256 Byte EPROM Support For SCELBI-8H

The standard SCELBI-8H doesn't include any EPROM memory.  This makes booting the system fairly laborious.  You must toggle in a boot loader program using the front panel, in order to load any significant applications.  This issue was solved by SCELBI with the development of a rather sophisticated monitor that was released with the SCELBI-8B.   That didn't help the original owners the 8H, but at least one owner, solved the problem by hacking an EPROM onto the back of one of his SRAM cards.  Here is the back of Steve Ciarcia's original 8H with switchable EPROM installed.

Steve Ciarcia's 8H with EPROM

Once I took the time to develop a simple 8008 monitor that would fit in 256 bytes, I had to add a 1702 EPROM to my SCELBI-8H.  8008 system owners should  see my MCMON page that describes the modified Creed Monitor that I adapted for the purpose.

Steve's idea of adding an EPROM on the back of the last SRAM card seemed to me to be the most logical solution to the problem.  The 1K SRAM cards used on the SCELBI-8H are quite easy to modify for EPROM support.  You add a daughter card that allows switching a bank of memory between the new EPROM and the original SRAM.  My implementation puts the 256 byte EPROM at address 017-000.  If you need to use this address range for SRAM in an application, you can always use the monitor to boot load a loader elsewhere in SRAM.  Then you can switch the EPROM range back to the SRAM function.  Here is how my implementation looks.

EPROM hack

The wiring is pretty simple.  You basically have to connect 8 data lines, 8 address lines, +5 volts, -9 volts and chip select.  Chip select for address range 017-000 needs to be disconnected from all the pins on the chips that it connects too.  You can lift all the pins out of their sockets or just cut the chip select line.  I choose to cut the chip select line.  This line is then connected to the SPDT switch to switch chip select of the last bank from EPROM to SRAM.  When the input is disconnected from the chips, you don't want the input floating, so I pulled each input up to +5 volts with a 3K resistor.  The +5 and -9 volt lines are connected to a convenient decoupling cap.  The address and data lines are connected to convenient vias.  An alternative is to connect address lines to the pins of an SRAM chip.  I choose to use vias because they are easier to solder to.  Here is a diagram of the wiring I ended up with.

EPROM connections
Red lines are the eight data bits.  Yellow lines are the 8 address bits.  Blue is chip select.  Purple is +5 volts and grey is -9 volts.  The two pull up resistors are omitted, but run from the two outputs of the switch to +5 volts.

The EPROM is mounted on a section of a Radio Shack proto-board.   Since it only had copper pads on the bottom, I completely wired it up before attaching it to the SRAM card.

EPROM daughtercard

The daughter card is attached to the SRAM card with a #6 screw and some washers to create clearance between the daughter card and the main SRAM PCB.  A thin piece of styrene plastic is also inserted in the middle to eliminate any chance of shorts.

Back to Mike's Hobby Home Page