IBM Bus & Tag channel protocol specs?

Sridhar Ayengar wrote: <snip> for what it's worth, the P/390 only ever ran tape drives and only drove selector channel protocol. It never ran any DASD due to protocol limitations, and the fact that it was too slow to be useful, when emulated storage thru the token ring ran faster. My friend who has both versions only remembered seeing a version of the Host version of the cards and cables for the Micro Channel version, and not for the PCI. Univac supported the host version on the PCI card set. IBM systems with P/390's were more likely to be in Escon farms than in bus / tag. The R/390's may have been more likely to be in sites with SSA available, which IBM was pushing at the time the P/390's were in vogue. Therefore less likely to see Escon versions of that hardware or the Bus/Tag versions. I don't think this is a bad idea, but I don't think that the solution exists in a form that can be bought, even if IBM had it in some product. Jim