Gustavo:
The purpose for the 13 MHz port is to adjust the RX clock (coming out from the E1/T1 line) with the internal clock inside the BOIA card.
when working with it you have to use a freq counter (10 digit precision), connect it to 13 MHz port, open Clock Control feature (under Objects menu), disable al previous adjustments and slide the bar until you have a reading of 13 MHz flat in the counter. Remember that prior to any adjustment you have to let the BOIA and freq counter to warm up, meaning that you have them powered up and connected for 1 hour at least, then you start the process.
if there’s any question, please let me know.
ricardo