Limitations on Using Chained Scheduling with Non-DASD Data Sets

The following are limitations on using chained scheduling:

Each data set for which chained scheduling is used must be assigned at least two (and preferably more) buffers with QSAM, or must have a value of at least two (and preferably more) for the NCP parameter with BSAM.
A request for exchange buffering is not honored, but defaults to move mode and, therefore, has no effect on either a request for chained scheduling or a default to chained scheduling. Exchange buffering is an obsolete DCB option.
A request for chained scheduling is ignored and normal scheduling used if any of the following are met when the data set is opened:
- CNTRL macro is to be used.
- Embedded VSE checkpoint records on tape input are bypassed (OPTCD=H).
- Data set is not magnetic tape or unit record.
- NCP=1 with BSAM or BUFNO=1 with QSAM.
- It is a print data set, or any associated data set for the 3525 Card Punch.
The number of channel program segments that the system can chain together is limited to the value specified in the NCP parameter of BSAM DCBs, and to the value specified in the BUFNO parameter of QSAM DCBs.
When the data set is a printer, chained scheduling is not supported when channel 9 or channel 12 is in the carriage control tape or FCB.
When chained scheduling is used, the automatic skip feature of the PRTOV macro for the printer will not function. Format control must be achieved by ANSI or machine control characters.
When you are using QSAM under chained scheduling to read variable-length, blocked, ASCII tape records (format-DB), you must code BUFOFF=L in the DCB for that data set.
If you are using BSAM with the chained scheduling option to read format-DB records, and have coded a value for the BUFOFF parameter other than BUFOFF=L, the input buffers are converted from ASCII to EBCDIC for Version 3 (or to the specified character set (CCSID) for Version 4) as usual, but the record length returned to the DCBLRECL field equals the maximum block size for the data set, not the actual length of the block read in. Each record descriptor word (RDW), if present, is not converted from ASCII to binary.

Related reading: See Using Optional Control Characters and z/OS DFSMS Macro Instructions for Data Sets for more information about control characters.