I wanted to run the 515 and the 525 side by side. It took me a while to find a serial cable to the pix that worked, but what really got me excited was ... the pix has a USB port! So why don't I connect to that instead of messing around trying to find a serial cable with the right characteristic?
Cables are either cross or happy. Most cables are happy, but sometimes you need a cross cable - for example, if you connect two computers back to back. With a happy cable between them, they sulk and won't talk, but if you use a cross cable, the computers are happy.
As with serial cables, so with ethernet, except that it's complicated by the fact that some clever switches work out for themselves whether the cable you're using is cross or happy, and adjust accordingly. And some don't. I, of course, have a mixture of these.
I try to keep track by saying "all red or yellow cables are cross, all others are happy". I just did some testing, and although I've mostly kept that rule, I do have five yellow happy cables, and one beige cross one. I've labelled the contrary ones so I don't get confused in future.
One nice thing about USB, is that any cable will do; you don't have to worry whether they're cross or happy. Of course, USB makes up for this by having umpteen sizes of connector; USB A, USB B, mini, micro and probably others I've forgotten about, and all in flavours of male and female. Serial only has two sizes; 25 pin and 9 pin. And, of course, two sexes. Unless you're a pix, in which case you have an RJ45 connector.
So you can see why, when I noticed the USB port on the pix, I was happy. But when I discovered, after a bit of googling, that the apparent USB port isn't connected to anything inside ... I was cross.