This page has first time setup instructions for SDX Plus.
Instructions :-
BOOTSDX.BIN into a 16KB EPROM,
and use to it replace the EPROM in the SDX
(note that the SDX circuit board is upside down,
and so pin 1 is top-right of the socket)
OSBASIC.BIN into a 16KB EPROM
and use it to replace the EPROM
on the motherboard at position 9H
sdp_gal1.eqn into a GAL16V8
and use it to replace the address decoder PAL14L4
on the motherboard at position 6A
sdp_pro1.eqn into a GAL16V8,
assemble it with 20 pin and 16 pin DIL sockets and a wire
(as shown below),
and use to to replace the 74S287 bipolar PROM on the SDX
(note the SDX circuit board is upside down,
and so pin 1 is top right of the socket)
boot.mfloppy and insert into the SDX drive
Rather than use a 16KB 27128 EPROM,
it makes more sense to use something like a W27C512 EEPROM instead.
These can be programmed using more readily available USB based programmers.
If you do this, given that W27C512s are 64KB in size, use the
BOOTSDX4.BIN and OSBASIC4.BIN files instead
(four repetitions of the same 16KB of data).
I've found it best to use the Opal junior 2.1 tools to compile the GAL equations to a JEDEC file, and use an EPROM programmer to program them.
Sourcing 74S287 bipolar PROMs isn't easy, and they're one-time-programmable, so this project uses a GAL16V8 in place of them. This obliges you to mess about with DIL sockets and a wire to make something that can be plugged in where the PROM was. On the left is the original 16-pin PROM, in the middle are the components unassembled, on the right is the assembled replacement :-