On Sun, Jan 22, 2017 at 4:27 PM, W2HX <w...@w2hx.com> wrote: > Hi friends. > I am 100% new to my pdp8e and I am troubleshooting a problem. > The problem is that whenever address bit 7 I asserted, I also see MD bit 4 > asserted. > > I am hoping there is a simple short somehow between these lines somewhere. I > should mention that my setup has known working boards with the exception of > M8310 does not work (all of my boards were tested in another machine one by > one). > So I am hoping the problem noted above might be occurring on the M8310 board > itself.
Could it be a short on the backplane? Or on the jumpers between the backplanes if you have 2 backplanes in the box (The standard 8/e box is designed to take 2 backplanes, I believe the rear one was optional but almost always fitted). > > I found a document that describes the signals on the bus located here: > http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp8/pdp8e/Omnibus_legenda.pdf > > I noticed that B1J (MA7) is directly adjacent to B1K (MD4). Could this be > related? > So I decided to pull my 8310 and check the resistance between these signals > on the > edge of the board. However, I cannot seem to square the signals named in the > PDF > and what I see on the card edge connector on the board. > > I am wondering if I am not understanding the PDF correctly. I have the board > sitting > on my table with the components facing up and the omnibus card edge is at the > bottom of the card. Going from left to right, do I have connectors ABCD or > is it > something else? Maybe DCBA? With the board like that, Connector A is on the right (so the order is DCBA from left to right, pin A is on the right of each connector, and side 1 is the solder side. > > Other than the reference PDF, is there another PDF that has a more detailed > description of the bus and the signals? Do you have the Engineering Drawing/Printset (same thing, just 2 different names) from bitsavers? There is also an excellent 3-volume PDP8/e (and /f, /m) maintenance manual on bitsavers (I have it on paper too). Volume 1 covers the CPU (which is what you need now), Volume 2 is 'internal options', Volume 3 is 'external options. In time you will need all of them. -tony
