New Pace RS232 - You Voted - Useful Treasure -Modded as a ROMBox

[Bryce]

He knows that. It's the third jumper "EXP" which he's enquiring about. I'd like to know what it does too

Bryce.

[TFM]

Damn! I didn't find that on the picture!

Found! I guess it's to set the EXP signal for the expansion port, this signal can be read using the PPI. So the CPC 'knows' if there is an expansion connected.

[IanS]

Interestingly the RS232 romboard and my Honeysoft board look very similar and are both labelled "HS-RX1" they must be quite different as the chips are not the same.

RS232 board
74LS137 3-8 line decoder with address latches
74LS32  Quad 2-input OR gates
74LS00  Quad 2-input NAND gates

Honeysoft romboard
74LS137 3-8 line decoder with address latches
74LS139 Dual 1-of-4 Decoder
74LS74 Dual D-Type flip-flop

I still don't think there is enough gates to fully decode the rom number.

[Bryce]

Not if they are just using the 3 to 8 decoder to select the ROM with the first 3 Databits, that would definitely repeat every 16 ROMs.

Bryce.

[IanS]

Not if they are just using the 3 to 8 decoder to select the ROM with the first 3 Databits, that would definitely repeat every 16 ROMs.

With only 3 data bits, it would repeat every 8 roms. I traced my Honeysoft romboard and it used one of the d-types to latch D3 (so it repeats every 16 roms). The Q or not Q outputs are used to select 0-7 or 8-15 by being linked to one of the gate inputs on the 74LS137. I wonder how the other design works.

[arnoldemu]

With only 3 data bits, it would repeat every 8 roms. I traced my Honeysoft romboard and it used one of the d-types to latch D3 (so it repeats every 16 roms). The Q or not Q outputs are used to select 0-7 or 8-15 by being linked to one of the gate inputs on the 74LS137. I wonder how the other design works.

please put this info on the wiki.

It is all good interesting information.

[Bryce]

With only 3 data bits, it would repeat every 8 roms. I traced my Honeysoft romboard and it used one of the d-types to latch D3 (so it repeats every 16 roms). The Q or not Q outputs are used to select 0-7 or 8-15 by being linked to one of the gate inputs on the 74LS137. I wonder how the other design works.

Doh, correct. It was late last night and had several beers before posting However, the fact that you can choose between 0-7 and 8-15, means that D3 is being read by some other gate so the device would still probably repeat every 16 ROMs.

@Arnoldemu: I don't really like putting "half information" on the wiki. If we get/make a schematic for the device, or if the owner has proven this, then we could add it to the wiki.

Bryce.

[IanS]

... so the device would still probably repeat every 16 ROMs.

I never suggested it wouldn't. I typed -

I still don't think there is enough gates to fully decode the rom number.

[Bryce]

That was actually referring to your post where you said: "With only 3 data bits, it would repeat every 8 roms."

I know what you meant, but I was trying to clarify my original post as to why it repeats every 16 ROMs.

Bryce.

[spookspring]

Ok well you guys go over my head, so Ill let you discuss while I get down to the practical task of modding this Romboard as per Bryce's idea.





The capacitor top left is a placeholder, bought a 100uf instead of nf.  Ordering a couple of ROM's from Urban Retro (ebay).
Parts £7. ROMS £14. I have never soldered before. I hope you are proud of my speccy job. If you see anything amiss or wrong before I turn it on, let us know!


Cheers, James

[Bryce]

The 100µf capacitor is also a Tantilum-bead type, which is poled. The 100nf part should be the ceramic type. You also need to make sure you put the EPROMs in the correct way. The sockets (assuming they are already soldered, are the wrong way around (The notch should be at the top), but this doesn't matter as long as you know which way to insert the EPROM.
If you remove the 100µf capacitor it should already work, even without the 100nf capacitor installed.

Congratulations on your first soldering. From what I can see (the selection links), it looks very neat.

Bryce.

[IanS]

Ok well you guys go over my head, so Ill let you discuss while I get down to the practical task of modding this Romboard as per Bryce's idea.





