A lot of the Very Cheap Z84C0020PECs (DIP40 type) are 'Remarked' old NMOS Z80A,Z80B and even Z80Hs.. out of 10pcs you usually find 8 work fine at 4MHz, 2 are DUFF (maybe Z80s that only work at 2.5MHz) and some will even work OK at 8MHz....but they are Not CMOS Z84C0020s !! If you're only using them to repair old 4Mhz Z80 computer or DIY/electronics then they're fine ..!
There are Genuine Z84C0020PEcs on ebay....I know because I've bought them and got them working at 20MHz...see link below.
different logo to the ones i got, mine just have a big Z ..... still waiting for the other lot to arrive......how did they test these for speed in the factory? surely they had a quick method??
gozzo wrote:how did they test these for speed in the factory? surely they had a quick method??
They do not test them, they just label them.
The high speed versions are not tested/rated, it is just a matter of production mask and chip technology used.
To distinct them, there are several methods:
1) logo
Even when Zilog did change the logo many times in the past, the logo with the "nuclear reactor sign" is very old and not used any more. When there are stated chips with this logo and a production date >2010 it is a fake.
3) location
If the seller is located in China you can be sure 99% of offered chips are fakes. This does not prevent european sellers to sell those chips, too (bought from China before).
I’m currently looking at design and building my own Hombrew Timex 1500+ computer , a sort of ‘Timex Turbo R’. The specification is pretty loose at the moment as I’m still doing the feasibility part with parts availability and costings to make sure I don’t hit any problem later down the line.
The General Specification is below.
Power Supply , 9-12V DC input, giving 5V 1.5A regulated to board.
CPU (Z84C0020) switchable between 6,8,12,16, & 24MHz !
Z80 KIO (Z84C9008VSC) giving counter Timer, SIO & PIO
Fast SRAM & Fast EEPROM
EPM7512 CPLD (surface Mount) as Glue and Video Logic (NTSC 60Hz)
8255 as CF card interface
SPI interface possible option?
512K Silicon Disc
TMS9918A as second video chip , as this has an external video input pin.
Sound YM2149 (or AY-3-8910) + SN76489 as these are cheap and easily sourced.
Real time Calendar & Clock
RS232 serial ports to connect to second CPU option
The most difficult part will be the video circuit as I’m wanting to use a TMS9918A as a second VDP (Video Display Processor) , though the TMS9918A has provision for an external composite NTSC video signal which it then superimposes it’s video signal on top making this task easier.
The EPM7512 will give all original video modes and some extra ones. These will be fed into DACs (Digital to Analogue Converters) as the CPLD (EPM7512) have no on chip DACs. These RGB signals will be fed into a CXA1645 (or similar) encoder to give NTSC composite video at 60Hz synchronised with the TMS9918A video signal.
This will give the following modes
32x24 Text (8x8 bit pattern) original mode
64 x24 Text (8x8 bit pattern)
85 x 24 Text (6x8 bit pattern)
85 x 48 Tile (6x4 bit pattern) each tile 256 will have colors
256 x 192 Graphics Mode bitmapped 4 Colors per pixel 12K RAM
512 x 192 Graphics mode bitmapped as original mode 12K
560 x 192 Graphics mode bitmapped with 70 x 24 color grid 14.5K RAM
640 x 172 Graphics mode bitmapped with 64 x 22 color grid 15.5K RAM
Then the TMS9918A modes on a separate graphics plane (superimposed).
It’s all just on the drawing board at the moment, but if anyone has any constructive comments or is doing something similar it would be good to hear from you ?
Costings
Z84C0008PEC 8MHz Z80 CPU DIP40………………………..$2.0
Z84C9008VSC 8MHz Z80 KIO (CTC,SIO,PIO) PLCC……….$2.50
UM61512 15ns Fast SRAM DIP42…………………………….$2.0
27Cxx EPROM faster than 150ns…………………………….$1.0
EPM7512 10ns Altera MAXCPLD... ………………………….$10.0
Intel 82C55 PIO CF Card ……………………………………...$1.50
Intel 82C55 PIO PATA Hard Drive……………………………..$1.50
Intel 8251 SIO Extra Serial Port………………………………..$2.50
Intel 8291 GPIB …………………………………………………$5.0
NVSRAM 512Kx8bit………………………………………….$3.0
DS12C887 RTC clock…………………………………………..$1.50
TMS9918ANL VDP…………………………………………….$5.0
DACs Digital / analogue………………………………………..<$10.0
AT89C2051 MCU for PS/2 Keyboard…………………………...$1.0
Total cost for ICs < $50
Extras
RS232 to USB adaptor … to be costed
SPI interface …. still in pipeline
PCB (Double sided) Approximately $20 size yet to be determined
Power Jack $1.0
On/Off Switch $1.50
D -type connectors Various $13.0
DIL Sockets $5.0
9V or 12V , 1.5A Switch Mode PSU (Cheapy) $5.0
Total cost < $100.00
Last edited by mdave4953 on Mon Feb 06, 2017 2:09 pm, edited 1 time in total.
Speeds above 16MHz are tricky, as there is not much time for address decoding logic to do it's job and allow time for the memory to respond.
Also, you need to be careful with the I/O chips, video processor and sound chips, as they are unlikely to work at the higher speeds. You may have to build a slow bus for them, or slow down the CPU when accessing them, as a lot more than a few wait states will be needed.
i think we had decided that if it could run "all" zeddy software without the need to modify said software then it can be a zeddy everything extra is just a bonus
what's that Smell.... smells like fresh flux and solder fumes...
I remember this discussion very well.
Andy and me decided anything that runs Zeddy Software is a Zeddy. Charly wanted it to be able to drive any Zeddy hardware without modding.....
Its no easy topic.
In theory, there is no difference between theory and practice. But, in practice, there is.
still waiting for the other lot to arrive......how did they test these for speed in the factory? surely they had a quick method??
Standby alert 