[cctalk] Re: STD Bus based Little Big Board

Hi Doug There was an unusual problem I came across on a STB bus board a number of years ago. It may be related but maybe not. The board used 64K RAM and a Z80 uP. Not all 64K DRAM chips do refresh the same. Some require 256 cycle refresh while others only require 128 cycle. The RAMs we had required 256 cycle but we were not using a separate refresh circuit, we were using the Z80's internal refresh. This meant every address ending in 80 to FF Hex was not getting refreshed. Thing would run for a minute or two after reset and crash. Once we identified the problem, we contacted the company we bought the CPU board from. The problem was it would take a week or more to replace the high demand 64K parts. We needed to get things running to check out our hardware and software. Luckily, Our system needed to do a 1 milli second interrupt to a check for a data bus activity. We added a read of the high 128 bytes with one of the Z80 block read commands to the interrupt. That satisfied the refresh requirements of the DRAM we had. I believe that even after we go 128 cycle parts we left that extra bit of code in what we shipped. Dwight ________________________________ From: Doug Jackson via cctalk <cctalk(a)classiccmp.org> Sent: Thursday, December 11, 2025 9:54 PM To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org> Cc: Doug Jackson <doug(a)doughq.com> Subject: [cctalk] STD Bus based Little Big Board Hi Everybody, I am on the search for a Pulsar Electronics Little Big Board (STD Bus Based) from the mid 80's. My board has some very weird bus faults that I am trying to diagnose. Having a working one would radically simplify my life. I am located in Canberra, Australia, and am happy to pay for the board and postage from Alaska if necessary :-) Kindest regards, Doug Jackson em: doug(a)doughq.com ph: 0414 986878