According provided by radio synchronizer Yongnuo RF-603N many thanks to Dmitry Vrachinsky.
Yongnuo RF-603N is a simple radio synchronizer, produced for Nikon (RF-603N) and Canon (RF-603C) cameras. A model for Nikon cameras came to my review. The set, in a dense cardboard box, includes two RF-603N modules, a YN-N3 cable and instructions in English and Chinese.
Each of the two Yongnuo RF-603N modules can work as both a transmitter and a receiver, which seems to be a rather convenient solution. Each module is powered by two AAA batteries. Both Yongnuo RF-603N modules have so-called. 'TTL pass-through', which allows you to use an external flash by placing it on top of the module mounted on the camera.
But in fact, there is no end-to-end TTL. If you install RF-603N on the camera and flash on it, the camera will not see the flash.
Often in online shops people are confused by the 'TTL' lettering in the name of the Yongnuo RF-603N, making users think that the Yongnuo RF-603N supports automatic TTL flash remote control. This is not the case, Yongnuo RF-603N can only set fire to an external flash, and the flash power will have to be set manually.
For some Nikon flashes, you can trick a bit, details here.
Depending on the configuration in the box, you can find different cables for remote control of the camera. Such a cable is connected to a special connector on the module, which serves as a receiver, and the other end to the camera port. Thus, you can remotely release the shutter. Of course, the Yongnuo RF-603N has a standard sync port for studio flashes.
It is stated that the Yongnuo RF-603N supports shutter speeds up to 1/320 sec. I worked with him on camera Nikon D700 and with flash SB-900. If you set the flash sync speed in the camera menu to 1/320 sec., Then with the Yongnuo RF-603N you can work normally at shutter speeds up to 1/200 sec. inclusive. But for 1/250 and 1/320 sec. Black stripes already appear on the image. Strange but model Yongnuo RF-602 / N worked great with shutter speed up to 1/250 sec, with the same camera and flash.
On top of that, I didn’t like the fact that you cannot test the flashes (by pressing a button) without attaching the Yongnuo RF-603N to the camera. In this case, the selection of the synchronization channel is in the battery compartment, that is, to change / install it, you need to remove the batteries from the synchronizer. Even worse, the Yongnuo RF-603N is only tied to Nikon cameras. For example, you cannot attach it to a Canon or Pentax camera and remotely fire the flash. Usually, simple radio synchronizers that do not have TTL sync can easily ignite an external flash using a central sync terminal, thus maintaining compatibility with a large number of cameras and flashes. In this plan Yongnuo RF-602 / N much more functional.
Another little thing that can sometimes be annoying - the Yongnuo RF-603N power button is too close to the shoe and, with the flash installed in it, turning on the RF-603N is very difficult or impossible at all! You have to shoot the flash, turn on the synchronizer and set the flash back.
The RF-603N modules do not have fastening latches for mounting them on racks, tripods, legs, etc., the instructions say that you should use heads with their own clip, but I personally rarely see such. Also, the instructions do not indicate restrictions on use with certain flash units and cameras, but I suspect that problems may arise, for example with Nikon SB-600.

Yongnuo RF-602 / N and Yongnuo RF-603N
The cheapest is to buy Yongnuo synchronizers on aliexpress.
Results
Yongnuo RF-603N is a simple and inexpensive radio synchronizer. For myself personally, I would be looking for some other option, since I often have to work with several systems (Nikon, Pentax, Canon, Sony), and the RF-603N is not universal in this regard. Most of the shortcomings were fixed in the new version - Yongnuo RF-603 II.
Material prepared Arkady Shapoval.
Good evening. There are two flares: Yongnuo YN-600EX-RT II and ancient Yongnuo YN-468 II. Is it possible to make the YN-468 II a slave from the YN-600EX-RT II flash via the Yongnuo RF-603C? I suffered, and failed. Has anyone tried something similar? Yongnuo YN-600EX-RT II is on the camera, “master” mode, channel “auto”, ID 0000, and RF-603C on the YN-468 II. I would be grateful for the answer
But in fact, there is no end-to-end TTL. If you install RF-603N on the camera, and a flash on it, the camera will not see the flash ... .. I disagree, it sees perfectly, both the D90 and D 7100.