ZT Zero tracking - this command enables or disables the zero tracking. Parameter = 0 disables the zero tracking and parameter = 1 enables the zero tracking. Issuing the command without any parameters returns the current ZT value. The command returns ERR and has no updating action unless it is preceded by the CE XXXXX command, where XXXXX is the current TAC. Zero tracking will be performed only on results less than +/-0.5d at a rate of 0.4 d/sec where d = display step size (see DS command). The zero can only be tracked to +/- 2% of max (see CM command). Factory default setting: 0.