Talk:S-100 bus

Comart (UK)[edit]

Comart Computers in Britain made a range of S-100 based machines: There was the single-user Z-80 based 'Communicator', and multi-user x86-based machines that ran Concurrent CP/M-86 and possibly Concurrent DOS 386. I wish I could remember more of the details, but the "C-frame" (was this a codename?) had Voltage regulators on each card and ran 18V and 9V down the backplane and the "K-frame" ran 12V and 5V down the backplane and had wire links where the regulators would have been (some cards were common to both series).

Power Suppy Voltage[edit]

The author of the S-100 page states "A deal on power supplies led to the use of +8 V and +18 V, which had to be "pulled down" on the cards to TTL (+5v) or RS-232 (+/-12 V) levels." I believe the actual reason for these voltages was to allow for on-card voltage regulation. +8v and +18v would have been sufficient to drive the linear regulators of the time (such as the 7805 and 7812). I'm fairly sure I can remember photos of S-100 cards that had these regulators onboard.

Yes. This sort of setup was quite common in electronics of the time. One advantage would be that with separate regulators for each card, the regulators could be smaller, simpler, and grow with the system (instead of having to build one big regulator for an entire system, including room for future expansion) and there'd be better isolation among cards because power transients from one card would have to propagate through both cards' regulators before getting to another card, instead of just through a shared and relatively unfiltered power bus. The article's current "ha ha, what idiots they were to design this way" slant is both stupid and unencyclopedic.

Now that DC-DC converters are common and cheap, there's less need for power to be distributed at a higher voltage than it'll be finally used to give the regulators headroom, but it's still the case that individual boards, subsystems, and chips in a modern computer will often recondition or reregulate incoming power before using it. 129.97.79.144 16:18, 2 October 2007 (UTC)[reply]

Also, on-card regulators eliminate a greeat deal of the risk which came from the placement of the different voltage supply lines on the bus (which are worngly called a "disaster" in the article). The danger was a short between the lower and the higher voltage line, possibly causing low voltage components receiving excess voltage. The regulators would provide some level of protection, probably failing at some point, but, being sort of inexpensive, not causing very much grief (I guess).

Modern busses would are more prone ot a short between a voltage supply line and ground instead, therefore more likely to lower/eliminate a supply voltage instead of raising it (with also a chance of "frying" the regulators in the PSU). --80.134.51.117 (talk) 13:32, 21 December 2007 (UTC)[reply]

In the absence of any evidence for the "deal on power supplies" theory of the voltage choice, I have removed it. Also, the article missed the important point that the 8v is not regulated; i.e. it isn't really 8v, it's anywhere from about 5.5 to 12 or so. If the bus had been designed with a 5v line, there would still be the same 8v power supply, just in a different place in the system. Also, RS-232 does not require 12 volts; +/- 18v would be fine. And disk drives were another big user of 12v. Bryan Henderson (talk) 16:16, 8 August 2008 (UTC)[reply]

There's at least one other very good explanation for the power supply voltage decisions. The standard 8800 PSU produces +18V & -18V (regulated) and +8V (bulk filtered). For at least one card, the all-important CPU card, supplies of +5V, -5V and +12V are required. In this case, the +8 bulk is reg'd to +5, the -18V to -5 and the +18 to +12; all of these regulators are local to the CPU card. Other cards may require +/-12V and +5V..

Yes, this increases regulator count, with a big bump to flexibility - and as others have mentioned, it provides a significant level of PSU line isolation between cards running on the same bus. Good Idea, for the times, anyway - and for the modern conservator / repair tech, these regulator parts are essentially ubiquitous and trivially inexpensive. Drlegendre (talk) 04:22, 13 March 2013 (UTC)[reply]

False claim and also irrelevant to this article[edit]

"were the zenith of the microcomputer world until the advent of the IBM PC"

What idiot wrote this crap? The fact that PC is today the 'zenith' by no means should lead to the automatic assumption that the very first PC ruled. In fact, the PCs of the 80s were extremely lousy and it was well into the 90s until PC clone machines actually became number one in graphics, audio and even raw computing power. --93.106.185.190 (talk) 20:32, 5 December 2009 (UTC)[reply]

I concur. Zenith is an astronomical term meaning the point in the sky immediately above an observer. It is also used to suggest something is at its peak, and anything else is downhill after that. That isn't the case with S100. It is shabby standard, just the 8080 signals taken to a connector the designer was able to get hold of a lot of cheaply. It was kludged up to cope with 16-bit processors. The STEbus was a far better standard, not a 'minor improvement'.

Bus width[edit]

Is it really appropriate to describe the bus width as 16 bits. The data busses (the usual metric) are most assuredly 8 bit and even though there's two of them asserting they amount to 16 bits is surely over-egging them. They were both unidirectional and couldn't even be in use simultaneously. ﬥ (talk) 03:36, 16 June 2012 (UTC)[reply]

Asynchronous bus[edit]

I think it is important to mention that this is an asynchronous bus. The clock on the backplane is not the system clock. Wizzy…☎ 08:15, 8 October 2019 (UTC)[reply]