datalink, then the device does not work, if we connect your switchbox in between, then everything works without problems.
I can’t understand what the problem is.
The good thing is that everything works with your switch 
It’s the connection scheme:
This kind of thing is usually down to a failure for devices to auto-negotiate properly. The Datalink-connector and device probably have some kind of incompatibility between the two ethernet transceivers inside each, causing them to fail to make a connection. If this is the case, then you should be able to confirm that by seeing no link lights on either device.
Unfortunately it’s very hard to nail these problems down. It could be due to literally bugs in the silicon of the transceiver, causing one to not conform correctly to the ethernet spec, and the other device not accepting that. This is more common in older ethernet PHYs.
It could also be that your datalink connector and device are using transformerless ethernet, some devices do use transformerless ethernet, which can cause issues, but usually the device manufacturer will tell you this.
Putting a switch in between the two devices solves this because while both devices seem unable to form a link together, they can each form a link with the switch. Often this hack is used to overcome these issues because, unless the manufacturer of both devices is willing to issue a firmware upgrade, you’re probably not going to be able to solve it without using a switch in between.
Of course, if you do see a link light when both devices are directly connected together, then the issue is higher level than auto-negotiation, and it could be something related to traffic flow.
Thanks for the analysis, I will contact the manufacturer of the devices.
