IBM 5155  -  Basics, for 5155 beginners

Although described by IBM as the 'Portable PC', the 5155 is actually a portable XT.  That would have been done for marketing purposes.
This is a slow computer.  BE PATIENT.  After turn-on, it can be minutes before an error message is displayed.
The IBM supplied floppy controller and 5.25" floppy drive are double density.  They will not read high density 5.25" floppies, such as 1.2M (2SHD) ones.
There is no 'CMOS SETUP'.  All motherboard configuration is done via switches/jumpers.
There is no real-time clock (RTC) on the motherboard or on IBM-supplied expansion cards.  Some examples of third-party solutions are at here.
After turning off the power supply, wait at least 5 seconds if you are planning to turn it back on.  Any shorter; the power supply may not turn on.
The RAM test (and on-screen count) done by the motherboard's power-on self test (POST), excludes any expanded memory.  It does conventional memory only.
Expansion slot 8 is different to the other slots.  Most cards do not work in slot 8.  More information at here.
The 5155 motherboard does not have a keyboard controller chip.  Instead, it uses discrete components - see here.
The power-on self test (POST) of an IBM 5155 motherboard outputs only a few POST codes and does so to port 60h.  I have yet to see a POST card that monitors port 60h.
Aged tantalum capacitors are known to explode.  That has happened to me many times, particularly when I have acquired something that has not been powered on in years.  If the motherboard, or expansion cards, are exposed to your face when you power them on, then consider wearing eye protection.  More information at here.