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RQDX3 hangs during a write


Experienced Member
Jan 20, 2014
I'm working on a program on an 11/73, that reads and writes an RD53, using an RQDX3 controller. This program runs without an OS - just code in memory. It works pretty well, but every 300,00 to 400,000 writes, it hangs up - never writes the response packet for the write command. The disk formats fine with ZRQC??, and the RQDX3 works fine when running RSX on another disk. I'm wondering if anyone out there that has had occasion to work on RQDX3s at the register level has experienced a hang condition like I'm seeing.


Veteran Member
Jun 19, 2012
UK - Worcester
We have our own operating systems and languages running on the PDP. Although this software is ‘mature’ and doesn’t get changed that often these days!

From what I remember there would be a couple of things to look for:

1. Accidental use if the Cache when referring to the I/O page.

2. Status bits changing state before your software is expecting them to. We had access to the DEC internal experts at the time we were developing our own hardware device drivers and I seem to remember quite a bit of internal documentation that we were provided with that was not generally available outside of DEC. We bought millions if pounds worth of DEC hardware, so we were a good customer in the UK.