I've never seen a proper spec or anything (unlike for SCSI) -- Because the standardization process is what created SCSI. Look in the Xebec, Shugart, and DTC manuals on bitsavers. There will be another batch of Shugart 14xx manuals up soon. The big difference is SASI has no notion of deconnect/reconnect. Once a transaction starts on SASI, no other device has bus access.