UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Gemini 80-Bus System

JohnBHanson · 6th Dec 2019, 8:35 pm

To help out - differences appear to be on the input port 0e5 bits 5 and 6. This is handled by IC2.

On the standard GM849A this is determined by IC14 from the outputs of port E5 bits 3,4,5,6,7 which come from IC19

According to the GM849A circuit IC14 is an 74LS21 but the circuit diagram has unused
pins of IC14 wired to the some of the outputs of IC19

I suspect that IC14 is a different chip on the MFB version of the board or there are some board patches in this area.

Looking at the youtube video of the tear down the board is a GM849 and not a GM849A but I cannot see if there has been any changes in the above area.

As this is in the SCSI area I am wondering if this is an attempt at protecting the MFB from being run on normal gemini machines.

I look forward to any additional info you can provide in due course.

6th Dec 2019, 8:35 pm	#269
JohnBHanson Heptode Join Date: Aug 2009 Location: Worthing, Sussex, UK. Posts: 661	Re: Gemini 80-Bus System To help out - differences appear to be on the input port 0e5 bits 5 and 6. This is handled by IC2. On the standard GM849A this is determined by IC14 from the outputs of port E5 bits 3,4,5,6,7 which come from IC19 According to the GM849A circuit IC14 is an 74LS21 but the circuit diagram has unused pins of IC14 wired to the some of the outputs of IC19 I suspect that IC14 is a different chip on the MFB version of the board or there are some board patches in this area. Looking at the youtube video of the tear down the board is a GM849 and not a GM849A but I cannot see if there has been any changes in the above area. As this is in the SCSI area I am wondering if this is an attempt at protecting the MFB from being run on normal gemini machines. I look forward to any additional info you can provide in due course.