emmo@moncam.co.uk (Dave Emmerson @ Monotype A.D.G.) (06/28/89)
We've had a fair response to Harry's letter (thanks), and they're still coming in. To avoid repetition and save the expense of lots of individual replies, here is the situation to date sorry it's long winded.. 4Mbytes total, optionally installed in 2 blocks. It is NOT necessary to remove the ORIGINAL memory chips, but any existing memory EXPANSION will need to be undone. A couple of flying leads will be necessary, I don't trust the power connections to the piggy back pins alone, omit them at your peril! Installation involves removal of the 64 pin MC68000 chip, and fitting a socket in its place. The expansion board plugs into said socket, and carries the MPU in its own socket. This is NOT a job for those who are inexperienced at soldering/desoldering, but it is considerably easier to do if you first snip all the pins off the chip and remove them one at a time. A replacement chip should cost you around 5-10 pounds (UK), but will vary with location, shopping around etc.. I realise that expansion boards already exist, but haven't seen any unpopulated ones which will accept 1Meg x 1 bit Zig-Zag DRAMs, -we have 32 salvaged ones we want to use. Given that these still cost around 17 pounds each, the cost of producing a pcb to use them is comparatively small - around 100 pounds for the first 6 (total) using my trade connections and dispensing with solder resist/legend. Since we don't actually need 6, we thought it a good idea to defray the cost by offering the remainder *at cost price* on the net. We do not intend to repeat it, but may be persuaded to supply details/photography to any *private individual* who does- sort of Public Domain Hardware. As the board was (still is for a few days) at drafting stage, we also asked if any potential purchaser had any requests for other additions. I'm not disappointed to say no-one has as yet. Only simple ones would have been included, and would have made the user's machine non-standard, since Atari have specified the mapping rigidly from the outset. The board can actually take 32 DRAMS, so you can expand up to the full 4Mbytes - NO, NOT 4MB + whatever you already have! (though yes, 2MB + existing is ok) This is due to The ST's designer using incomplete address decoding, common practice in many machines, to reduce production costs. Essentially, only the highest address bits are used to decide whether RAM, ROM or devices are being addressed, thus addresses 00000 to 3FFFF are RAM. Since the firmware ROMs will *almost certainly* only refer to the first device register at address 40000, not (say) 50000, where it may 'echo', you cannot simply apply further address decoding to restrict the device to 50000 to 5FFFF and map in a further Mbyte, 5MB total, since the device drivers expect to find device registers there, not RAM. It would be possible to map in the 'lost' 512K/1Meg of memory at (say) 50000 .. 5xFFF, but what would you DO with it? The OS wouldn't recognise it. If you REALLY need to go beyond 4MB, you should buy a bigger machine, and to get the best from your extra *expensive* memory, it probably aught to have a 32bit data bus, ie 68020 based, but this is not the place to discuss that issue. The ST is fine for its intended market, and is capable of much more than most users know how to ask of it. (If you want to get more from your memory, try using assembly language rather than compilers. Compare a simple 'C' programme's binary file size with an assembled version, 100 : 1 ratio is not uncommon.) The board uses a couple of PALs, a jedec listing can be posted for these or -possibly- we may be able to supply them at cost with the PCBs, approx 12 pounds. The remaining standard 74Fxxx chips should be readily available worldwide. Using 1 meg drams should not impose an excessive strain on the power supply when the average current drain is calculated, if sufficient smoothing/decoupling is provided. It will certainly be less than if the same expansion was achieved with 256K devices. WARNING - note that ALL drams are STATIC SENSITIVE, and you should only handle them under static safe conditions. Failure to do so seldom actually destroys the device, but often reduces its life expectancy. The cost of drams is expected to drop slowly now that supply is catching up with demand and 4Mbit devices have been launched onto the market (at 172 pounds EACH, in medium quantities!) The problem with the MPU clock speed was traced to a faulty crystal by the way. It works fine now it's been replaced. --- Dave E. & Harry F. SnailMail to : Harry Fearnham, Monotype Advance Development Group Unit 16, Science Park, Cambridge ENGLAND CB4 4FQ -Disclaimer- Well that's my opinion, and I'm sticking to it! (but my employer doesn't necessarily agree..)