emmo@moncam.co.uk (Dave Emmerson) (07/25/89)
Thanks to all those who posted enquiries/suggestions, the board has been modified to accomodate two of the best, and is now at the PCB manufacturer. We expect to build and test one this weekend. In response to requests, our 68000 section will be cut off of the boards, and a position has been added to fit into the shifter chip's location instead. On our revision C ST board this is the 40 pin device nearest the keyboard connector, to the right of the board. In all probability this will already be socketed, so there will be no need to desolder it. This location takes care of the 16 databus connections without resorting to flying leads, and provides mounting for the board. A position has been added for a 16 pin DIL IC style ribbon cable connector and one 256K DIL DRAM socket. This allows the option of removing the 256K DRAM nearest the expansion board, fitting a socket in its place, and using a *short* ribbon cable to pick up 9 of the 10 multiplexed address lines. The 256K DRAM then sits in the new socket on the expansion board for 2MB expansions, or can be discarded if you fit 4MB. The ribbon cable also provides an extra pair of 5v supply connections. Alternatively, you can just fit 9 flying leads to the expansion board and solder these directly to the pins of the nearest 256K DRAM. Using a socket is tidier, slightly safer, and makes removal of the expansion board simpler should you need to have your machine serviced. 3 or 6 flying leads will be needed anyway, to pick up the RAS & CAS lines. Upgrading to 4MBytes will involve cutting 3 tracks and linking them to +5v to disable the original 1/2 Megabyte on the motherboard. Details will be supplied. I have a few 256K DRAMS I can pass on (one per board) at 5 pounds each, and can supply the short cable and its 2 connectors for 4 pounds, assembled. I generally recommend that the pins of the 256K DRAM be cut off and then desoldered one at a time, rather than trying to desolder the chip intact. It may not survive the prolonged heating, and you risk damaging the mother- board otherwise. The PAL device and TTL mentioned in an earlier posting are not needed for the memory expansion, all you will need are 16 or 32 ZIG ZAG type 1M x 1bit (page mode) DRAMs, a 40 pin socket, a 40 pin adaptor plug, decoupling capacitors, solder and thin insulated wire. The DRAMS normally have 20 pins, but some makes omit pin 10 which is unused anyway. Most 150nS devices are suitable, a few examples are : Hitachi HM511000zp-15 (or -12, -10 etc..) Toshiba TC511000z-15 (or -12 etc..) Micron MT4C1024z-15 etc.. 18 pin devices (DIL) are NOT suitable for use with this board. 120nS parts are often similarly priced to 150nS, and are preferred, as are 100nS or even 80nS, but offer only a margin of reliability, no increase in machine speed. We will post the price next week, as soon as we have verified the design, this is still expected to be around 15 pounds + carriage. There are only a couple of spares, priority will be given to those who have already expressed an interest. Any reserved boards not paid for by the end of August will be sold on a first come first served basis. We will not be initiating another batch, but anyone who wishes to borrow the photography for *PRIVATE* use may do so. The boards have solder resist on the underside, but no silkscreen. A clean photocopy of the layout will be supplied, along with fitting instructions (in English only). No special skills are needed beyond proficiency with a soldering iron. ---- Dave E. -Disclaimer- Well that's MY opinion and I'm sticking to it! (But my employer might not agree..)