RC2014 Pro troubleshooting ... at a bit of a dead end

[Pete Baker]

Hello, RC2014 noob here, with another nearly complete but non-functioning RC2014 Pro!

Disclaimer: I'm very much a beginner when it comes to electronics. However, it's been great fun troubleshooting this so far, in a way I prefer that it didn't work first time, I've already learned so much. But it WOULD be nice if I could get it to work! ...

So I've got to the point where I want to test the first five modules together (CPU, Dual Clock, ROM, RAM and Dual SIO/2), but no matter what I do I can't seem to get any response at all from the TTL serial port. Like, nothing at all, not even garbage characters, just completely dead.

Starting with the basics, I've done a pretty thorough run through of all the troubleshooting advice on Spencer's website, continuity tested the backplane and all of the boards (verifying everything against the schematics). I've made sure that all the jumpers are set accordingly, using the instructions on the site.

Finally I've pointed my oscilloscope at it. I have a nice strong clock signal at 7.37MHz (Clock.jpeg), and when I hit reset then probe everything else on the backplane I can see activity on RD, A0, A1, M1, MRQ, and D0-D7 (MREQ.jpeg, M1.jpeg).

This is where it gets a bit weird. The "Testing with an Oscilloscope" page on rc2014.co.uk states that after hitting reset there should be some activity on Tx, and that A7-A14 should settle down to a consistent stable state of no activity.

However, there is also CONSTANT activity on all of the other address lines, whilst the Tx line is completely dead after reset, even with the oscilloscope zoomed in at 200us/div.

I've also probed RTS and RTSB on the two serial ports on the dual SIO/2 card, and I can see constant activity on these as well (RTS.jpeg). Sorry if I'm misunderstanding this, I'm not 100% on the purpose of RTS / CTS, but this sounds to me like the SIO card continuously going "I have something to send!", but never actually sending anything.

After googling this problem I thought it might be an issue with the 1k ohm resistors on Rx / Tx / RTS, so I replaced them with 100 ohms, but this made no difference. I've also tried three different USB / FTDI cables, including the one that came with the kit ... again no difference. I also ordered another 24886009 ROM and swapped it out, thinking perhaps some ROM problem (I thought it unlikely) ... again no difference.

So after all the continuity testing and scope probing, I'm starting to wonder if there's an issue with the RAM, or even the Z80 itself, but as there is activity across both the address and data ports I think this is unlikely too.

Finally, my suspicion is that I've damaged the Z80 SIO somehow. And if so, how would I verify that it's damaged? I'd like to try and swap it, but I'm struggling to find new chips online, I can only see second hand ones on ebay, etc.

I guess another next step forward would be to buy a logic analyzer and start having a play with that, but before I start down that rabbit hole maybe there's something obvious I've missed, or something easier to check that someone smart on this group would be kind enough to suggest?

Thanks in advance,

Pete

[Mark T]

200 us is too slow to see the real shape of the clock signal. 1MHZ would be 1us. I think you are seeing aliasing of the clock to the sample rate of the scope. Though the frequency measured by the scope looks correct so your clock is probably ok.

Try testing your usb ttl serial adapter, connecting the tx to rx and check that typing in the terminal emulator will echo back to the display.

Also make sure that tx and rx are connected correctly, some adapters expect tx is output and rx is input, so tx on the adapter connects to rx on the target.

[Robb Bates]

As I understand, A8-A15 will not settle down to zero activity, since during an IORQ, the I/O port address gets placed on those address lines... or something... I'd have to dig.

Robb

[Pete Baker]

Hi, unfortunately this is a Fnirsi beginner cheaposcope (fine for my needs), and it only goes down to 10us!

I've connected Rx to Tx on all three of my serial adapters, and they all echo back just fine.

I've also tried swapping Rx and Tx on the adapter connection, and it's made no difference. Besides, shouldn't there be activity on the Tx bus line regardless of whether there's an adapter connected to the port? Or does there need to be something connected?

[Pete Baker]

So interestingly there's also constant activity on IORQ (see image), which lines up with there being activity on A8-A15.

All this activity, but no output from the Z80 SIO ... I'm starting to get very suspicious of the chip!

[Mark T]

10us sweep time should be ok for most signals but maybe won’t give a good indication of the clock, but it seems to be running so the clock should be ok.

Activity on IORQ looks like it might be checking status of the SIO. Was this on pin 36 of the SIO?

Check if the CE input of the SIO pin 35 is also active, and the RD input on pin 32.

[Pete Baker]

On Friday, October 17, 2025 at 5:48:38 AM UTC+1 Mark T wrote:

Activity on IORQ looks like it might be checking status of the SIO. Was this on pin 36 of the SIO?

 

Yep, I can see this signal both on pin 36 of the DIP and on the RC2014 bus.

 

Check if the CE input of the SIO pin 35 is also active, and the RD input on pin 32.

 

I see activity on both Chip Enable (35) and Read Cycle (32) pins (see screenshots).

