Z80 Homebrew Computer - fault finding

[nockieboy]

It says that something has been loaded into the transmit register which has not transmitted yet - normally I would expect that is because it is still being sent. The only reason I can see for the current situation, assuming the Z80 is correctly wired to the 68B50 is that it does not have a transmit clock.

I find myself wondering whether this 68B50 does not like being overclocked?

What clock are you currently running things at?

It's running at 7.3628 MHz at the moment. I was trying to run it at 3.6864 MHz, but without the correct 33pF caps I switched back up to the 7.3628 MHz crystal.  I'm going to have a stab at getting the clock divider (D-type flip flop x 2 to divide the clock by 4) working again to drop the clock speed down to 1.8407 MHz (2 MHz is the quoted top speed for the 68B50 in the datasheet.)

I do have a Z80 SIO chip lying around. I was considering swapping it for the 68B50 later once I'd got it all working and was a little more experienced at this sort of stuff.  Might have to consider doing that sooner than later at this rate.

[nfmax]

What do the signals E at the 68B50 pin 14, and CSO at the 68B50 pin 8 (which should just be M1) look like on the oscilloscope?

[grumpydoc]

It's running at 7.3628 MHz at the moment. I was trying to run it at 3.6864 MHz, but without the correct 33pF caps I switched back up to the 7.3628 MHz crystal.  I'm going to have a stab at getting the clock divider (D-type flip flop x 2 to divide the clock by 4) working again to drop the clock speed down to 1.8407 MHz (2 MHz is the quoted top speed for the 68B50 in the datasheet.)

I'd put the clock divider in there and run at 1.84MHz initially then you will guarantee to be within the 68B50 spec.

The datasheet says 1MHz for the clocks but also says that the high/low pulse width can be down to 280ns which implies that you can run it OK at 1.8Mhz.

Also the data access time might be marginal - the IO cycle gives you about a clock and a half (from memory, I will check when I get home from work) which would be just over 200ns at the 7.3Mhz clock - the 68B50 has a specified access time of 180ns which should be fine but....

Where did you get it from by the way? eBay or a reputable source?

I do have a Z80 SIO chip lying around. I was considering swapping it for the 68B50 later once I'd got it all working and was a little more experienced at this sort of stuff.  Might have to consider doing that sooner than later at this rate.

The SIO is a bit more complex to set up so there is more opportunity to get it wrong - I haven't actually used the 68B50 before in a design but in theory it looks relatively simple and dare I say (probably not in the current context) bombproof.

[nockieboy]

What do the signals E at the 68B50 pin 14, and CSO at the 68B50 pin 8 (which should just be M1) look like on the oscilloscope?

I don't have a trace handy for E but will try to get one later tonight.  M1, however, is as below:

Okay, some oscilloscope traces from the 68B50 ACIA chip.  The SBC was running at 3.6864 MHz (albeit with 22pF caps and a 1K resistor, where these should be 33pF and 5K resistor.)

First trace is on pin 8 (CS0) which is the ~M1 signal:

image upload

I'd put the clock divider in there and run at 1.84MHz initially then you will guarantee to be within the 68B50 spec.

The datasheet says 1MHz for the clocks but also says that the high/low pulse width can be down to 280ns which implies that you can run it OK at 1.8Mhz.

Also the data access time might be marginal - the IO cycle gives you about a clock and a half (from memory, I will check when I get home from work) which would be just over 200ns at the 7.3Mhz clock - the 68B50 has a specified access time of 180ns which should be fine but....

Where did you get it from by the way? eBay or a reputable source?

The 68B50's were from eBay... 

[grumpydoc]

The 68B50's were from eBay... 

OK, well.

Now, I have bought a good few NOS chips from ebay for repairing ancient 1980's electronics and they have been absolutely fine but you do have to consider the possibility that someone has remarked a straight 6850 as the higher speed "B" part, maybe not likely, but possible.

Slow everything down such that you are well within the spec of the chip and see where you get to.

[nfmax]

I bought a 68B50 off eBay seller Littlediode, which works fine in Grant's 6809 design. YMMV

[grumpydoc]

Yes, I've used Littlediode, they seem legit, if occasionally expensive.

[nockieboy]

Oookay. 

