2Mb/8Mb RAM disc board with 24bBC bus ===================================== This hardware project consists of a ready made printed circuit board (available seperately), which when populated by the following list of parts forms a 24bBC address decoding board. Building such a board from scratch on strip board makes quite a large final board so by using surface mounted parts the physical size of the board can be reduced to 3" by 3.5", anyone wishing to build one from scratch can do so by consulting the 24bBC document. A 24 bit address bus has 16 million unique addresses, half of which have been set aside for a fast RAM disc with associated RAM filing system (available seperately). So about a third of this 3" by 3.5" board is occupied by sockets for RAM chips for this purpose. Operation of this design is covered in the 24bBC article, elsewhere. PARTS... Two different versions of the printed circuit board exist, the more recent revision simplifies the design a little by saving some logic gates. Boards marked "REV 1.01" use the following components: 3x 74ALS990 in SOIC package (U5, U6, U7) 4x 74HC02 in SOIC package (U11, U12, U13, U9) 1x 74HC139 in SOIC package (U8) 1x 74HC00 in SOIC package (U14) 1x 74HC138 in SOIC package (U3) 1x 4075BE or 74HC4075 in SOIC package (U15) 1x 470uF capacitor, radial leads (C1) 1x 1.3mm PCB mount power socket (SK4) 8x 100nF capacitor in 0805 outline (Grey shaded areas) 1x 34 pin IDC PCB transition header (SK2) 1x Fuse holder plus 200mA fuse (FS1) 2x Turned pin 32 pin DIL chip sockets (SK9, SK10 - can be omitted) 1x 34 pin IDC plug 2x One foot length of 34 way ribbon cable [Option when no other devices will be used on the 1MHz bus] 4x Turned pin 9 way SIL socket (SK5, SK6, SK7, SK8) 4x 9 pin SIL array 2.2k resistor packs [Option if the board coexists with other 1MHz bus devices] 4x Turned pin 9 way SIL socket (SK5, SK6, SK7, SK8) 1x 34 pin board mount non locking IDC socket (SK1) [Optional expansion parts] 1x 74HC138 (U4 - upper 4 bit address decode) 1x 7805 voltage regulator in TO220 case (U10 - for non 5V supplies) Boards marked "REV 2.00" use the following components: 3x 74ALS990 in SOIC package (U107, U108, U109) 4x 74HC02 in SOIC package (U100, U101, U102, U103) 1x 74HC139 in SOIC package (U105) 1x 74HC154 in SOIC package (U110) 1x 4075BE or 74HC4075 in SOIC package (U104) 1x 220uF capacitor, radial leads (C100) 1x 1.3mm PCB mount power socket (SK100) 8x 100nF capacitor in 0805 outline (Grey shaded areas) 1x 34 pin IDC PCB transition header (PL101) 2x Turned pin 32 pin DIL chip sockets (U111, U112 - can be omitted) 1x 34 pin IDC plug 2x One foot length of 34 way ribbon cable [Option when no other devices will be used on the 1MHz bus] 4x Turned pin 9 way SIL socket (RP100, RP101, RP102, RP103) 4x 9 pin SIL array 2.2k resistor packs [Option if the board coexists with other 1MHz bus devices] 4x Turned pin 9 way SIL socket (RP100, RP101, RP102, RP103) 1x 34 pin board mount non locking IDC socket (PL102) The amount of RAM fitted is optional and is inserted into the two 32 pin DIL sockets. 1 or 2 of KM681000 (128k x 8bit SRAM) will provide either 128kB or 256kB, alternatively 1 or 2 of KM684000 (512k x 8bit SRAM) can be used for 512kB or 1024kB of RAM total. Note that KM681000 and KM684000 can't be mixed on the same board. BENEFITS... *Tiny design is even smaller than a 3.5" floppy disc *Very low power, drawing only 125mA *Special 21 pin expansion header gives the designer the option to add other peripherals to the 24bBC bus or even more RAM *Potentially up to 1 megabyte of storage on board (or 8 megabytes using the expansion header) BUILD... Assemble the printed circuit board in the following order to allow plenty of room for the soldering iron tip when placing neighbouring devices. 1.Fit the surface mount integrated circuits. 2.Fit the surface mount decoupling capacitors. 3.Solder in the DIL sockets for the memory (if used) and SIL socket for the bus terminators (if used). 4.Add the power connector and electrolytic capacitor. 5.Depending on which RAM configuration is being used, there are some option links to alter. For use with KM681000 the links should be made in the '1' position as shown on the diagram, and for KM684000 the links should all be made in the '4' position. Links can be made from an off cut of the capacitor's leads 6.Fit any remaining leaded components. 7.If you are not running the board from a 5v supply there needs to be a regulator fitted in the place of U10. To enable the regulator you must first cut the 'U' shaped track on the rear of the board going between pins 1 and 3 of U10 which bypasses U10 when it is not required (the default). HINTS... *Soldering surface mount components is easy - it requires a steady hand and a good soldering iron tip. Don't try to solder with one shaped like a marker pen or a chisel, you need a pointed one shaped like a very slim cone Smear a little flux or solder paste onto the pads where the device is to go and sit the chip in place so that you're happy it lines up with the pads. Hold the chip still by pressing on the top with a flat head screwdriver, and dab a little solder on the diagonally opposite corner pins (eg. pin 1 & 14). This anchors the device. Make any minor adjustments by melting one of those two pins and nudging the chip into line, then when happy put solder on the remaining pins. Now clean the excess flux off and inspect the joints. *To crimp the 34 pin IDC header without crushing the pins put the ribbon cable through the plug and satisfy yourself it is lined up straight. Put the assembly into a vice, and put a few matchsticks between the pins. Then, when closing the vice the pressure is applied to the matchstick which transfers it to the plastic casing without bending any pins. *If only one of the possible two RAMs is installed, put it in the right hand socket nearest to the board edge first. *With the later "REV 2.00" boards the upper 4 address bits are decoded by a chip which is always present, on "REV 1.01" boards U4 must be purchased and added separately. In both cases the option links detailed in step 5 of the assembly process must be set in the '4' position. FROM... Robert Sprowson. Happy soldering!