               Model 4P Memory Modification for Gate Array Boards

mod4pmod/txt

How about expanding your model 4P to 512K ram or even to 2 Meg. Sound expensive
? Well for the 4P we will Need 10 Chips, a roll of Wire Wrap wire and tool. And
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 goodluck. The last set I bought were $3.54 ea. So a 512K system should
cost you $75.

This Mod is for the GATE ARRAY board ONLY. If you don't have a gate array board
and have schematics that logic used here will also apply. The chip numbers will
change. Actually the original memory mod was first done on my old Model 4 which
has 1 Meg installed

We will need to add a New port and it was nice of Tandy to leave two decoded
addresses on U28. I used U28-10 (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
addressed by port 84H. So we have 5 bits in port 94H D0-D4. This will give us 1
to 10 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. In the 4P however you will need to cut away some of
the metal sheld so the rams will clear. You can get 1 meg in a model 4 without
cutting any metal.

I don't own a Model 4P (Don't get Worried) so Jerry Johnson's 4P was the first
guineapig. Jerry says it nice to have 352 Grans (512k) for a system disk.

The installation for the 4P is as follows:

1. Open up the machine and remove the main CPU board
2. Mount the following Chips on the board. We will Piggy back them and only
solder the pins that we dont bend up
#1 74LS32 -  U105P - Bend all pins Except 7&14
#2 7486 -    U24P  -  "    "          "   7&14
#3 74S08 -   U129P -   "   "          "   7&14
#4 74LS32 -  U87P  -   "   "          "   7&14
#5 74S08 -   U114P -   "   "          "   7&14
#6 74157 -   U111P -   "   "          "   8,15&16
#7 74157 -   U110P -   "   "          "   1,8,15&16
#8 74LS244-  U125P -   "   "          "   3,5,7,9,10,12,14,16,18,20
#10 7442 -   U116P -   "   "          "   8,16
3. Connect the following:
U85-16 to U24P-2
U85-12 to U105P-9
U85-15 to U105P-10
4. Place the 74LS273 on U85 with all pins Bent up except1,3,4,7,8,10,13,14,
17,1,8,20
5. Make all connections except U114-3&5 and U115
Then power up the board in the machine and check for operation. At this point
the only change noticed should be a new port 148 (94H). Set and reset the bits
and read the port to verify operation.
6. Then remove the Main Cpu board and do the following:
a. Pull all Rams
b. Wire all pin 1 of the Ram sockets together and connect to U129-6
7. Remove R21
8. RAS MOD - Lift one end of R23 that goes to U115 (See Drawing). Add jumper
from R23 on board side to pad on R21 opposite to U115(See Drawing). Connect
lifted end of R23 to U114-11. Connect remaining Pads of R21&23 to U114P-12&13
MAKE SURE IT IS LIKE THE DRAWING !!!!!!
9. CAS MOD - Find the feedthrough next to RP1 between pins 15&16. This should be
the CAS line for the rams. MAKE SURE - it should connect to pin 15 of the RAMS.
(Use Digial Ohm meter only or LOOK). Turn board over on the Non Component side
(Bottom) and follow the trace to another Feedthrough about the same distance fro
from the rams as the first feedthrough. Cut the trace that connect the Two
gh. The Feedthroughs will now be the RAM CAS connections. U133-140 will be "Cas1
" & U153-160 will be "Cas2". Now follow the trace from the first feedthrough on
the bottom of the board to U136 & U156 - 15. Cut the trace that connects U136-1
& U156-15. Follow the trace to a feedthrough next to pin 16 of U136. Cut this
trace near feedthrough. Now we should have separated the two rows of Rams on pin
15. Check With a Digital Ohm meter that the two rows are separated. Make Sure
you understand which traces to cut !!!!
10. Now connect U114P-3 to a 47 Ohm and place the other end in the feedthrough "
Cas1". Connect U114-6 to a 47 Ohm and place the other end in the feedthrough
"Cas2."
Now to add more RAM just Stack the Ram and Bend pin 15 out and connect all
pin 15's per row of Rams to a 47 Ohm and to Cas3-7. Walla 2 Meg.
I recomend you run 512K for a month before you stack Rams.
11. Refresh is found on pin 5 of U87. Connect it ( You can run the wire through
any avalible feedthrough ) to U114-2&4
12. Replace the Main board and power up
If you Get Trash on the Screen Check the Wiring You GOOFED.
If it runs but is Unreliable then you will need a good Scope and look at the
RAS line. Check for Ringing. Add resistance to the series resistor if you have
ringing. If the transitions are a good 4.75V then reduce the resistance.
Check the Cas line also. 47 Ohm maybe to much.

On My model 4 I have No resistors in the Ras or Cas.

If you Use NewDos/80 then I have a RamDisk program that will add a 5th drive and
use the Memory we just Added.
The RamDisk uses the fourth Pdrive, so if you have 512K ram then set Pdrive #4
as follows
Pdrive 0 4=0
Pdrive 0 4 tc=99

Then type:

Ramdisk 4 ' I renamed it to RM so it would be " RM 4 "
If the NewDos/80 4P system has two Directorys then we need two steps (I
calculated the Directory starting and ending points so I can return Directory
Protected Status)
Copy 0 4,,fmt cbf ndmw /sys
Copy 0 4,,nfmt cbf usr ndmw
If you Made the ModelaIII a SYS file then a full disk copy will work
Copy 0 4,,fmt cbf ndmw
then to mount as drive 0:
Ramdisk 0
thats it. ( Yes Jack we "Can" CHAIN through this Version )

The Ramdisk program will support a standard Model 4 or 4P with Just 128K if You
set the Switches accordingly

I have Mods for the Model 4 and Have 1 Meg in mine But the design incorporates u
sing a HD64180. This Makes the Mod a little Complicated. The Model 4 Version board that I have is one of the older Non Green screen type and I had to
Redo the the RAS/MUX/CAS circuits in order to Run the 1 Meg rams . However if
there is some interest for the model 4's I will send in the Mods I have done.
interest for the model 4's I will send in the Mods I have done.


Model 4P Hd64180 Mod.

This is an additional mod to add the HD64180. If you are doing this all at once
then you must use the MOD4PHDx/TXT files for reference. The difference between
the two mods is really the need to:

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 A1
9 & A20 will have no effect.

What did he Say Hmm.. Well the best way to Have your cake and eat it to is
to make two options available 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 addressarea 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
board. It works like this. Get an adaptor socket like the SAMTEC Model
APA-640-G-D.

This is a Wire Wrap Adapter that plugs into the Z80 socket and will fit through
a perf board. Then wire across Per the Parallel connection list. The Last thing
to do is add the Crystal freq. We used DCLK on J7-10 and connected it to pin 3
of the HD64180. This has a freq. of 10.1376 Mhz in 64x16 Mode and 12.672 Mhz
in 80x24 Mode. Now if you use NewDos/80 then the 64x16 mode is the most used.
But if you want 80x24 then a different clock must be obtained. All you need is acrystal for 10 Mhz and place it across pins 2&3 of the HD64180.

See Readme/txt for Limitations..
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