I've got the clock divider working again.  After some messing about and trying to work out why it was producing a lovely clean clock signal when it wasn't connected to the CLK bus line, but a 1V-high line of fuzz on the 'scope when I plugged it into the CLK bus line, I've worked out that I need to disconnect the CLK line from the single-step clock circuit for some reason.  Looks like you were right, grumpydoc, the CLK pulse was the issue after all.

Once the single-step clock is disconnected, the CLK signal is marvellous (running at 1.84 MHz).  So marvellous, in fact, that my serial program, Realterm, logged a 468K text file of garbage from the serial port recorded over about 10 seconds or so.

Here's an abbreviated version of the log file:

   ” À ø   ü 8   ø    ø <  ð< ø  88 ü  8 8ÿ€ 8 ø <   ø œ   øà   | x  x    ø ø   ü8    ü   < øx>   ø€|  ÿ ø8 ü ÿ <ø  ø8   8x <?   À|>ÿ   pà  p 8   ø   | ðüx  àààøð  øððàø  ààp   ø ððø  ðð<  x   ? ð ðø  øðð à  øðÀx   Àððøà  ðÀÀ<  àà ð  øð< |  ø | ÿ >À ð  <p à  ð ðøÀ  < xÀ ðø||  ü  ? <€ |~  øx < ÿ ððx  àððøø  ððø8  ø8à  ø  àðð8   øðx <  øøÀ ð   ðððð  ðððð<À ððøð|  ðððøx  ð8 x<  x>þàð  <? <   ÿð ? ððx    <þð  ðøð    >~ x ? ðøø x   <ÿð  ððàx  àà ðð  ðàø €  ðððð  àð |ü   ðððx€ øøü|    |øðð  øððàð  àðx|  ð|8 ð   øøøð  ððð8    øð p   ø| ø  xxüðø  ø ðð  ððð  ÿ  ðøxx  px |ü  ðððð>  ð< <ÿ  |üððx  ðø ||   øøðð  ðø <   øøøx€ ðøpx  x € ð xxü  øøx>  x p   >ø  <ÿx>  ð8 À |  ÿ à <x  à Ki°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-°i8´i-°K8´i-°K8´i-°K8´i-°K8´i-°K8´i-

The K8 pattern repeats until end of file.

Erm.. seems the ACIA is working.    All I need to do now is work out the port settings and maybe swap the original monitor program ROM back in.

[CJay]

Well if that K8 pattern really does repeat then it should be a simple matter to watch the terminal window and step through the serial speeds, if it was meant to be 57600bps at 7.6864MHz though I'd take a guess at 14400...

[nockieboy]

Yeah, got it - 28800 bps.  Took me a little longer as Realterm provides a drop-down list of speeds, of which the one I needed wasn't there.  Luckily I was able to type it in and lo and behold, it's working!! 

Thank you very much everyone for all your help, it has been much appreciated!! No doubt I'll be back later when I start trying to upgrade this or working out how to swap it over to a more permanent PCB. 

Thank you again. 

UPDATE:  SBC is working at 7.3728 MHz too!  One thing I've noticed at this speed, though, is that characters are getting dropped in the terminal:

Z80 SBC By Gant Seale

Cold o am tat (C o W)? C

Memory top
Z80 BASIC Ve 4.7b
Copyight (C) 1978 by Micooft
56958 Byte fee
Ok

Doesn't seem to drop chars when listing a program that I've entered, though, but it certainly seems to be a timing issue perhaps.  Might roll it back to 3.6864 MHz and see if it's more stable there.

UPDATE:  It seems that the letters S and R (and likely a few others too) are being dropped.  Whilst entering a program, I noticed the letters S and R were being dropped from a line. I rewrote the line, doubling up the S's and R's and when I listed the program, they were present as single characters.  Odd, but no doubt something to do with how the characters are represented in binary and transferred across the serial interface, perhaps?

[CJay]

Yes, I've used Littlediode, they seem legit, if occasionally expensive.

'Occasionally'? If I could get the prices they ask for some of the parts I have here NOS I could retire 

[nockieboy]

Yes, I've used Littlediode, they seem legit, if occasionally expensive.

'Occasionally'? If I could get the prices they ask for some of the parts I have here NOS I could retire 

Have you started on a Z80 build yet then CJay?  You mentioned being tempted back into it some posts back... 

[CJay]

I've got about this far:
https://goo.gl/photos/WveGmjvTenpGQvp27

There's a Z80 in the antistatic box, bottom left corner, all the HC TTL and RAM in the upper section (628128 and 68B50 on order) and a pile of mixed 'other' stuff including a few 6502 parts, some 805x parts inc a couple of BASIC52 EPROMS.

