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"MTX Plus+" Video Board

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Software

The VDPs are controlled through a number of I/O ports, for MTX and MSX computers (TMS9918), there are two ports, a data port and a command port. The data port is used to read or write single bytes to the Video RAM (the VRAM is not directly accessible to the CPU, all read/writes must be through the VDP). The command port is used to setup the data port address register and to read/write one of the VDP registers.

MSX2 (V9938) and MSX2+ (V9958) use two additional ports for access to the enhanced features of the VDP.

VDP Port Dir

Function

MTX MSX2 MTX+ Dir
0 In/Out VRAM Data 1 98h tbc Out/In
1 In Set up data port address register 2 99h tbc Out
Write to VDP Register
Out Read from VDP Register In
2 Out Palette Registers   9Ah tbc In
3 In Register Indirect Addressing   9Bh tbc Out
Out In

To support software compatibility with the MTX, the VDP must be capable of responding to Z80 I/O ports 1 and 2, for software compatibility with the original MSX, the VDP must be capable of responding to Z80 I/O ports 98h and 99h and for compatibility with MSX2/MSX2+, the VDP must be capable of responding to Z80 I/O ports 98h to 9Bh.

I/O Port Address Decoding

The modularisation of MTXPlus+, coupled with the desire to minimise the number of signals on the system bus drove the decision to perform I/O address decoding on the individual I/O cards, rather than centrally as in the original MTX design. The draft design for the MTXPlus+ video board was based on the use of discrete TTL logic to decode the VDP and CSG I/O addresses.

However, since Lez's kits of Z80 parts included a number of GALs, address decoding will be done using a GAL, greatly reducing the chip count on the video board. This also makes it possible to easily reconfigure the I/O port addresses for the MTXPlus+ video board so that they do not clash with the original MTX port numbers, and therefore, allows the video board to be tested on an MTX computer before the MTXPlus+ CPU board is available.

Hardware

The V9958 was manufactured in a 64 pin shrunken DIP (SDIP) package, the legs on this package have a hole pitch of 0.07" rather than the 0.1" of a normal DIP component. Whilst this would not be a problem for a custom made PCB, it does mean that they are not directly compatible with prototyping boards which presented a little bit of a problem.

The image shows the comparative sizes of sockets for a 64 pin, 0.1" x 0.9" DIP package and an SDIP 64 socket. It can  be seen that the footprint of an SDIP package is much less than a standard DIP, in this case, about 40% less.

This reduced space would have a cost benefit when producing PCBs, but presents some difficulty when developing prototypes such as the MTXPlus+ video board.

It would be relatively easy, and not too expensive, to get an adapter board manufactured to allow an SDIP64 to be mounted on 0.1" pitch prototype boards.

I am considering getting this made, it would not only be useful for MTXPlus+, but would also help in any other designs that we may do that uses one of the V9958s that Lez has supplied.

In the meantime though . . . . . .

Martin has worked out a way to "force-fit" an SDIP-64 socket onto 0.1" prototyping board, describing it, he says :

"Running from pin 1, 1 fits, pin 4 is at 0.21 and is close enough to fit the 0.2 hole, the 8th pin at 0.49 fits through the hole at 0.5, and the sequence begins again with the 11th pin. That gives 3 pins in 7 holes on the inner row, and 7 holes on the next row out to feed wires through for the other 7 connections keeping it relatively neat."

SDIP 64 Adapter Board

I decided to go ahead and get some adapter boards made - here is the result.

I was a bit concerned about putting the rows of pins too close together, but having seen the board, I could have reduced the distance between them, as well as the distances to the edge of the board, making it perhaps 10-15mm narrower - maybe next time.

[Update: Version 2 of the design ordered, which is 0.5" narrower]

I did a little tweaking of  the pad sizes for the SDIP pins to give a little more clearance between them for hand soldering, as you can see, they are still very close, but the spacing should be OK.

There is a minor cosmetic issue - I meant to delete the header connector IDs from the board as it looks messy, but I forgot.

Test fitting the socketed SDIP and 0.1" pitch header pin strip to the bard board - everything actually fits!

I'm quite relieved (surprised ?) as I had to create the SDIP footprint myself - it is not in the standard KiCad footprint library.

Profile view - I may mount the headers on the top side of the board to reduce the wasted space below the adapter board, unless I use the space to mount some passive components on the main board.

 

Board Design

Draft version 0.6 of the video board design

The main board will accommodate the VDP, VRAM, sound generator and the supporting chips.

The small daughter boards are for two different signal conditioning boards for the audio and video which will be cabled to the pin header at the left hand side of the daughter boards

This is Version 1.01 of the Video board design.

The "input" side of the board, including the RAM and clock generation, is relatively straightforward, the same basic design was used in computers in the MSX2 range (V9938) and the MSX2+ and Turbo R machines (V9958).

The other chips on this page are the sound generator, buffer and I/O decoder that will be installed on the video board for convenience.

The Video output from the VDP is a bit trickier though, some V9938 machines used transistors to amplify the RGB outputs and others used a Sony CXA1445P RGB encoder. All MSX machines that had a V9958 installed used the Sony encoder.

The CXA1445P is obsolete and difficult to find, my design uses the later CXA1645P - which has more video output options. It is just as obsolete, but Lez has been able to source some for us.

Martin was a little uncomfortable using such potentially hard to source decoders, so we decided to make the video board flexible enough to cope with a transistor based design as well as one using the Sony encoder.

The area at the left hand of the main board has headers to allow an appropriate daughter board to be installed.

Although I have worked up a transistor based design, because there needs to be some modifications done to the transistor schematics used with a V9938 to work with a V9958, I am less confident of the "analogue" design.

At this point, we have parked the analogue daughter board and will probably revisit if/when the "digital" board has issues that we can't resolve.

A close up of the "digital" daughter board, using a Sony CXA1645P. The other IC is a 74LS04 inverter package used for the optional PAL clock for the encoder.

The default output from the video encoder is NTSC, the jumpers on the board allow PAL to be selected - provided that the PAL clock components are added. I am hoping that this won't be needed as most modern TVs can handle NTSC as well as PAL.

 

Firmware

The only firmware on the original video board design described on this page was the GAL responsible for I/O port decode and generation the "normal" VDP and PSG control signals (MODE 0, MODE 1, VDPWR, VDPRD, IN3 & OUT6).

The basic design of the video board proved to be fine for operation of MTXPlus+ at speeds comparable to MSX 2+ systems, but as the speed of the system increased, the I/O devices, including the VDP, PSG and PIA, were not able to keep up and software waits had to be added to the ROM to allow the system to operate at higher speeds.

Whilst adding software waits to the ROM did allow the system to run at higher speeds, these waits could only be changed by rebuilding the ROM and configuring waits suitable for operation at higher speeds meant that the speed of operation at lower speeds was compromised by the inclusion of additional waits that were not necessary at lower speeds.

Tony Brewer devised a method of automatically inserting hardware wait states as required based on the system clock frequency, this functionality required some modifications to the existing GAL and the addition of a second GAL on the video board.

For details of MTXPlus+ firmware, including the GAL fitted to the video board, see the MTXPlus+ firmware overview page.

 

 

References:

Roger Samdal's Spectravideo site, VDP page

GR8BIT project, Eugeny Brychkov, GR8BIT Knowledge Base article on adding Composite and S-Video output (KB0014).

 

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