The negatives are stored with a sign in the rightmost position of the field – the sign is normally a letter and is dependent upon the rightmost digit in the value. This sounds more confusing than it is and you don’t really need to know too much about it in practice.
To enter negative values in packed fields you could use DFU or SQL or any other data entry tool and just enter the value as a normal negative – the system will handle the packing and unpacking for you.
Expanding Martin explanation, to store a packed field the format in hex for number 12345 will be:
For -12345 it will be :
Well, the last byte (‘5D’) will represent character ‘)’ but if you put this negative number in screen it will be displayed like :
because system will convert hex ‘12345D’ to hex ‘F1F2F3F4D5′ and ‘D5′ is letter ‘N’
To expand on Wilson’s answer… in his example it will only
display an N if you display the value with without consideration
for the sign. use a display field that allows for negative numbers
and you can display ‘-12345′.