Were do i find a list of what the codes mean ,for a peterbilt on board diagnostics scan?

we have a 1999 ,600 cat , peterbilt. It does over the road self diagnostics, and flashes a code at you.Where do i find out what these codes mean ?|||Manufacturer will be able to give you these. I suspect that they might be the same as those used on OBD automotive ECUs, so that would be another approach.