Hello Rik_Rodeny;

I have worked for a long time with S7-400 CPUs, and to my knowledge the only way to power them was through the dedicated PS 407 and PS 405 units. 

These are specifically designed to provide both 5 VDC and 24 VDC over specific pins on the rack backplane, among other designed signals and protections.

Even Siemens' Sitop Division, dedicated to providing power 24 V supplies for industry, has no equivalent for the S7-400.See the attachment below, taken from the Sitop Catalog KT 10.1:

https://support.industry.siemens.com/cs/ww/en/view/109745655

You can find more information on the specialized S7-400 power supplies in the following S7-400 Module Data manual (Chapter 3, Power Supplies):

https://support.industry.siemens.com/cs/ww/en/view/1117740

Hope this helps,

Daniel Chartier