Using big ROMs on Q40

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Using big ROMs on Q40

Postby HAOUI » Fri May 19, 2017 5:13 pm

With Peter Graf's indications and some AM27C4096 ships supplied by Derek Stewart, I realized a little circuit permitting to have a global ROM size of 1 MegaBytes on Q40 with 2 bigs OS ROMed together (up to 512KB each) and selectable with a case switch (3 wires, 2 positions) :

Code: Select all

PIN 39 (A17)--+        O---- Vcc +5v (Switch pos 2)
ROM 1 (LW)    |       .
              |      .
              |      \
              |       \
PIN 39 (A17)--+        O---- Ground (Switch pos 1)
ROM 2 (HW)

*Pins 39 bent outside sockets
*ROM selector jumper set to default (1-2)

With switch on pos 1, lines A17 are grounded, only lower 512KB usable
With switch on pos 2, lines A17 are selected, only upper 512KB usable

I used following program to build the ROM image :

FOR i=0 to (1024*1024)-1 : POKE a+i,255
LBYTES win1_SMSQE_Q40_OS2_ROM,a+(512*1024)
SBYTES win1_SMSQE_Q40_ALL_ROM,a,1024*1024

Then, I processed the ROM image file with WROM_BAS program supplied with SMSQ/E to split ROM file in LowWord and HiWord files and burned them on the M27C4096 ships.

This worked nicely and permitted :
-Confirm usage of 1MB ROM size on Q40
-ROMing of recent SMSQ/E versions greater than 256KB
-Alternative DIY ROM selector working with any ROM size

Have Fun

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Re: Using big ROMs on Q40

Postby Paul » Fri May 19, 2017 5:41 pm

I would add two resistors between switch and 5V and between switch and 0V.
Any value between 1K and 10K would be good.
I have seen cheap switches that made short circuits while switching and catching fire at high amperes.
If you just need a high or low level other than fixed on pcb (meaning using a wire) its good practice to reduce possible short circuit ampere with a resistor.
Please excuse my bad English. I hope you get what I mean.
Kind regards Paul

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Re: Using big ROMs on Q40

Postby Dave » Fri May 19, 2017 10:15 pm

I would suggest adding the resistors too. The issue is switches that make contact before breaking contact, so they create a short from Vcc to Vss.

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Re: Using big ROMs on Q40

Postby Peter » Sat May 20, 2017 1:06 am

You only need one pullup and a switch between A17 and GND.

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Re: Using big ROMs on Q40

Postby HAOUI » Sat May 20, 2017 7:25 am

Well, in fact I was going to do a pullup as Peter said, but given that my case switch is already 3 wires and has to be selected once before powering (no transition) I thought that a simple straight switch wiring will be enough. Am I wrong ?

Anyway, I will put resistors when I will be able to find adequate (10K) resistors (no hardware man here ;)

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