Roger that, I will study the 80286 board for this master/slave idea and come back to this later.
I have many hours to go to finish this design as I see it.
Subject: RE: [N8VEM-S100:3480] 10 Pin RS-232 connector
Date: Sat, 3 May 2014 09:53:32 -0700
Josh just got to this e-mail. I find the safest way to do RS-232 connections is a dual row of jumpers before the final socket. That is how many “old” S100 boards did it, precisely because of the numerous combinations out there.
On the Phantom line, originating for the (current) master, it’s simply an open collector output with (one somewhere/anywhere on the bus), 1K pull-up. The buffers on the (current) master CPU are not tri-stated by phantom. They are however by CDSB*, ADSB*,DODSB* and SDSB*. It’s on the slave boards (RAM, ROM, etc.) that phantom has its effect, when activated on that board it renders that board invisible to the master CPU and other boards.
An onboard ROM/RAM setup on the current master CPU board is a kind of special case situation. When addressed it short circuits the CPU ability to see the S100 bus data and simply connects the ROM/RAM directly to the CPU with the relevant Rd/Wr signals. How much the S100 boards “see” varies from manufacture to manufacture. On our own Z80 master/slave CPU board the bus sees the address lines but no RAM data is ever exchanged. In fact for read operations RAM board(s) are putting data on the bus but it never gets pass the Z80 boards input buffers.
That is why you cannot single step with our SMB a Z80 ROM monitor on the Z80 board. (Actually you can, if you copy the ROM to the underlying RAM and then single step, but I may have lost you in that detail).
As I said previously. Once you get the master mode working, consider splicing in the master/slave circuit. By this time its fined tuned and bullet proof – been use on numerous boards. Best is the 80286 circuit layout.
Hope the above helps.
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