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

    Default: 4             Value: 0-255

    Number of sync characters that precede each message. This
    attribute is supported only if the characteristic protocol is
    set to bisync, chips, ddcmp, genbyte, or swift. You can modify
    this characteristic only when the entity is disabled.