IBM SC33-1683-02 - Manual (Page 795)

from a back-end application program whose session is in SEND state (and which
has never issued a SYNCPOINT), the session will be put into RECEIVE state. If the
code invoked by DFHEDAP then issues a SYNCPOINT, an abend occurs. This can
be prevented by all DTP applications issuing a SYNCPOINT request when they get
into SEND state (on all of their sessions) and before they issue the LINK DFHEDAP
command.
Do not attempt to use LINK DFHEDAP when more than a pair of DTP application
programs are involved—that is, one front end and one back end.
The general rules for using LINK DFHEDAP within a simple DTP environment (one
front end and one back end) are that all sessions in a DTP environment should be
in SEND state when the LINK DFHEDAP command is issued, and they should
revert to SEND state in the event of a SYNCPOINT ROLLBACK being issued by
the DFHEDAP code.
DFHEDAP in a DTP environment
Chapter 31. The programmable interface to the RDO transaction, CEDA 763