> -----Original Message----- > From: Ian S. King [mailto:[email protected]] > Sent: 25 June 2017 01:07 > To: [email protected]; Rob Jarratt <[email protected]>; General > Discussion: On-Topic and Off-Topic Posts <[email protected]> > Subject: Re: DECstation 220 - Out of Ideas? > > Rob > I don't recall from this thread - do you have prints? I'd try to divine > whether > this is a wire-OR bus (likely). If so, I'd suspect the P82C206. It may not > be toast > itself - be sure it has sufficient power to drive the OC transistor that's > bringing > down the bus, and that its connection to that bus doesn't have excessive > resistance, and that its ground (earth) connection is also low-resistance. A > long > shot: it could be something else on that bus that's sourcing rather than > sinking, > but the scenario seems unlikely. > > I'd agree that it's probably not the ROMs, from the symptoms you're describing > - unless, again, one of them has an internal short/leak to rail on its > read-enable > pin. A thought: can you measure the current and its direction on the P82C206 > pin? > > That's my troubleshooting thinking given that I don't have it in front of me. > :-) I > hope this is helpful -- Ian > > >
Thanks Ian. Unfortunately I don't know of any schematics. I suspect you are right about it being a wire-OR because the documentation for the P82C206 says its output is tri-state. As everything is soldered on rather than socketed (except the ROMs) I am not sure I can measure any currents, nor can I think of a way to check if the other chip (Olivetti GA099-B) is trying to drive the output the other way :-(. I will check the resistance between the outputs of the P82C206 and the ROM inputs and of the Vss pins. Regards Rob
