5 Apr
2005
5 Apr
'05
3:22 p.m.
Subject: Re: The SC/MP is finally alive!
From: "river" <river at zip.com.au>
Date: Wed, 06 Apr 2005 07:44:02 +1000
To: <cctalk at classiccmp.org>
Hi,
Yes, for it's time (1976) the SC/MP had the logic that allowed it to share
the bus with other processors etc. The 8080 chip could also do this, but
this device required 3 separate power supplies and you needed three chips
to get the CPU working properly. The 6800 also offered similar multiprocessing
abilities and, like the SC/MP was a single chip system, but I'm not sure if it
came out before or after the SC/MP.
Commnet:
The biggest differnce between the SC/MP bus intnerface was that
the sc/mp didn't assume it was the bust master. So the bus
interface pins did a priority resolution with other potential
bus masters.
All of the other cpus (8080, 8085, z80, 6502, 6800 etal)
assumed they were the bus master and you took the bus only
after being granted DMA access.
While the difference is a small one it's notable in that all
of it's peers of the time were different.
The SC/MP I (ISPA/500) was a PMOS device and required
+5 and -7 volt power
rails. The SC/MP II (ISPA/600) was NMOS and required only +5 rail (same as the
6800), it also had three of it's CPU control lines inverted (as compared to the
SC/MP I) and though it could take 4 times the speed of the clock of the SC/MP I,
due to internal clocking it ran only twice as fast.
What made the SC/MP popular back in those days was the cost. It was about a
quarter of the cost of the 8080 and 6800 processors.
The cost factor was insignificant by 1978. The CPU was only part of a
systems cost.
Finally, it's good to see some other old SC/MP
dudes around and also others who like
to build and program their systems from scratch.
I started a SC/MP cpu in TTL and Bit slice (2901) and then
abandoned it for a design that expanded it in some obvious
ways.
the sc/mp was in the same class for cpu hackers as the 1802,
6100 (PDP8 in cmos), and a few others.
A significant part of my collection and expeimentation is with SBC
(Single or Small Board Computers). I have SBCs for 1802, SC/MP 8a/500,
Nibble basic 8073 sc/mp, 6800, 6809, 6502 Kim-1, 6100 Intersil sampler,
IMSAI IMP-48 an 8035 SBC, Ti9900 Technico Superstarter board,
NEC TK80 (8080), several 8085, Z80, Z280 and Z8001 sbc of my
design, and T-11 (a 40 pin dip pdp-11) homebrew. The only one
I haven't played with is the 2650, 16032, 68000... yet.
Allison