This is due to two different Digi drivers being installed on the same system. If RealPort and MPI drivers are installed, be sure to use the appropriate ditty binary. The RealPort ditty binary is located in /etc/drp. The syntax is as follows:

# /etc/drp/ditty onstr AAAA offstr ZZZZ ttyx##

The MPI ditty binary resides in /usr/bin (default executable). The syntax does not require the full path:

ditty onstr AAAA offstr ZZZZ ttyx##