>C=Y TM=1 BM=66 LM=2 RM=80 J=NModel 4 Memory Modification for Non Green Screen types>C=NMOD4HD/TXTWell for the 4 we will Need 18 TTL Chips, a roll of Wire Wrap wire and tool. And 16 to 32 120ns 256K rams. (Yea I know 150ns will work Just a well but... If you want to speed up the machine faster that 4 Mhz then get the 120ns) I use NEC and have had good luck. Now I said 16 to 32 or 512K to 1 Meg. the addressing is good to 2 meg. I have had up to 1.5 meg in my Model 4.We will need to add a New port and it was nice of Tandy to leave two decoded address on U50. I used U50-10 (94H) as the Ram control port.  To be fully compatable with all model 4 software port 94H Must have at least 1 of the lower 5 Bits set. For example if you set 94H to a 1 then this will enable the second group of 64K addressing in the first 256K ram to be used for the Alt 64K ram bank (32K high & 32K low).  The way this works is simple. In the model 4/4P the Alt 64K ram is accessed using port 84H, this will not change after the mod.  Port 94H will select which 64K group within the 2048K Max Ram will be used as the alt 64K ram bank that is accessed by port 84H. So we have 5 bits in port 94H D0-D4. This will give us 1 to 31 pages of 64k Ram that we can select for port 84H to access.  To get more than 512K ram we will need to stack ram chips and bend up pin 15 of the stacked chips. This is the CAS* control line and will be connected to the 74S08 chip in the drawings. You can get 1 meg in a model 4 without cutting any metal.  The addition of the HD64180 will require the following  1. Be able to switch between an address decoding of 64K & 256K.  2. Disable Memory map decoding when Address lines A16,A17,A18 is active but A19 & A20 will have no effect.  What did he Say Hmm.. Well the best way to Have your cake and eat it too, is to make two options avalible for address decoding. We can use the "Z80 Bank Method" in which you use the port 148 to select the starting address for the 64K alt memory.Port 148 - D0  D1  D2  D3  D4  D5  D6  D7           A16 A17 A18 A19 A20     WP  Z80/HD64180                                  Mod 4 When D7 is Set HIGH then the HD64180 MMU will control the first 512K of addressing and we can select the starting address for the Upper 256K Ram address area of the MMU.Port 148 - D0   D1   D2  D3  D4  D5  D6  D7           Not  Not  Not A19 A20     WP  Z80/HD64180           Used Used Used            Mod4  Now D6 is Reserved for the model 4 Z80/HD64180 mod because the model 4 doesn't have a Write Protect Bit for the Lower Ram if Used for Rom Image.  Ok A16-A20 is as follows. A16 65536-131070 A17 131071-262140 A18 262143-524287 A19 524288-1023999 A20 1024000-2047999 The Parallel Connection List on Page 2 is a wiring diagram for the daughter borad. It works like this. Get an adapter socket like the SAMTEC Model APA-640-G-D for the Z80 then get a Robinson Nugent ICS-6475-2-TG30 for the HD64B180R0P. The Wire Wrap Adapter plugs into the Z80 socket and will fit through a perf board. Then wire across Per the Parallel connection list.The DUP1 is the HD64B180R0P. The Last thing to do is add the Crystal freq. We used DCLK on U3-16 and connected it to pin 3 of the HD64180. This has a freq. of 10.1376 Mhz used in 64x16 Mode and 12.672 Mhz for 80x24 Mode is found at U17-12. The Model 4P doesnt provide the seperate clocks before being switched like the model 4.  The reason we use the 10 Meg clock is that the Model 4 has some slow PAL chips and the placement of the MREQ in relationship to T1 is different for the HD64180 than the Z80. I believe that if the PALs were replaced with some fast 74F chips then we could run at 6.144.  See Readme/txt for Limitations..  I am sorry but I have not put together a step by step procedure for modifying the model 4 so if you can't figure out what must be done then PLEASE don't do it.  These mods were done because the people who manufacture add on boards would not provide Schematics. I was not going to BUY a $200 or $300 board and not get schematics.  I have tried several times to contact a Certian Company for some technical info but was always lost in the answering service. I would much rather plug in a PC board than wire wrap this mod but if it dies and you don't have schematics the so do you. Parts List:Chip #    Piggy back #       Type 1         U41P               74LS2442         U37P               74LS2733         U35P               74LS1574         U63P               74LS1575         U25P               74LS326         U53P               74S747         U39P               74LS328         U12P               74S869         U27P               74S0810        U14P               74S0811        U75P               74LS15712        U76P               74S4213        U63P               74LS15714        U64P               74S0815        U29P               74S26016        U31P               74S1117        U61P               74S0418        U20P               74S32DUP1      On Daughter Bd.    HD64B180R0P The 74LS Chips could be 74S Chips. I would advise spending the money for them however I have used LS where shown and they work. However some are close. Now I am sure that when you buy "Quality" chips from the discount hous that they are "PRIME". I have found that there is a large difference between the Tol. of the S and LS series. So if you get a "SLOW" LS chip And donot have a good SCOPE (At least 40MHZ) you will go nuts.  So do the mod in pieces. 1. Do the Port mod - U41P,U37P,U25P and check it out 2. Do the rest except for the Ram change and the HD64180 board    ( of course dont remove the jumpers at E11&12) 3. Add the Ram (256K) 4. Last build the Daughter board.  Make the daughter board large enough for the Z80 adapter,HD64180 socket, and the nessary 1488,1489 level converters for the 2 serial ports. ( You can design this as I am tired of typing ) I hope this is enough Information. If not call or Dont write                                   Dave Huffman                                   1345 Williams St                                   Des Moines, Iowa 50317                                   (515) 266-1759                                