Library /sys$common/syshlp/nclhelp.hlb set, hdlc, link, maximum_unsequenced_pdus  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: 1             Value: 1-127

    Maximum number of unsequenced I-frames that a primary or
    secondary station can send in a single transmission. This
    characteristic is not used if the link type characteristic is
    balanced.