What's the difference between M26FA and M26FB

What’s the difference between m26FA m26FB.
I mean does some documents available like this one?

The main component PA (power amplifier) is different, other are consistent

the main problem I faced with is:
I could not upload the Opencpu SDK and got this error
FAIL, META_ConnectWithTarget_r, [36], BootROM start command passed, but target has no response until timeout

why did this error occur? and what does it mean?
I couldn’t find it in any documentation.

This baudrate is too big… try 460800

I have checked it out with all baud rates!

it really does not change!

QFlash_V5.8,Please try again!

It does not create that error, but the application doesn’t start.

I worked with M66 before and I know that after “Boot meta in brom stage…” step.
It turns to a Clear NVRAM. this version doesn’t do this step!

even if I upload the core (which is located at QFlash_V5.8\Release\dl_src\M26 ) it succeeds but it won’t work!

I have tested with 2 modules!

M26 M66 … RX & TX are exchanged

I know that sir! if it is not correct, it won’t program from the first step!
I Think the SDK core of the module that I bought might have a problem!
the core was:

do you have the core for M26FA ?!
I found a core of the M26FB in some Chinese forums. but It does not upload with FlashTools of the MediaTek.
the soc of module is MT6261. it means we could program the core with it. since it does not work.
although this core program could write on the module without any error with FW_Upgrade_Multi_V4.51_MCx0
and it is so Odd!

It will not work only if the SOC has some physical defect, test with other module
SDK has nothing to do with flashing
There should be a Mediatek Header at the beginning of the Application BIN file ( build targe t)

MT6261 have small “smart” ROM boot-loader inside
at beginning uploader connect and read some info from the SoC
first stage, uploader send small BIN file to RAM ( Boot Loader )
second stage uploader send big BIN file to RAM ( Download Agent - real flash application )
stage 3 uploader change baud rate to HI speed
next stage is real flashing

if you understand from Python you can use this too

BTW: Old Mediatek uploaders it will not work - not support “all” Flash chips