Planning to build it on this cheap Chinese proto board:
https://goo.gl/photos/Bq7zDLMv7fRAb59q8

[nockieboy]

I've got about this far:
https://goo.gl/photos/WveGmjvTenpGQvp27

There's a Z80 in the antistatic box, bottom left corner, all the HC TTL and RAM in the upper section (628128 and 68B50 on order) and a pile of mixed 'other' stuff including a few 6502 parts, some 805x parts inc a couple of BASIC52 EPROMS.

Planning to build it on this cheap Chinese proto board:
https://goo.gl/photos/Bq7zDLMv7fRAb59q8

That looks like a little treasure trove of parts! I've never used proto board (strip board is the closest) - do you use a wire pen to link the component legs on those? Almost tempted to try it myself.

My next steps are to build a PCB once I'm happy the extra bits I want to add work (bus buffers, extra IO, maybe keyboard/monitor interface) - I'm thinking of getting a commercial company to build it from my design (I use DipTrace). I doubt my PCB-etching skills are up to it!

[CJay]

I've got about this far:
https://goo.gl/photos/WveGmjvTenpGQvp27

There's a Z80 in the antistatic box, bottom left corner, all the HC TTL and RAM in the upper section (628128 and 68B50 on order) and a pile of mixed 'other' stuff including a few 6502 parts, some 805x parts inc a couple of BASIC52 EPROMS.

Planning to build it on this cheap Chinese proto board:
https://goo.gl/photos/Bq7zDLMv7fRAb59q8

That looks like a little treasure trove of parts! I've never used proto board (strip board is the closest) - do you use a wire pen to link the component legs on those? Almost tempted to try it myself.

My next steps are to build a PCB once I'm happy the extra bits I want to add work (bus buffers, extra IO, maybe keyboard/monitor interface) - I'm thinking of getting a commercial company to build it from my design (I use DipTrace). I doubt my PCB-etching skills are up to it!

Oh that's just one box. There are many more where that came from, if pressed I might have to admit to being a bit of a packrat but I've been salvaging components from boards since the 80s, there are boxes and boxes of parts that were tested and stored away 'for projects'.

Home etching is easy at this scale, I'd encourage you to practice a little because it really is very satisfying to be able to take a design and manufacture it from start to finish.

While not my design, the board under the 6502 stuff is one of a batch I etched back in the early 90s (it's a board for an Elektor 8052 BASIC computer).

I etched a few for friends but never used that one as it was a little scruffy, there's nothing wrong with it electrically it just wasn't as good as the rest.

The better examples have probably long since passed into the dustbin of history as friends moved away or, in a couple of cases, died but I do have all the bits to build that one up...

https://goo.gl/photos/7M6VcVUfee9SJZVQ6

[nockieboy]

Home etching is easy at this scale, I'd encourage you to practice a little because it really is very satisfying to be able to take a design and manufacture it from start to finish.

I've only ever really tried very small circuit designs when I was at school, with the traces drawn in pen. Seem to recall the etching results being very variable and so am a little anxious about trying it with a board as complicated (to me anyway) as the Z80 SBC would be.

Clearly transfers are the only way to go to lay the tracks and pads down. Maybe I'll give it a few tries and see if I can make something myself before going the expensive route.

While not my design, the board under the 6502 stuff is one of a batch I etched back in the early 90s (it's a board for an Elektor 8052 BASIC computer).

I etched a few for friends but never used that one as it was a little scruffy, there's nothing wrong with it electrically it just wasn't as good as the rest.

The better examples have probably long since passed into the dustbin of history as friends moved away or, in a couple of cases, died but I do have all the bits to build that one up...

https://goo.gl/photos/7M6VcVUfee9SJZVQ6

Looks good to me - certainly better than anything i could produce!

You've persuaded me though - I think I'll give it a go myself. Still got the drill and some old copper boards (though I'll probably get some new ones to work on).

It'll be a while before I need to worry about the board, I've got some additional work to do with the prototype first (such as an SD card reader) but when I do are there any supplies or design tips I need to keep in mind?

[CJay]

It's not expensive to have them made if you use one of the Chinese fabs but when you've got an idea or need to respin a prototype waiting three weeks or longer is a pain in the nethers.

