Prop-ZX81
blittled
Posts: 681
I was looking at Dennis Ferron's Prop-6502 entry, which is a great idea, and was thinking that it should be feasible to replace the 6502 with the Z80 and use it for a replacement of the ULA chip in the ZX81. I figure with the control lines (NMI, HALT, CLOCK, M1, RD, WR, MREQ, IOREQ, RESET) and the addressing/data scheme Dennis has (12 I/O pins) there are enough I/O lines left for a 3 resistor video DAC, PS2 Keyboard, mic in and cassette out and leaving I/O 27-31 for serial and eeprom interfacing. Also by using the Propeller to drive the Z80 clock the t states of the cpu cycle would be synchronized better to the Propeller driver software for video.
Also there could be a front end running a DOS that permits loading ROM images and programs from upper memory locations of the eeprom.
Does this seem like a feasible idea?
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What electronics need - MORE POWER!!!!!!!
Also there could be a front end running a DOS that permits loading ROM images and programs from upper memory locations of the eeprom.
Does this seem like a feasible idea?
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What electronics need - MORE POWER!!!!!!!
Comments
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http://www.propgfx.co.uk/forum/·home of the PropGFX Lite
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The N8VEM is a working CP/M board with 448k of battery backed ram (drive A) and is completely open source and bare boards and built boards are for sale.
A Z80 and a 512k ram chip still cost less than a propeller chip. I guess it is a matter of taking what works and building on it, rather than destroying working things in order to make progress. More and more things could be transferred over to the Propeller.
The current project is to build a dumb terminal using the propeller. The problem is the word "dumb" kind of implies simple, and it is not simple. Scrolling and VT100 codes just don't seem to have been coded anywhere yet. But I'm sure it can be done.
The current N8VEM can talk to hyperterminal running on a PC. It also has bit banged code to read a PS2 keyboard and display on a 20x4 LCD display. So in its current configuration you can have two keyboards and two displays working at the same time.
It would be great to cast off from a PC though. 20x4 just isn't big enough, and with a propeller you can drive a vga screen and in my part of the world, CRT VGAs are freebies on the side of the road at rubbish time.
What else can a prop do? Well, it can do serial. So a Z80 system doesn't need a uart. It can do general I/O, so there goes the 82C55.
Now we get right down to the core of a bare bones minimum 8 bit micro system - an eprom, a ram chip, disk storage and the CPU.
The N8VEM cunningly combines the working ram for the CPU with the disk storage space, so it has just three chips at the core - the eprom, ram and CPU.
The prop could certainly emulate the eprom. With 8 data bits and 8 address bits it could transfer enough code over to ram to start up a Z80 and run a simple program to transfer more data over.
Maybe down the track a whole CPU can be emulated. But there are some technical problems. A ZX81 had 1k of ram I think. Pukka CP/M uses a full 64k. Could a propeller emulate that? If it can, the programs could exist on a SD ram, and maybe down the track the whole thing could exist as a simulation on a propeller?
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Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, uOLED-IOC, eProto for SunSPOT, BitScope
www.tdswieter.com
What type of board is brilldea looking for?
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www.fd.com.my
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I am not necessarily looking for boards. But I see that there is interest in the designs dicsused above. Sometimes a PCB is made by the project designer, but they don't want the hassle of selling and shipping boards though other enthusiast would love to duplicate or improve the project. I am willing to be a resource for those that don't want to sell the PCB themselves by using Brilldea to sell the PCBs and maybe even kits.
I like the idea of having the Propeller emulate some of the old classic chips and systems. I haven't spent much time in this realm, but it sounds very neat.
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Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, uOLED-IOC, eProto for SunSPOT, BitScope
www.tdswieter.com
CP/M will run in a very small amount of RAM so only the Prop is required to get it booted and run small programs. For anything more serious an external RAM will be needed.
Still looking for any suggestions to ring a little more speed out of it.
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For me, the past is not over yet.