24 Jan
2021
24 Jan
'21
12:44 p.m.
I still have a perfectly functioning BBC Model B, the only concession being a floppy disc
emulator and video output improvements. It?s one of the fastest 6502 based machines we
used at school and university. The computer science department had classrooms of them with
their own VT100 ROMs for connection to the VAXen via Gandalf boxes. 24 lines for the
terminal and the remaining 12 lines for a programmable ?crib? to display the mapping of
the function keys. Other ESC sequences were eventually added to allow full access to
graphics and to allow download of binary images into memory for execution. Our VAXen could
download all the games! Those were the days.
The computer centre authored their own 3270 emulator ROM for connections to their beloved
4341 and 3083. Funnily enough, nobody wanted a great big noisy (and sometimes smelly)
Memorex
terminal in their offices.
Keith
On 24 Jan 2021, at 10:06, David Moylan <djm at
wiz.net.au> wrote:
?
From: owner-hecnet at Update.UU.SE [mailto:owner-
hecnet at Update.UU.SE] On Behalf Of Dave McGuire
Far be it for me to disparage Nanny Beeb, but
this type of dependency
is indicative of poor design.
Oh yes. Poor design is certainly the case here. In order to keep everything simplistic
they wanted to avoid BUS contention while running the CPU at 2MHz. They had trouble
finding RAM which would match this design and Hitachi was the only manufacturer with a
suitable component. The NS 81LS95 multiplexer was the only component that was stable in
this configuration.
To make matters worse, they were exceeding the CPU's specifications which cause the
machine to crash. The designers worked out if they put their finger on a certain spot on
the board the machine became stable. They updated the design to include a resistor pack
across the BUS which kept the machine stable. This kludge was nick named "the
engineers fingers".
A dependency on a particular manufacturer's
implementation of a
standard and common design pattern like the 81LS95 is a bug. I sure
hope they fixed it in a later revision.
Absolutely not. They produced around 1.5 million machines with both of these kludge fixes
in place and the machine made it to end of life with no revision changes.
Cheers, Wiz!!