Hugo Holden
Veteran Member
I have a 64k RAM card in my SOL-20 (deactivated by its own address decoder above 48k) I needed to deactivate it over address range 6000h to 7FFFh to be able to run a Bytesaver board over that range.
Initially I made an address decoder on an s-100 prototype board, using logic gates, it worked but used up a slot in the SOL-20.
I thought there must be a better way to do it, but I did not want to modify my 64k memory board or damage it in any, like cutting tracks or soldering into plated through holes etc. Also, it seemed to take at least two or three packages of standard logic gates to alter the address decoder to have the board still disabled over 48k and the window disabled over 6000h to 7FFFh.
But then I was looking at the N* disk controller that used OTP PROMS for address decode. This was the answer to a "one IC address decode" where a number off addresses need to be activated & deactivated. Recently I had bought a BP1400 programmer so I could program the 74S287. So I made an address decoder from a 74S287, it just required 16 bytes of it to be programmed and just using the LSB as the output and 4 of the address input lines.
Also, I made it so it can be jumpered out to get rid of the 6000h-7FFFh window by moving two jumpers. I mounted the board with the 74S287 by machining a tiny brass spacer into which I cut a full length 1mm threaded hole. So it attaches with two 1mm dia metric screws to an existing hole in the pcb. No tracks were cut and the wires pass through existing plated through holes to attach to IC pins on the rear of the board. So there is no pcb track damage at all to make the modification. It could be removed with no trace it was ever there. I avoided using adhesives and double sided tape as the glues can attack the pcb surface. The PTFE coated wires won't cause any harm or corrosion to the plated through holes they pass through.
I have attached some photos.
Initially I made an address decoder on an s-100 prototype board, using logic gates, it worked but used up a slot in the SOL-20.
I thought there must be a better way to do it, but I did not want to modify my 64k memory board or damage it in any, like cutting tracks or soldering into plated through holes etc. Also, it seemed to take at least two or three packages of standard logic gates to alter the address decoder to have the board still disabled over 48k and the window disabled over 6000h to 7FFFh.
But then I was looking at the N* disk controller that used OTP PROMS for address decode. This was the answer to a "one IC address decode" where a number off addresses need to be activated & deactivated. Recently I had bought a BP1400 programmer so I could program the 74S287. So I made an address decoder from a 74S287, it just required 16 bytes of it to be programmed and just using the LSB as the output and 4 of the address input lines.
Also, I made it so it can be jumpered out to get rid of the 6000h-7FFFh window by moving two jumpers. I mounted the board with the 74S287 by machining a tiny brass spacer into which I cut a full length 1mm threaded hole. So it attaches with two 1mm dia metric screws to an existing hole in the pcb. No tracks were cut and the wires pass through existing plated through holes to attach to IC pins on the rear of the board. So there is no pcb track damage at all to make the modification. It could be removed with no trace it was ever there. I avoided using adhesives and double sided tape as the glues can attack the pcb surface. The PTFE coated wires won't cause any harm or corrosion to the plated through holes they pass through.
I have attached some photos.
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