Hi! The LM4250 is an op-amp that monitors the load of the backplane pins and sets the voltage of any pin not pulled high or low to 2.5VDC. Since it is always pushing the pins to the mid-range it dampens any sort of reflection or transient signal. It acts a bit like a shock absorber in a mechanical system.
From: Crusty OMO <cru...@hotmail.com>
Subject: RE: [N8VEM-S100:1419] S-100 Bus Terminator
To: n8vem...@googlegroups.com
Date: Sunday, February 10, 2013, 9:10 PM
I'm curious, please excuse my ignorance, but what is the DC to AC converter for? (LM4250)
From: LYN...@YAHOO.COM
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1382] S-100 Bus Terminator
Date: Sun, 10 Feb 2013 09:37:58 -0500
Hi Paul! Thanks! I like the idea of adding the prototyping area. That would be easy to do and generally useful. There is nothing wrong with being frugal!
My intent with kicking off the S-100 bus terminator board design is to start the discussion on topic. Partially we can find out how much interest there is for a bus terminator and if the concept is viable. The board is really only useful for those with large legacy S-100 backplanes. We do have an S-100 backplane but it is small and already includes bus termination circuitry.
The S-100 bus terminator design is not final by any means and meant to be a starting point. I’ve made some updates to the S-100 bus terminator and attached are the new schematic and PCB layout files. Please review and send me any changes and/or corrections. As always, comments, questions, ideas, thoughts, are always welcome! This is a home brew computing community so peer to peer design and review is essential!
I don’t really like the probe and bus extension concept though since we already have an S-100 bus extender board which does those things and the design is pretty solid. In addition, I am working with Neil to make a basic boot board tool (64K SRAM, EPROM, UART, and parallel IO) called the S-100 64KIO board so that’s redundant.
I can send the S-100 64KIO board information to the group for review as well however noting that Neil is in the process of totally redesigning the board. Please keep the comments and questions more at the concept level instead of the particular implementation. The design is likely going to change quite significantly if I understood Neil correctly.
Great commentary and questions! Please let’s discuss more. Thanks and have a nice day!
Andrew Lynch
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Paul Birkel
Sent: Sunday, February 10, 2013 3:38 AM
To: n8vem...@googlegroups.com
Subject: Re: [N8VEM-S100:1375] S-100 Bus Terminator
Andrew:
Being a congenital penny-pincher, as well as a "tweaker", it strikes me as a lost opportunity to define a PCB like this with *lots* of unused space (and yet sucking up a full S-100 slot). At a minimum, how about filling the remaining space with a through-hole proto-area array? It would/could be accompanied by appropriate regulators along one edge but the setup/layout would be spatially similar to the buffered proto-board (replacing the buffer-area with the line-clamp area). This would leave opportunities for experimenters (with legacy S-100 backplanes) to wire-up small accessory circuits -- else no impact on the termination function?
Alternatively, place the bus-terminator circuitry onto the unused area of the bus-extender card and get three-in-one functionality with extension, termination, and probe functions. An example of this approach of combining termination and
extension was the VTE100a "EXTERMINATOR"; see:
http://maben.homeip.net/static/S100/vamp/cards/VMP%20VTE%20100-A%20Terminator.pdf
http://maben.homeip.net/static/S100/vamp/photos/VAMP%20VTE-100%20Exterminator.jpg
http://maben.homeip.net/static/S100/vamp/photos/VTE%20100a%20Exterminator.gif
If trying to go 3-in-1 ends up running into real estate issues then just combine termination & extension like the VAMP.
Another possibility would be to commit all/part of the (above) proposed proto-area to specific generally-useful functions. Two possibilities would be a
configurable basic UART serial I/O port and/or a configurable 8 or 16-bit (E)EPROM handling up to 1 MB chips. I believe that someone pointed out recently that it would be useful to have a board with these sorts of basic functions in order "bring-up" legacy CPU PCBs in a minimal configuration. One could possibly swap one of these options for the probe in a 3-in-1 configuration? Unfortunately it seems to me that when including the extension-function that the resulting routing problems when adding something like I/O or ROM might require a lot of accessory over-board wires to effectively provide a (minimal) third connection-plane. So that might not be a good idea. But the VAMP illustrates that termination+extension go together quite well.
My preference would be to leave the existing probe+extension PCB alone, and instead look towards combining termination, Serial I/O, and (E)EPROM into a single card. This plus
legacy CPU and RAM cards would result in a functional system on any "naked backplane". If some RAM could be squeezed into this multi-purpose board, so much the better :->. We have all of these circuits in tested-form, the only issue would seem to be routing. This sort of board would also be useful to a wider audience who could only populate the subsections of immediate need (e.g., not actually use the termination section if employing a self-terminated backplane) -- and this would increase the board-orders and therefore the likelihood that we'll quickly reach a good ROI threshold.
The bottom line for me is that a pure termination PCB seems like a lost opportunity. And might not easily reach critical-mass for a board-order :-<.
Attached are the schematic and PCB layout files for the S-100 Bus Terminator.
Please review and send any changes and/or corrections to me.
Thanks and have a nice day!
Andrew Lynch
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