PDP-9 Restoration at the RICM

Warren Stearns and I have been working on the PDP-9 at the RICM. Yesterday we got the core memory working and started debugging the processor. Quite a milestone for a 44 year old system. All of the operate instructions seem to work. The JMP and ISZ instructions are flakey, even when single stepping. This could be a microcode problem, or a problem with the data multiplexers. It is an interesting machine because it is microcoded with read-only core memory. I am not sure why they used core for the microcode. Maybe just a board with a bunch of diodes would have done the same thing and would have been a lot simpler? I am finding the schematics a challenge because they were created before ANSI or IEC standard logic symbols were invented. Eventually I will understand the symbology. Some of the design is a little strange because you can abuse DTL logic in ways that TTL will not tolerate. Lots of delay lines where you can adjust the delay by changing wire-wrap on the backplane. Lots of pulse amplifiers to improve fan-out and reshape signals. You can read about our debugging progress here. http://www.ricomputermuseum.org/Home/equipment/dec-pdp-9/pdp-9-restoration We need to fix the paper tape reader so we can load some of the diags. We are a long way from connecting the TC-59 and the TU20. I suspect that getting the TU20 working will be a challenge. -- Michael Thompson