Vixen - Motherboard for Vic 20

[Robbie]

Hello.

Just picked one of these up on ebay, and can find few details about them.

It seems functional so far, and I'm going to have a mess with it to see what I can figure out. If anyone knows what the switches do then please shout - presumably they're related to where the PROM appears in memory.

[Robbie]

Here's the innards:

[srowe]

I have one, I made a facsimile of the sheet that came with it originally. Two switches independently switch +5v to slots 3 & 4. The other two reverse BLK1 & 3 on slot 4 only so that a standard 8K RAM cartridge can be used to give 24K expansion.

[Robbie]

You're a God-send srowe! I was just prodding away with a 35kB RAM extension cartridge wondering why I was only seeing 19967 bytes free.

I'll have a mess and see if any other RAM is there but hiding (like it says in the sheet).

If I need to put a cartridge in slot 4 then I fear I'm going to need a bigger desk / spare-room / boat!

[srowe]

I think both the blk1:3 switches must be in the same position, otherwise you're shorting the lines out of the VIC together.

[Mike]

I was just prodding away with a 35kB RAM extension cartridge wondering why I was only seeing 19967 bytes free. I'll have a mess and see if any other RAM is there but hiding (like it says in the sheet). [...]

Not all the RAM of the bigger expansions can form a continuous block for BASIC memory. The power-up message only tells the amount of RAM available for BASIC, which is 28159 BYTES FREE maximum on an unmodified VIC-20. More details here: VIC-20 Memory Layout.

If you want a more thorough test, try my RAM check. You should set aside a pot of coffee though.

I think both the blk1:3 switches must be in the same position, otherwise you're shorting the lines out of the VIC together.

The BLKx multiplexer in the VIC-20 can cope with this (it is a LS-TTL part and the single output on low level will only need to sink slightly more current as part of an unintentional wired-AND, nothing harmful), but that condition will result in mirrored memory blocks and this will be detected by my RAM check.

...

Besides: a BLK1 <-> BLK3 swap doesn't buy one much. The VIC-1110 +8K RAM expansion could also be moved to BLK3 via DIP switches. A BLK2 <-> BLK5 swap would be much more useful.

Anyhow, a +35K RAM expansion covers most of the possible RAM needs (and should be considered a full RAM expansion). Putting extra RAM in the I/O range is rather uncommon, as this resource normally is allocated to cartridge firmware: 3rd party programs have no idea what could be there, and inconsiderate writes (and even reads!) can have side-effects very different from normal memory. That's the reason my RAM check explicitly asks the user whether it should include the I/O area for tests.

Two switches independently switch +5v to slots 3 & 4.

This is actually a very bad method to deactivate the cartridges! It is generally not recommended to separate the power supply yet still connect signals to I/O of digital logic chips. That can lead to all sorts of funny/undefined behaviour of those chips or even damage them.

[Robbie]

I was just prodding away with a 35kB RAM extension cartridge wondering why I was only seeing 19967 bytes free. I'll have a mess and see if any other RAM is there but hiding (like it says in the sheet). [...]

Not all the RAM of the bigger expansions can form a continuous block for BASIC memory. The power-up message only tells the amount of RAM available for BASIC, which is 28159 BYTES FREE maximum. More details here: VIC-20 Memory Layout.

If you want a more thorough test, try my RAM check. You should set aside a pot of coffee though.

Thanks Mike, I shall give that a go - I suspect there's at least 3kB hiding.

I am getting a different response with and without the 'motherboard' expander (28159 without it, 19967 with it), which suggests it's more complicated than just what BASIC has available. There's no real electronics in the expander (other than the switches) and the manual explicitly references 3kB, 16kB and 8kB cartridges in ports 3 and 4, so I suspect they never thought they'd see 35kB RAM, or more than 16kB in a single slot.

[Mike]

I shall give that a go - I suspect there's at least 3kB hiding.

You can quickly test this by POKEing into and PEEKing from addresses 1024..4095. This address range is 'locked' away from BASIC use as soon as there is a RAM expansion in BLK1, which takes precedence. For hardware reasons, the screen memory cannot be moved all the way down to 1024 (unless you apply the VFLI mod to your VIC-20), and so the screen memory instead starts at 4096 with +8K RAM or more.

I am getting a different response with and without the 'motherboard' expander (28159 without it, 19967 with it), which suggests it's more complicated than just what BASIC has available.

This is what happens, when BLK3 gets 'missed' underway. Either by an unfortunate setting of the BLK1 <-> BLK3 exchange switches, or because there's an open on the BLK3 signal trace on the motherboard.

There's no real electronics in the expander (other than the switches) and the manual explicitly references 3kB, 16kB and 8kB cartridges in ports 3 and 4, so I suspect they never thought they'd see 35kB RAM, or more than 16kB in a single slot.

TBH, there's not much gained from this motherboard expander given you already have filled up all available RAMx (<- likely at least) and BLKx slots.

Just try my RAM check and look how it fares. Directly at start you will be prompted with all RAM ranges that are 'fully there', so that should give you a good idea what RAM is active.

[Robbie]

I am getting a different response with and without the 'motherboard' expander (28159 without it, 19967 with it), which suggests it's more complicated than just what BASIC has available.

This is what happens, when BLK3 gets 'missed' underway. Either by an unfortunate setting of the BLK1 <-> BLK3 exchange switches, or because there's an open on the BLK3 signal trace on the motherboard.

Note to self: I've just paid a bit of attention to the manual, and the BLK1 <-> BLK3 is because the 8kB and 16kB RAM cartridges normally both use the same area. These switches move the cartridge in socket 4 to BLK3, hence the 28159. With these switches in the 'normal' position 28159 should be available (if I stick the 35kB expansion into skt 1)... which it is!

Mike, you were right, it was "an unfortunate setting of the BLK1 <-> BLK3 exchange switches". (Nice way of saying I messed up!)

TBH, there's not much gained from this motherboard expander given you already have filled up all available RAMx (<- likely at least) and BLKx slots.

I think I just like playing with cartridges!

Just try my RAM check and look how it fares. Directly at start you will be prompted with all RAM ranges that are 'fully there', so that should give you a good idea what RAM is active.

You're right about the long wait. I shall give this a go later on this afternoon.