| Posted By
Vimfuego
on 2011-06-16
07:38:23
|  Plus/4 - PAL/NTSC switchable mod
I just read over on this website:
http://www.zimmers.net/cbmpics/roundrock4.html
That he has a Plus/4 that is PAL/NTSC switchable, is this a modification anyone knows how to do?
Cheers.
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Posted By
Csabo
on 2011-06-16
09:51:35
| Re: Plus/4 - PAL/NTSC switchable mod
Unfortunately there's nothing specific on that page about what he means. All computers in the 264 family (C=16, Plus/4, etc) are PAL/NTSC switchable by default, without the need of any modifications. So he may be referring to that fact, or perhaps he did mod the Plus/4 in some way, though I'd say the latter is less likely.
The actual switching is done via one of the TED registers. The simplest way is MONITOR, >FF07 08 will switch to PAL, >FF07 48 will switch to NTSC. The BASIC equivalents are POKE 65287, 8 for PAL and POKE 65287, 72 for NTSC.
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Posted By
Tomster
on 2011-06-16
15:02:00
| Re: Plus/4 - PAL/NTSC switchable mod
This only switches the TED to NTSC mode  Unlike the C64's VIC-II, the 264er series' TED doesn't have dedicated PAL and NTSC versions. Ok not completely true if you take the rare Drean TED into consideration 
Turning a PAL machine into a proper NTSC machine requires an NTSC ROM and a different x-tal for the timing. A switchable machine would require both ROM versions and x-tals respectively.
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Posted By
TLC
on 2011-06-16
15:28:00
| Re: Plus/4 - PAL/NTSC switchable mod
Vimfuego: yeah, that actually needs some hw hacking, but it's pretty simple. You don't need a different TED chip, they support both tv standards all by themself, yet, you need to do some modifications. If you check the schematic diagram of the Plus/4, you'd find out that PAL machines have a 17.734470MHz crystal, whilst NTSC machines have a 14.31818MHz one. In order to make the machine "switchable", you need to hack in the respective "other" type of crystal + the small trimmer capacitor (you'd use that one to finetune the oscillator frequency) + a switch to select between the crystal circuits. Two things to note: 1.) the TED is capable of producing both PAL and NTSC (off the respective crystal frequency, that is), but, as to which one is set upon booting the machine, is determined by the Kernal ROM. So, in practice, a "switchable" machine would also need some sort of ROM switching (setting a PAL or NTSC rom image active, together with setting oscillator frequency). 2.) Even though you get standard PAL and NTSC by this method, PAL and NTSC have some ties with RF broadcasting standards, too. If you check a PAL and an NTSC motherboard, you find out that they also have different RF modulators. To keep the story short: the RF connection (using an RF cable between the computer and a TV's aerial outlet, and tuning in to a free channel) will most probably not work with the "newly implemented" video standard (ie. an NTSC machine in PAL and vice versa). Video signals (those found on the video port) are perfect, though...
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Posted By
Vimfuego
on 2011-06-16
23:05:12
| Re: Plus/4 - PAL/NTSC switchable mod
Thanks folks, I figured it couldn't be a simple jumper selection on the PCB with the different clock rates in play. Switching the Kernal ROM isn't a big deal, but switching the XTAL might pose some problems, it's not like you can (should) run XTAL signals from one side of the PCB to a switch and back again.
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Posted By
TLC
on 2011-06-17
15:49:40
| Re: Plus/4 - PAL/NTSC switchable mod
Vimfuego: I have actually done that hack once. The machine wasn't a Plus/4 but a C16. You can find a photo of that hack here: http://coroners.no-ip.hu/stuff/DSC00219.JPG . Well, the actual way that I did this hack might look pretty crude , I also wouldn't make a bet if it'd comply with FCC's rules of EM interference emission ), but it certainly worked (and works to date...). You can recognize the extra 14.31818MHz crystal on the top of the original 17MHz one (this is obviously a PAL machine), and the extra trimmer cap beside it. The complex at the left is a simple 128k flash board that (amongst other things) has the extra NTSC kernal image. All in all: you can most probably just use simple wiring and SPDT switches, it'll most probably work (...I'd keep the wiring as short as possible, though).
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Posted By
Vimfuego
on 2011-06-20
21:33:08
| Re: Plus/4 - PAL/NTSC switchable mod
Thanks TLC, food for thought.
On your ROM board, why so many connections to switch it? Wouldn't you normally just switch the highest address line high or low?
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Posted By
TLC
on 2011-06-21
12:22:39
| Re: Plus/4 - PAL/NTSC switchable mod
Vimfuego: the majority of the connections are ROM CS' lines, the rest are probably R/W', CAS', and some other control signals. In short, the board has been a "prototype" of a universal 128k flash module ("universal" = it'd emulate 8 pieces of 16k ROM slices, ie. the ROM size decoded/mapped by the standard ROM CS' logics of the Plus/4). You can connect a maximum of 8 ROM CS' signals to the board from the mainboard and/or your own TTL logics. For example, in this C16 all original ROMs were just replaced by this single flash chip ie. the pages of the flash acted as Basic, Kernal, and the extra NTSC kernal (...and even the 3-plus-1 function roms originally only present in the Plus/4). A TTL priority encoder chip (which is located under the flash rom) encodes the individual ROM CS' lines into a 3-bit page address ie. the highest 3 bits of the address which is then fed to the flash chip. The rest were needed by the in-system-programmability function (ie. you could rewrite any pages of the flash chip using a small ML routine without removing the chip from the socket). ...It just ended up like yet another prototype that never really "made it" , but it became handy (at least, for this hack) nevertheless...
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