2 Dec
2008
2 Dec
'08
3:06 p.m.
On Wed, Dec 3, 2008 at 8:48 AM, Chuck Guzis <cclist at sydex.com> wrote:
As long as this isn't a "vintage"
project, why not add a few useful
bells and whistles? For example, one of the reasons that I like a
PeeCee as a debugging terminal is because I can scroll back hundreds
of lines to see what I did, or even save the particular segment long
after it's occurred. The ability to set bookmarks and split-screen
history with current output would be very useful. As would being
able to freeze and save history across power cycles.
Interesting ideas, but right now, we are trying just to get basic
terminal functionality.
The cost motivation that dictated the features of the
original
VT100/VT220 no longer apply.
True. This project has other cost motivations.
The cost difference between 128Kx8 and
512Kx8 SRAM, for example, is negligible today.
For this project, the SRAM needs to be, IIRC, 70ns or faster to keep
up with the CPU which is slinging bytes out an I/O port to bit-bash
the video. The cost of a fast 256kbit SRAM is a bit more than a
slower one, and I don't know how that compares to a 2Mbit SRAM.
Another cost issue here is address pins - the VT-4 supports 32Kbytes
in one socket, and can handle 64Kbytes with a second SRAM (the
prototype has a second SRAM socket), but there aren't enough address
bits available to do more. There's also the cost issue associated
with board space. This is intended to be a small board with no
surface-mount (hobby-level product, not a modern "you can't service
this" product). Even adding a couple of more DIP spaces to latch
address bits is a tricky prospect.
The issue of more RAM was hashed over a few times amongst the firmware
developers already, and I am reasonably certain that the VT-4 is not
going to have wads of memory, just what it needs to display and scroll
an 80x24 or 80x25 screen.
Using a Z80 today
would seem to be a quaint anachronism where there are so many fine
(and power-miserly) microcontrollers available.
I don't know where Z80s came into this. The VT-4 uses a 30ish-MIPS
microcontroller (to keep up with the demands of squirting out VGA
video). The video is essentially software-generated, keeping the
part count way down. Low part-count == low price.
-ethan