The Question is:
Is there a DSSI bus based CD that could be installed in a Microvax 3500?
The Answer is :
Via one of the HSD DSSI-to-SCSI adapters, possibly.
Another option is the KZQSA Q-bus to SCSI. (KZQSA support is limited
to specific SCSI CD-ROM and tape devices; other devices and particularly
disks are not supported and not reliable.)
Clustering and InfoServer devices are also possibilities.
There is and was no CD-ROM DSSI device.
It might be simplest to replace the entire MicroVAX 3500 system with
a less-old MicroVAX or VAXstation with more direct SCSI connections, or
with a more recent Alpha system. These are far faster systems, too.