16C550 Twin Serial Module Issues

[Martin Sidgreaves]

Hi all, I’m looking for a little advice with the 16C550 Twin Serial Module. I’ve been struggling to get it working and after several days of ripping my hair out, I have reluctantly admitted defeat and decided to post here for the first time.

So I have the RC2014 Pro with numerous modules including the 512k Module and am running RomWBW (3.6.0). Until now I’ve not experienced any real problems with configurations etc and have been able to figure out the issues I have experienced without too much drama.

Recently I bought a 16C550 Twin Serial Module, assembled it as per the instructions on the site and set the jumpers to A0/A8 as suggested. Initially I was planning to leave the SIO2 board in place and believed it would work happily with the Dual Serial board set A0/A8. When I booted and hit reset I saw nothing. Blank screen. It refused to boot until I put the jumpers down at E0/E8. It booted but wasn’t recognised at all by RomWBW (which didn’t surprise me as I figured it wasn’t built with those settings enabled).

So I rebuilt RomWBW, I set the UART settings as suggested on the Custom RomWBW build section on the product page and changed the ports to match E0/E8. Still wouldn’t boot. That started a mad cycle of changing the config/jumpers, blowing the ROM, refitting it, booting, resetting and despair!!

I removed all modules apart from the CPU, ROM/RAM board, Clock and have tried with and without the SIO2 board. So a minimal config to reduce the chance of interference from other modules. Regardless of what tried, still got nothing.

I then got the oscilloscope out and started probing the board. From what I’ve seen, the ICs seem to be working fine. There’s pulses and activity but no / or very limited output on the output pins.

Has anyone else come across issues with this module? I’ve checked it electrically and it seems ok. I’ve checked the obvious such as bridges, component orientation etc. I’ve even reflowed the solder joints.

Any help or advice on what to try next is much appreciated. Thank you in advance

Martin

 

[Wayne Warthen]

Hi Martin,

I happen to be testing that exact setup right now.

I suggest using the pre-built RomWBW ROM as-is and strapping the Dual 16C550 UART for A0/A8.  The SIO can be left strapped at it's default of 80/82.  This combination is a good known working setup.

When you put a 16C550 UART in RomWBW it will take priority over the SIO, so the initial console will be routed to the primary port of the Dual 16C550 UART if RomWBW detects it.  So, are you connecting your serial dongle to the primary port of the Dual 16C550 UART?  If left connected to the SIO port, you will get a blank screen.

Thanks, Wayne

[Martin Sidgreaves]

Thank you for that Wayne, I tried with the minimal setup (CPU, Clock, Rom/RAM, SIO – all standard ports presumed) and popped the 16C550 back in (A0/A8) and after blowing the pre-built ROM back on….. still no bananas unfortunately. I tried all four of the serial ports, I tried with the SIO, without the SIO and the only life I saw was the attached screenshot when I had the adapter plugged into PORT B, on both cards. Other than that it was just a flashing cursor and no boot.

Seems I have got a hardware issue however, I’ve gone over the board so many times and can’t find a problem…. I shall persevere! In the meantime, I have at least got the system working with the standard SIO.

Thanks again for your help. I really appreciate it.

Best wishes

Martin

[Spencer]

Hi Martin,

With the pre-built ROM it works with the SIO/2 Module?  Do you get any POST messages on the SIO/2 with the Twin UART plugged in? If so can you post them here.

Also, if you just get a blank screen with a flashing cursor, try double-tapping the Space Bar. Then see if you can get any response. If so, it is booting with a different serial port as the main one.

Spencer 

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[Wayne Warthen]

Hi Martin,

The recurring BAD INT messages indicate that something is asserting the /INT line.  When RomWBW sees /INT asserted, it checks all the devices that could cause an interrupt.  If none of them indicate they own the interrupt, then RomWBW diagnoses a "bad interrupt".  Your mission is to figure out what is causing /INT to be held asserted.

Since everything works fine until the UART board is inserted, that board is the likely culprit.

Thanks, Wayne

[Martin Sidgreaves]

Thanks chaps.

Spencer - Yeah, tried all that with no luck. I didn't even get a boot screen however I think that something was going on in the background as the LEDS on the Digitial IO (when it was plugged in) were dancing away when I hit reset. Just wasnt' seeing anything on the console.

Wayne - I've got a Saturday, Oscilloscope and several cups of coffee ready to go! I've thought all along that it's a problem on the board itself. I've got replacement ICs which I may try too. I'll report back at some point. 

Thank you both for your help

[Wayne Warthen]

On Wednesday, January 28, 2026 at 11:20:43 AM UTC-8 Martin Sidgreaves wrote:

Wayne - I've got a Saturday, Oscilloscope and several cups of coffee ready to go! I've thought all along that it's a problem on the board itself. I've got replacement ICs which I may try too. I'll report back at some point. 

That is the way!  Note that the RomWBW UART driver does not enable interrupts nor does it rely upon them (unless you generate a custom build that does so).  You could lift pin 3 of U2 to prevent the board from driving /INT on the bus.

Good luck, Wayne