Can I detect a subunit abnormality (cable, subunit main body) on the DR232 (expandable)?

You can with the following methods.
If you short any channel of any module attached to the subunit (always 0 V in voltage measurement mode) and set an alarm and relay output on the channel, the alarm occurs constantly, and the alarm output relay turns ON.
On the other hand, if an abnormality occurs on the subunit, the measured data becomes "±******", and alarm determination cannot be made on the measured data. Therefore, if an abnormality occurs on the subunit the alarm is cleared, and the alarm output relay automatically turns OFF.