I really hope this is not the SIO, seems that the Z84C4208PEG is pretty hard to get hold of on its own. Does anyone know if the 10MHz version (Z84C4210PEG) will work with the RC2014?

Pete

[Alan Cox]

On Fri, 17 Oct 2025 at 16:01, Pete Baker <evo...@elencher.com> wrote:

On Friday, October 17, 2025 at 5:48:38 AM UTC+1 Mark T wrote:

Activity on IORQ looks like it might be checking status of the SIO. Was this on pin 36 of the SIO?

 

Yep, I can see this signal both on pin 36 of the DIP and on the RC2014 bus.

 

Check if the CE input of the SIO pin 35 is also active, and the RD input on pin 32.

 

I see activity on both Chip Enable (35) and Read Cycle (32) pins (see screenshots).

I really hope this is not the SIO, seems that the Z84C4208PEG is pretty hard to get hold of on its own. Does anyone know if the 10MHz version (Z84C4210PEG) will work with the RC2014?

It will: you need a part that is 8MHz or faster.

Alan

 

[Spencer Owen]

Hi Pete,

It looks like you've dived pretty deep in to this. 

You don't mention what ROM image you're trying to boot or what the jumper settings are. I would suggest trying either the 32k BASIC ("2") or SCM ("9") as these have the least hardware dependencies. It sounds like you've found the jumper settings, but just check them against this https://rc2014.co.uk/wp-content/uploads/2021/05/RC2014-Pro-24886009-Jumper-Settings.pdf

Things do seem to be pointing towards the SIO/2 chip. I've sent out a few thousand of these over the years and never had a DOA one yet - there's always a first though! Has anything else happened to it, such as being inserted backwards or the module being plugged in off by a pin?

Spencer

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[Pete Baker]

Hi Spencer,

I've tried selecting all of the different ROMs, but yes it's currently set to SCM as per the Jumper Settings PDF (see attached image for my jumper positions).

I inserted the SIO/2 like I did the Z80, ROM and RAM chips - very gradually and carefully, ensuring no bent pins or misalignment.

I'm actually super doubtful that it's the SIO/2, but after all my prodding and poking I'm kind of now at the stage that all I can think of is swapping out chips and seeing what difference it makes, and this seems like the best first candidate.

I've managed to find some Z84C4208PEG / Z84C4208PEC chips online, I'll see if they make any difference. If not, then at least I'll have some spares!

[Phil G]

Whereabouts are you Pete, maybe theres a fellow RC2014 enthusiast nearby?

[Pete Baker]

Hi, I'm in Milton Keynes, and I am fully aware that there was an RC2014 event at TNMoC (of which I am a member / supporter) a couple of weeks ago!

Frustratingly, I was busy that weekend and couldn't attend (really wanted to).

But this does indicate that there might be enthusiasts nearby who might be able to help, or swap a couple of modules out to help narrow down the problem?

Also, currently waiting for my replacement SIO/2 chip, hopefully will be here next week.

Cheers, Pete

[Christer Karlsson]

Hi Pete,

I looked at your picture on the jumper settings.  I have the same ROM (24886009), same clock card, and the same memory, but my jumper for page selection is A13=A14=0 and A15=1. And I use the serial card. It might be worth a try?

To view this discussion, visit https://groups.google.com/d/msgid/rc2014-z80/7d7f1d5a-23af-498e-91d6-b3979bb2aec2n%40googlegroups.com.

[Christer Karlsson]

Forget my post, I had missed you wanted SCM and not CP/M

On Sat, Oct 25, 2025 at 6:45 AM Pete Baker <evo...@elencher.com> wrote:

To view this discussion, visit https://groups.google.com/d/msgid/rc2014-z80/7d7f1d5a-23af-498e-91d6-b3979bb2aec2n%40googlegroups.com.

[Peter Onion]

On Sat, 2025-10-25 at 05:45 -0700, Pete Baker wrote:
> Hi, I'm in Milton Keynes, and I am fully aware that there was an RC2014 event at TNMoC
> (of which I am a member / supporter) a couple of weeks ago!
>
> Frustratingly, I was busy that weekend and couldn't attend (really wanted to).
>
> But this does indicate that there might be enthusiasts nearby who might be able to help,
> or swap a couple of modules out to help narrow down the problem?

Yes there is :-) I'm at the museum most Saturdays (though it will be Sunday next weekend)
and sometimes during the week (Tues or Thurs). I have a working system and test equipment
(and I know how to use it!).

PeterO

[Pete Baker]

My replacements came ... and it's definitely not the SIO/2!

Tried both the older PEG and newer PEC version, and the symptoms are exactly the same.

I didn't think it would be this. I think I've done something daft during the assembly, but damned if I know what that is ...

So yeah ... I'll see you at TNMoC, Peter! Thanks again for the offer of help!

Pete

[Peter Onion]

Ok, I'll see you on the 8th.  I normally arrive at 10:00.  Hopefully we can find the problem with your machine.

PeterO.