There really is no need to have a user manual with this stuff. As Fleeced said, it's all about the software and pretty much the functionality of one box to the next is the same, the main issues would be DVR or Non DVR or older MDN boxes versus newer ODN boxes. The 8642 and 8742 are practically identical functioning boxes, just different looking physically and the 8742 has the external power brick. So if you absolutely must have a manual check out the one for the 8642, but looking at it, it doesnt tell you how to use any of the functions, just how to hook it up to various types of equipment.
Since Cisco (along with Samsung and Motorola) just manufacture the set top boxes, each MSO puts their own firmware on them. Especially with firmware being updated all the time, it would be near impossible for the hardware manufactures to put out manuals. Ive never bothered to read a user manual for anything electronic, but when I got my DirecTV DVRs last year, DirecTV provided documentation with them and briefly looked it over, the problem was it was outdated, I could tell that just by flipping through the pages.
Anywho, heres the closest thing to a user manual youll find for the 8742.
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www.cisco.com/en/US/docs ··· 345C.pdf