Re: M2 mini ssd vs Nvme

On 11/15/22 2:38 PM, Paul wrote:
> On 11/15/2022 12:16 PM, philo wrote:
>> Without reading the specs, I purchased an M2 to SSD adapter which of 
>> course did not work with the Nvme I have, so I had to purchase an 
>> adapter specifically made for Nvme.
>>
>>
>> Just curious as to why the M2 ssd and Nvme have the same physical form 
>> factor.
>>
>> For a guy who started out in the punch card days, I find it slightly 
>> challenging to keep up with all this :)
> 
> A number of the low voltage differential signaling standards,
> share similar characteristics.
> 
> This means, if you "join the wrong things", there is no magic smoke.
> The circuit may not work, may not perform a function, but it is not
> blown out. The items are "voltage compatible". The terminators
> might be 100 ohm differential in all cases. Even HDMI and DP
> could be similar in that regard (but not a topic of discussion for
> this post).
> 
> Differential circuits have a "common mode voltage range". Perhaps
> any voltage between 0 volts and 3 volts is acceptable.
> 
> When they made Wifi modules, Wifi has both USB2 and PCI Express.
> They put those signals on separate pins. The USB runs at 5V,
> the PCI Express has that lower range to work with.
> 
> Well, when they made NVMe modules (M.2), someone figured out
> "we can put the SATA signals on the same pins as the PCI Express
> signals". Because they shared similar common mode ranges. The
> packets will look slightly different.
> 
> To provide some control of "what mates to what", they included
> keying slots. The circuit board on a plugin module, could be
> B or M or B&M. My NVMe here is an "M", which means it is
> PCI Express x4 only. There is no SATA option on that Samsung controller.
> 
> A Wifi module would have a keying arrangement, so the
> module would not plug into an NVMe slot.
> 
> This means, when an engineer builds a PCB for a computer, the
> BOM (bill of materials) has different part numbers for the
> NVMe socket versus the Wifi socket (on high end desktops). The
> part numbers have different keys molded into the insulation.
> 
> PCI cards had keys. AGP cards had keys. The keys back then prevented
> wrong voltages from being mixed. For example, my P4B motherboard,
> had a feature to check what voltage the AGP card wanted to use.
> If a certain mis-keyed SIS video card happened to be plugged
> into the AGP slot (the keys weren't able to protect),
> there was a transistor to sense that the voltage indicator
> on the vid card was wrong. And that's what happens, when somebody
> makes a bad mistake, and other engineers have to come up with
> adhoc means of protecting their products from damage.
> 
> I'm not going to analyze the various pinout options, but will
> leave it at "keying prevents not-compatible things from being mixed".
> Considering at least some of the standards on there, a PCIe or
> a SATA could use the same wires, and chances are, not blow anything out.
> That's why the pinout table says that PCI x2 or SATA could be
> presented to the signal pins, and it would work.
> 
>    PCIe_0  TX+       SATA TX+   \
>            TX-            TX-    \
>            RX+            RX+     \
>            RX-            RX-      \___ How two different things can
>    PCIe_1  TX+                     /    share interconnect.
>            TX-                    /
>            RX+                   /      The SATA option is a lot slower.
>            RX-                  /
> 
>     Paul
> 
> 
> 


As always, thanks for the great information.

In the future, maybe I will read the data sheets better.


anyway, I was able to do a data recovery for my friend