Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XLIII

Canon XLIII.

The subdeacons may not leave the doors to engage in the prayer, even for a short time.

p. 153 Notes.

Ancient Epitome of Canon XLIII.

A subdeacon should not leave the gates, even for a short time, to pray.

On this canon the commentators find nothing to say in addition to their remarks on Canons xxj., and xxij., except that the “prayer” is not their own private prayer, but the prayer of the Liturgy.  It has struck me that possibly when there was no deacon to sing the litany outside the Holy Gates while the priest was going on with the holy action within, subdeacons may have left their places at the doors, assumed the deacon’s stole and done his part of the office, and that it was to prevent this abuse that this canon was enacted, the “prayer” being the litany.  But as this is purely my own suggestion it is probably valueless.