6ES7422-1BH11-0AA0 Siemens S7-400 Digital Output Module Problem

I'm experiencing a problem regarding Voltage output of channels in S7-400 6ES7422-1BH11-0AA0 DO Module. I've connected the positive in Terminal 3 of DO module and negative to terminal 23 of DO Module to a 24vdc power supply.
When a channel is OFF, the reading is 21vdc. and when it's ON the reading is 24VDC. Did i left something about the wiring or the module is damaged? thanks!