There is no way to send a negative number directly to a digital PI tag. Or more accurately, sending a negative integer to a digital state tag will result in a system digital state being written to the tag. For more details on why that happens, see this KB article.
Scott Robertson recently introduced me to a clever work-around for this issue. Instead of sending the negative number directly to the digital state tag you can instead do the following:
If you do not have PI AF Server at your site, then you have one option:
If you are new to enumeration sets, then these videos will help you get started: