I changed the DALLAS 1287 chip and reconfigured it with reference diskette, 
but
now the system boots directly in IBM BASIC.
I tried to SYS C: and refresh MBR, I also checked autoexec.bat and
config.sys with no result.
Until now the only way to boot to DOS is from diskette and then transfer
control to fixed disk with COMMAND C:
May it depend on the CMOS configuration ?

This finally happened to me...

Instructions on how to fix this have been posted to the group. The basic
procedure is to get a DOS boot disk put together. You'll also need your
system's reference diskette. Put a copy of DEBUG on the DOS boot disk.

Boot the disk and start DEBUG. Type in the following:

- o 70 2E
- o 71 FF
- q


Power off and restart. Now you'll need your reference diskette.

This worked fine for me with a 55SX and a DS12887+. I've heard that some
55SX still won't boot after this--if that happens, I don't know what to tell
you...you might try the more complex Debug script shown here:

http://www.computerhope.com/rdebug.htm#1