The capacitor top left is a placeholder, bought a 100uf instead of nf.  Ordering a couple of ROM's from Urban Retro (ebay).
Parts £7. ROMS £14. I have never soldered before. I hope you are proud of my speccy job. If you see anything amiss or wrong before I turn it on, let us know!

As Bryce says, make sure the tantalum cap is the right way round if you leave it in.


Can you take a picture of the other side whilst you have it out of the box?

[Bryce]

I was thinking about that EXP jumper. It may be there to set the /EXP signal going to the CPC. Nobody every really used this signal, not even the DDI-1, which was the original reason for its existance as far as I know. But maybe Pace wanted to auto-detect whether the device was connected?

Bryce.

[IanS]

I was thinking about that EXP jumper. It may be there to set the /EXP signal going to the CPC. Nobody every really used this signal, not even the DDI-1, which was the original reason for its existance as far as I know. But maybe Pace wanted to auto-detect whether the device was connected?

The DDI-1 uses the internal pull-up resistor on the EXP signal as the pull-up for LK1 (rom 0 or 7 selection). In it's default state (rom 7) the DDI-1 pulls the signal low.


Part of the reason for asking for the picture of the other side was to see if I could see where the EXP pads were connected to.

[Bryce]

Yeah, I was aware of that usage, but I meant using the EXP signal for what it was really meant for. As far as I know, the CPC doesn't ever check the signal?

I thought you just wanted to see the bottom of the board to check his soldering skills

Bryce.

[IanS]

Yeah, I was aware of that usage, but I meant using the EXP signal for what it was really meant for. As far as I know, the CPC doesn't ever check the signal?

Because the DDI-1 uses it for this purpose it's almost impossible for anything else to make sensible use of the EXP signal.


If you had a DDI-1 then EXP would be permanently pulled low. So nothing else can use it.
If you had cut the link LK1 on the DDI-1 to always boot into CP/M, then any other device that pulled EXP low would set the DOS rom back to number 7 stopping it from auto-booting into CP/M.


Effectively the signal has become "DDI-1 fitted with Rom at no. 7".

[spookspring]

Romboard flip side

If there is a prize for amateur welding, I'll take it.


Interface flip side.



Also fixed an FD-1 with a snapped board near the red read light, its not as messy as when it came in, plus it works.
No copper trace rolls available anywhere, or ebay.

[IanS]

Thanks for the flip-side pictures.

So the EXP pins are connected to the EXP signal on the expansion bus (and GND).


I can't imagine what possible use they though they could get from it. Maybe the fact that the link isn't made tells us all we need to know.

[Bryce]

That's seriously impressive work for someone who says they've never soldered before!

You can use standard solid wire to fix broken tracks, it doesn't have to be copper traces.

Bryce.

[Bryce]

They may have wanted to auto-detect the hardware, but as you said above, it would only work if a DDI-1 wasn't attached.

Bryce.

[TFM]

I was thinking about that EXP jumper. It may be there to set the /EXP signal going to the CPC. ...

That's exactly what I posted before.

[spookspring]

Oh regards that missing EXP jumper, that's why it does not work on my 6128 maybe. So if I put a wire there and enable the 8-15 ROMS...it should work on the 6128?

[Bryce]

No that would have no effect on the 6128, or at least NOT having it wouldn't be a problem. Adding the jumper would mess up both the 464 and the 6128. I've no idea why it wouldn't normally work on the 6128 though?

Bryce.

[TFM]

Oh regards that missing EXP jumper, that's why it does not work on my 6128 maybe. So if I put a wire there and enable the 8-15 ROMS...it should work on the 6128?

Did you check that? I see no reaseon (as long as you own the right cable) why it shouldn't work with a 6128. All ROM/Flash cards I know work with all CPCs.

[IanS]

Adding the jumper would mess up both the 464 and the 6128.

Why? On the 6128 it is already linked to ground via LK1, same as it would be if a DDI-1 was fitted to a 464.

I can't see any reason or advantage to adding the link nor can I see it doing any harm.