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"MTX Plus+" - System Bus

 

Hardware Design

System Bus Design Schematic (Version 1.02)
The system will be based on a Eurocard type backplane, with individual boards for the major functions such as CPU & memory, video processor & VRAM, I/O etc. I bought this used backplane for 5.00 off ebay for development & testing the concept, the finished system will require a much smaller bus - even allowing for expansion, probably 6 slots at most.
DIN 41612 connectors come in various forms, type "C" shown here, has up to 3 rows (a, b, c) of up to 32 pins, giving a maximum pin count of 96.

The connectors that I shall be using have 2x32 rows of pins in the "a" and "c" positions, i.e., the middle row is unused.

Male connectors as shown here will be used on the PCBs to mate with the female connectors on the backplane.

(Using 96/96 way connectors, screening is provided on row "b" between each signal track on the backplane and, via the connector, through onto the individual cards. If a 64/96 way connector is used, the 0V screen is still a feature of the backplane)

A close up of the first 12 slots on the solder side of the backplane.

If you look closely, you may be able to see that the metal areas on the top and bottom edges provide separate planes, suitable for dual voltage systems. On this backplane, the Vcc planes are connected to pin 2 (second pin from the top) and pin 31 (second pin from the bottom).

The backplane has an RS part number (435-068) and was manufactured by Vero Technologies as part number 222-2470.

The front side of the backplane, on this side, the metal areas on the top and bottom edges provide separate ground planes, connected to pin 1 (top) and pin 32 (bottom). (It is possible to cut a track on the board and assign one of the ground planes to a third power rail if required.)

I don't think that this is standard and have seen pins 1 and 32 used for power instead.

Of course, a pre-assembled backplane is not essential.

Martin is building his prototype from scratch - this is his backplane in the process of being assembled. You can see that Martin's connectors are spaced further apart than on my backplane, this will allow the cards to include taller components and also make soldering of the connections a bit easier.

And the completed version . . . .

The power for Martin's "backplane" will be supplied by a 40W "wall wart" and the regulators on the board provide the 12V, 5V and 3.3V regulated supplies for the PCBs.

The solder side of Martin's backplane, showing the intricate soldering required for the DIN 41612 connectors.

Makes me glad that I invested a whole 5 in a pre-built backplane!

To cater for different backplane power distribution, my PCBs will have link options to allow the Vcc lines to be connected on pins 2 and 31, or 1 and 32 and corresponding selection pins for GND.

The default configuration will have +5v on pin 2 and grounds on pins 1 and 32. Although not used in an MTX, the other VCC plane will be used for +3.3v should it be required for any of the newer logic chips.

A +12v supply will also be distributed on pin 17c.

The 16 lines for the Z80 Address Bus are assigned as shown :

 

Address

Pin

Address

Pin

A0

3a

A8

7a

A1

3c

A9

7c

A2

4a

A10

8a

A3

4c

A11

8c

A4

5a

A12

9a

A5

5c

A13

9c

A6

6a

A14

10a

A7

6c

A15

10c

The Z80 Control Bus and system specific control lines (*) are assigned as shown :

Function

Pin

Function

Pin

/IORQ

11a

DIAG*

17a

/MREQ

11c

+12v

17c

/WR

12a

/BUSRQ

18a

/RD

12c

(Spare)

18c

/WAIT

13a

/BUSAK

19a

/HALT

13c

(Spare)

19c

/M1

14a

SER01* 20a
/INT

14c

PHI4* 20c
/NMI

15a

(Spare) 21a
/IEO

15c

(Spare) 21c
/VDPINT* 16a SER02* 22a
PHI 16c /RESET 22c

* These signals are described on the CPU board page

The 8 lines for the Z80 Data Bus are assigned as shown :

 

Address

Pin

Address

Pin

D0

23a

D4

25a

D1

23c

D5

25c

D2

24a

D6

26a

D3

24c

D7

26c

The 8 lines for the MTX Page Port are assigned as shown :

 

Bit

Pin

Bit

Pin

P0

27a

R0

29a

P1

27c

R1

29c

P2

28a

R2

30a

P3

28

/RELCPMH

30c

Starting to put the system together . . .

Testing the bus power - the power board is on the right, wired to a PC ATX connector for its power source. The board on the left is the bus diagnostic card - showing healthy bus voltages.

 

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