Mikeselectricstuff has a good tutorial (we disagree about etchant though, I loath FeCl, persulfate is much nicer)

http://www.electricstuff.co.uk/pcbs.html

Find a local office services place that can copy your design from an inkjet printout onto transparency (the Elektor board was made from a direct photocopy from magazine to transparency) or, if you've a decent laser printer, print it direct to transparency, don't dick about with pens unless it's a trivially simple design, ditto transfers unless it's onto a transparency.

A nail curing UV lamp will do the job for exposing the board but it's a good idea to experiment with exposure times, I think BigClive had a video about that too. You can make a UV box for not much money.

Fresh UV laminate is always best so try and get it from somewhere that's got turnover, it degrades with age and takes longer to expose, develop and strip.

I've got a roll of the dry film UV resist stuff you find on eBay but I haven't used it myself yet, I believe it works well though.

Don't be tempted to use sodium hydroxide for processing PCBs, it's dangerous stuff and a PITA to get right anyway. Sodium Metasilicate is much safer and way simpler to get reliable results.

eBay is your friend for chemicals, you can buy all you need there, common or garden kitchen plastic tubs work well as process tanks but obviously don't use them for food afterwards.

I reckon you could be up and started for under £20 in materials, tools (basic) and trays, you'd need a PCB drill (with stand if you use carbide bits) but the very simple ones can be had for a couple of quid.

[grumpydoc]

Glad to see things are now working

I confess I'm not that great a fan of home etching.. I  admit that his is mostly my fault but I just have not managed a set-up which gives consistent results. I did acquire a UV box (for a whole £5 it works but boy does it need a clean) which I haven't tried yet so that might be the magic key to good results.

But even then safe handling of etchants and developers in a domestic environment is something which bother me quite a bit.

I think I just need to learn a PCB design tool and use the cheap Chinese PCB houses - the results will be way better than I can achieve and not that much more expensive given the price of PCBs (especially with photoresist on) and chemicals. It also means that 2 layer and even 4 layer boards are possible.

The latter is probably significant for something like a Z80 SBC - having finally decided I really need to address this as a prject I have finally sat down to design a system (still need to finish the I/O bits but there is a separate post with the main system). I've got this far on laying it out on a 6"x4" perfboard



I haven't quite finished but it looks like I have enough board room for the passives that I need plus 1 or 2 MAX232's, some 9-pin D connectors, power in and a bit more glue logic if needed. 6"x4" is a nice size for the project so I'd like to keep it at that. Sadly I don't think I can quite squeeze in two extra 74xx74's to add in the Nascom-2 style hardware single step but we'll see.

But I don't think I could route it on a single sided PCB that side, maybe on double sided but that's much harder to do at home and lack of through hole plating is a pain (there's a guy somewhere on the 'net who shows how to do double sided with plated holes and solder mask - I'm amazed at his work but know I'm just not skilled enough to replicate it).

[nockieboy]

Glad to see things are now working

Thanks in no small part to yourself. It was a step-by-step fault-finding process and what you said about the 68B50 Tx/Rx clock that revealed the problem in the end. 

I confess I'm not that great a fan of home etching.. I  admit that his is mostly my fault but I just have not managed a set-up which gives consistent results. I did acquire a UV box (for a whole £5 it works but boy does it need a clean) which I haven't tried yet so that might be the magic key to good results.

But even then safe handling of etchants and developers in a domestic environment is something which bother me quite a bit.

Hmm - well my experiences of home etching have been very inconsistent (and it's something I've not done in about 20 years) so I'm a little reticent about the whole process.  I don't recall the UV method of doing it back at school, so that's new to me and seems pretty good - but it's the initial outlay of trying to get hold of (or make) a UV box etc.  Offset that against the high-quality wonder of having a double-sided (or more) PCB with silk screening and solder masking and I'm still on the fence about getting one made professionally - they don't seem that expensive really.

I think I just need to learn a PCB design tool and use the cheap Chinese PCB houses - the results will be way better than I can achieve and not that much more expensive given the price of PCBs (especially with photoresist on) and chemicals. It also means that 2 layer and even 4 layer boards are possible.

I've been using DipTrace to design my Z80 board, can't recommend it enough.  It is much easier to use than Eagle or KiCAD.  The full software isn't free, but the free version will go up to 300 pins and, if you write a nice e-mail to software company asking for a home licence, you can use it to design up to 600-pin circuits and boards.

