That Texas Instruments SN74LVC161284DL parallel port transceiver chip? It has two mystery signals that are not well explained, HOST LOGIC and PERI LOGIC. What are these for? Well, this took a lot of searching around to find out what they’re for, but they are designed for detecting whether the device on the other side of the link is powered on. Essentially, its a Vcc power source of a sort. However, these pins are not exposed in the standard DB25 connector, they are only available on those stupid nonstandard 36-pin micro Centronics connectors that nobody uses. But, the most interesting thing about this? I found an online source that said the micro Centronics connectors are recommended for new applications! Ha! They never showed up on PC back panels before the parallel port became obsolete, who would ever use them?
20201028/DuckDuckGo ieee 1284 device power pin
20201028/http://www.interfacebus.com/Design_ieee1284c_Connector_PinOuts.html
20201028/DuckDuckGo ieee 1284 host logic high
20201028/http://www.efplus.com/techref/io/parallel/1284/1284conn.htm
I think a lot of things about the IEEE 1284 were a “wannabe” standard. This standards committee got together and thought they could create this new high-speed communications standard that would then take the market by force, but that simply never happened. So, in the latest versions of IEEE 1284, we ended up with all kinds of bespoke functional specifications that were never used in practice.