Are you b building or thinking of building an Acorn 6502 BBC B/Master

General "chat", not necessarily MTX related
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lezanderson
Posts: 175
Joined: 14 Aug 2012 15:31

Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by lezanderson » 20 Dec 2018 13:55

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Ideas and Musings about Design & Building a homebrew 6502 Acorn BBC Computer


These are just ideas and thoughts and should perhaps be taken with a little pinch of salt. But they are a starting point for anyone possibly thinking of such an endeavour


Design ideas:

CPU:
As in the original a 6502 , but a newer CMOS 65C02 such as the R65C02Pxx,W65C02Pxx or even a W65C816 running at up-to several times the original 2MHz speed.

Speed. : Clock Speed could be switched/varied from 2MHz to as high as 16MHz ??

Memory :
Fast SRAM and EPROM/EEPROMs would be used to allow for much faster clock speeds. SRAMs such as UM61256/UM61512s 15ns type and 27C256-45 45ns are examples. Giving 128K SRAM or more.

Video :
As in the original machine a Motorola 6845 CRTC would be used (HD45605,R6545 etc) in conjunction with Dual Port SRAM such as IDT7007 (32k x 8 bit). This would allow the CPU and Video circuit to work independently. Allowing much higher CPU clock speeds without causing video disruption.

Sound:
As in the original a SN76489 would be implemented but also a YM2149 , SAA1099 or even speech synthesiser giving multiple sound generators and numerous sound channels, with stereo output. As these chips are cheap this could be done relatively inexpensively.


I/O
As in the original , namely 6522s,68B21,68B50,68B54setc to give compatibility with the original BBC B / Master . Floppy disc controllers could also be kept . This would probably means most I/O would be done with a relatively low clock speed of 2Mhz or possibly 4MHz ? Glue logic such as CPLDs (EPM7128,EPM7032s) or GALS could be used for the function of I/O chip selection, these would obviously need programming.

Teletext :
Again as in the original .

Glue logic :
Glue logic would be needed to interconnect all the various ICs, components this was mainly done by old TTL ICs and some dedicated ICs in the original. These could be replaced with more modern (programmed) GALs & CPLD (Complex Programmable Logic) in order to be more efficient . This would do such functions as Memory Mapping / Selection, I/O selection and various other low level functions. It could also be used to create the ‘Tube’ 1MHz (or More) to allow a second CPU to be connected.

Keyboard/ Mouse
This could be kept as original or a PS/2 or even USB keyboard connected with the aid of a Modern MCU ? I’ve seen several PS/2 circuit using MCUs so that should be relatively easy to implement. A USB keyboard would be more complex, but could be done.

Hard Drive / CF Card (Storage)
A hard-drive interface could be added using a PATA type . I’ve seen this done with Intel 82C55s and I’m sure there’s plenty of schematics for a CF Card interface on the web.


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lezanderson
Posts: 175
Joined: 14 Aug 2012 15:31

Re: Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by lezanderson » 20 Dec 2018 14:01

So if your seriously thinking of or currently building a 6502 BBC B/Master system then I might be able to help with ICs

65C02 (R65C02P4 or W65C02Pxx) DIP40
SRAMs , 32K,128k,512K : HM628128-7 or similar 128K SRAM DIP32
16K 32K or 64K ROM DIP32 27C512 etc
R65C22 VIA
68B21 PIA
MC68B50 ACIA
MC68B54 ADLC DIP
6845 CRTC (MC68B45,HD68B45,HD46505,R6545) or similar
SAA5050 DIP Teletext Chip
D7002C DIP ADC 12 Bit
MC146818 RTC DIP
MB8877 FDC IC DIP
CPLDs EPM7128SLC84 / EPM7032SLC44 EPM7064SLC44 CPLD Glue logic PLCC44/PLCC84
SN76489 Sound Chip DIP
AY-3-8910 Sound Chips
7805 Regulators
Plus other bits and bobs like IDT7005IDT7006/IDT7007 Dual Port SRAMs

Just email me and I'll see Which ICs I have left.

lezanderson
Posts: 175
Joined: 14 Aug 2012 15:31

Re: Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by lezanderson » 20 Dec 2018 14:03

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For those that don't already know : My email address is lezanderson@gmail.com.

Martin A
Posts: 417
Joined: 09 Nov 2013 21:03

Re: Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by Martin A » 21 Dec 2018 11:19

On a more Memotech theme, there's always a 6502 co-pro board with BBC Basic to consider http://primrosebank.net/computers/mtx/p ... xcopro.htm

Basic requirements for that are:
MTX "Side"
8K or larger eeprom or flash
16V8 Decode GAL

6502 "side"
1K or larger Dual port ram - the faster the better
65C02 CPU
16V8 Decode GAL
128k static ram (44k usable)
32k or larger eeprom or flash.
Can oscillator for the 6502 CPU clock.

Speed isn't critical on the MTX side anything made in the last 30 years should be fast enough! They just have to be no slower than the original MTX parts (350 ns or so).

A 90's vintage Roackwell CPU would probably be limited to 4 or 5 Mhz, the Modern W65C02 are rated for 14mhz and can be pushed further, but that requires the other components to be working fast enough to keep up.

As Dave mentions on the page I linked, the MTXplus boards were able to run at 16Mhz, but that required 65C02 side 7ns logic and 55ns memory (and that would have had to have been running faster than specified!) 8 to 10Mhz should be achievable with the more normal 70ns memory and 25ns GAL logic.

lezanderson
Posts: 175
Joined: 14 Aug 2012 15:31

Re: Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by lezanderson » 21 Dec 2018 14:11

MTX 6502 / BBC Co-Proceesor ICs.

I might be able to help with ICs for the MTX co-processor...just tell me which ICs you need



R65C02P4
IDT700x (usually IDT7005 8kx8bit) Dual port ram
16V8 Decode GAL (various GALs)
32K,64K,128k static rams
27c256,27c512, 27C010 etc EPROMs, I also have some eeproms/flash ROMs.
No oscillators I'm afraid


Hope this is useful ??

>> P.S.

Is there a PCB for the MTX 6502 BBC Co-Processor unit ?

Martin A
Posts: 417
Joined: 09 Nov 2013 21:03

Re: Are you b building or thinking of building an Acorn 6502 BBC B/Master

Post by Martin A » 22 Dec 2018 00:08

lezanderson wrote:
21 Dec 2018 14:11
Is there a PCB for the MTX 6502 BBC Co-Processor unit ?
No PCB, all 3 prototypes (1 MTX, 2 MTXplus) were hand wired, with only 7 chips and a few other components it's not a complex board.

The schematic is on Dave's site if anyone want to do their own.

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