The latter is probably significant for something like a Z80 SBC - having finally decided I really need to address this as a prject I have finally sat down to design a system (still need to finish the I/O bits but there is a separate post with the main system). I've got this far on laying it out on a 6"x4" perfboard



I haven't quite finished but it looks like I have enough board room for the passives that I need plus 1 or 2 MAX232's, some 9-pin D connectors, power in and a bit more glue logic if needed. 6"x4" is a nice size for the project so I'd like to keep it at that. Sadly I don't think I can quite squeeze in two extra 74xx74's to add in the Nascom-2 style hardware single step but we'll see.

But I don't think I could route it on a single sided PCB that side, maybe on double sided but that's much harder to do at home and lack of through hole plating is a pain (there's a guy somewhere on the 'net who shows how to do double sided with plated holes and solder mask - I'm amazed at his work but know I'm just not skilled enough to replicate it).

Looks awesome.   The link CJay posted earlier about PCB etching might be worth a look - it mentions vias using pins and things, but I'd be surprised if you weren't already aware of the options.

I'm starting to think that having an I²C interface on my board will be useful - could certainly simplify adding things like an SD-card reader, etc.  Anyone have any experience of interfacing this with a Z80?

[james_s]

I use toner transfer from a laser printer to etch at home, usually using that blue transfer film. I routinely do 10 mil traces, 0603 and SOIC parts with consistent results and on a few occasions I have managed 0.5mm pitch SMT ICs but that is pushing the process.

If I don't mind waiting or need more than one board I usually just send it off to China though. Especially if if needs to be double sided.

[CJay]

I've never tried toner transfer, mainly because I think the setup I have/had worked well enough (haven't used it for a while) but I've seen great results from it.

It has the same inherent problems for double sided as pretty much any other home method, you still need some way to get signals from one side to the other and alignment is awkward though I have considered etching two 0.8mm boards and then somehow fixing them together using some alignment holes and a thin layer of epoxy but it all seems rather fiddly when you can get such high quality boards for cheap nowadays.

I'd still recommend it as a learning experience and would still prototype on home etched stuff if it needed to be done on a PCB, I'm just not patient enough to wait for a PCB from China if I know I can make it at home, once I know a design works I'll order boards, I usually go from protoboards to fab though I confess I've not done anything for a while now and most of the 'things I've worked on are still at proto stage.

[james_s]

The best way I've found to get the alignment right is to take the two sheets of transfer film, place them face to face and align over a strong light, then tape one edge. Slip the board in and make sure the seam is straight so the layers stay aligned, drill vias, poke a piece of stripped wire-wrap wire or a scrap of resistor lead through the hole, snip on both sides and the cut will usually deform the wire enough to keep it in place. Then I crimp or punch it to mushroom the ends flat with the board and solder. It's a pain but it's doable.

[CJay]

My 68B50s arrived today, I now have all the parts I need and a large enough breadboard to test it all on before committing to solder and wiring on proto board.

Or, perhaps, it might get a PCB.

*potters off to find EPROM programmer and eraser*

[nockieboy]

Good luck with the project, CJay! 

I've cut a 7404 chip from my board (down to more efficient positioning of the clock circuit and better usage of an existing 74HCT04) but I'm mostly playing with the software at the moment.

Does anyone know a good forum where I can help modifying Microsoft BASIC?  My biggest task at the moment is sorting out the line buffer - each new line is entered into a buffer before being tokenised or executed, depending on if the line is a numbered program line you've entered or a command entered in direct mode.  The issue I'm having is that the line length is limited to 74 characters.  I'm trying to increase it to 90 as a particular type-in I'm entering has a couple of lines longer than 72 chars.

Yes, I can modify the program to make the offending lines shorter, but that defeats the object of me learning Z80 assembly.    I'd have though extending the line length would be easy, but hey ho, apparently not.  It seems whatever I change the max line length to, it still cuts short at 74 characters.

Anyhoo, I've probably gone into too much depth already as it's not really an electronics-related question! 

[CJay]

Damnit, I'm missing a 74LS32.

I have found the EPROM programmer along with a prototype 6502 board I started and I'm looking at a pile of GAL chips and wondering about sticking all the glue into one of those.

As for line buffer, why off topic? It's a technical board and software is a part of it so ask away unless anyone can offer a concrete reason why not.

I'll apply my limited Z80 skills if possible and maybe we can come up with a solution.