5 Mar
2014
5 Mar
'14
12:18 p.m.
So it sounds like for character display, it's
basically worthless. What
about for bit-mapped displays? I'm afraid I don't know much about the
6845/6847 (I've not dealt with any sort of video generators yet), but is
the 6847 not more powerful than the 6845? I seem to see more computers
using the 6845, from best I can tell.
The 6845 and 6847 are totally different devices. One is not in any sense
a superset of the other.
The 6845 is the video timing chain. That is, it dvides dow na master
clock to produce the sync signals and the signals to address the video
RAM appropriately. It does not do anythign with the data in the video
RAM. That part has to be added by the designer, using other devices.
The 6847 is mostly the video data circuit. It contains the character
genrator ROM (uper case only), dot shift register, colour encoder (to
colour differnece signals IIRC, not to NSSC or anythign liek that), and I
think a simple timing chain. The last does not have ot be used (if it
exists), it was common in 6809 systems to pair the 6847 with the 6883
SAM (Synchronous Address Multipelxer, basically a RAM controlelr and
video timing chain). I cna't rememerbr what the Acorn Atom did, that used
a 6847, but no 6883.
-tony