Are you talking about the Teltonika FMB 125 device ?
You can capture the vehicle battery voltage through the Analog Input 1 of the device.
The device itself will give you the the vehicle speed through GPS calculation.
If you want to capture the Vehicle CAN Data, you need to use additional Teltonika Vehicle CAN adapters (LV- CAN200 & ALL-CAN300) with FMB 125.
RS232 port in the FMB 125 is used for connecting RFID reader,LLS Sensors(Fuel Level) and Garmin devices.