TRS-80 Gebruikers Vereniging

Actief sinds 1 oktober 1978

You can't buy new Level II ROMs for the Model I anymore, so you'll have to put (burn) the ROM in EPROM yourself. Maybe you could find someone with new Level II ROMs, but I guess you want the hard disk boot ROM found somewhere else on this website to put in EPROM. Find out if your EPROM programmer needs hex files or bin files. Now build the ROM file (bin or hex). Burn the file in the EPROM. Before doing that you must decide what kind of EPROM to use.

In the TRS-80 Model I there are two IC sockets to place the ROMs in. These sockets are suitable for a 2364 (8k, ROM A) and a 2332B (4k, ROM B) ROM. Level II totals 12k ROM. You can replace these ROMs by one or two EPROM's. For instance in a 2764 (8k) plus a 2732 (4k), or in a 27128 (16k) from which only 12k will be used.
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Examine the ROMs and EPROM above carefully. Look at pin numbers and the signals from the (EP)ROM's. The IC sockets on the motherboard of a TRS-80 Model I are wired for the 2332B / 2364.
The differences between a ROM A (2364) and a 2764 and 27128 are:

pin 1 at the 2764 = VPP, only for programming
pin 2 at the 2764 = A12
pin 20 at the 2764 = CS*
pin 23 at the 2764 = A11
pin 26 at the 2764 = nc / A13 at 27128
pin 27 at the 2764 = PGM* only for programming
pin 28 at the 2764 = +5V
The differences between a ROM B (2332B) and a 2732 are:

pin 18 = CS (2732) in stead of A11 (ROM B)
pin 21 = A11 (2732) in stead of A12 (ROM B)
The pins 3 until 26 at the 2764 match, with exception of pins 20 and 23, with pins 1 until 24 of the ROM A. By placing the 2764 in the IC socket in such a way that pin 14 of the 2764 will match with pin 12 of the IC socket Z33, we accomplish that most part of the pins will be connected to the right signal.

From the differences and similarities between the ICs one can conclude that the ROM IC socket on the motherboard will changes (get a other function) only at two places, pins 18 and 21. Pin 18 needs CE* and pin 21 needs A11. Moreover the four extra pins (1, 2, 27 and 28) of the 2764 must be provide with the right signals.

Preparation of the 2764:

Pins 1, 28, 27 and 26 all need +5V. Pin 26 of the 2764 already gets +5V as it corresponds with pin 24 of the ROM IC socket. So just connect these pins (1, 28, 27 and 26) with each other. Attach a piece of wire to pin 2 of the 2764 to connect it later on to A12 on the motherboard.

Preparation of the 27128

This is basically the same as the 2764 except that pin 26 has to be bent out at forehand as it will get the signal of A13 later on. So attach a piece of wire to it first.

The following instruction is for replacing the ROM into EPROM on a American motherboard with modification "G".

Cut through the print traces (under and upper-side of the main circuit board) at pin 21 of Z34 (ROM B, isolating pin 21 from A12)
Pin 18 of Z34 (ROM B) is connected to A11 at the underside and this trace goes through a hole to the upper-side were it can easily be cut (ROM B, releasing pin 18 from A11)
At point 1 we also interrupted the A12 trace so we have to fix this by reconnecting Z38 pin 5 (A12) with Z51 pin 10 (A12).
Connect the metal hole at pin 18 of Z34 (ROM B) with GROUND, e.g. pin 87 of Z51. This pin (was A11) now becomes CS* and stays low all the time.
Pin 21 of Z34 (ROM b) must now be connected with Z51 pin 6 (A11).
Install the EPROM's (the 2764 in Z33, the 2732 in Z34). Pin 14 of the 2764 should be placed at pin 12 of the IC socket. Pin 2 of the 2764 must now be connected with the metal hole right beneath pin 13 of Z34 (ROM B). This metal covered hole (A12) is connected with Z51 pin 10 (A12) at the components side.
Confirm that in jumper X3 the following connections exist: 9 to 8, 10 to 7 en 11 to 6. Also make sure that the connection 16 to 1 is broken.
For installation of a 27128 EPROM follow the same instructions as above (point 1-7) and furthermore change the following too:

Pin 26 of the 27128 must connect to A13. This pin should not go in the socket hole 24, because that one is connected to +5V. It's extremely difficult to disconnect the socket pin from the +5V. So before hand bent out pin 26, and connect it by means of a piece of wire to Z35 pin 3 (A13).
The pins 1, 27 and 28 of the 27128 EPROM can easily be connected with each other and than to the +5V.
Pin 2 of the 27128 must be connected with A12 likewise as was done to the 2764.
At X3 now connect 1 to 16. Place the 27128 in one of the ROM IC sockets.
There is no guarantee that the hardware project will work. This are roughly translated parts of larger articles published in Remarks. So no warranty.