INFO: SCC's S8 rom and CTC+SIO can't using port A for CTC
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Joel Burton
Jan 8, 2026, 7:29:53 PM (3 days ago) Jan 8
to retro-comp
I don't have a question; I've solved this already. Posting this in case someone searches these terms.
Problem: my SCC CTC+SIO card didn't work when jumpered to use-CTC-for-port A.
It worked fine when port A directly used a clock, but I wanted to have software-adjustable baud rates (I have a real terminal that requires specific rates).
Exploring, I found that it worked fine for CTC-for-port-B ("baud $2 $96", "console 2" switched to using port B at 9600 baud).
I assumed this was a hardware problem and scoured the board for bad soldering and swapped out a CTC and SIO/2 for known good ones; none of this changed anything.
Digging into the software (SCMonitor/Source/Config/config_S8_SC150_151.asm), I found that it disallows SIO+CTC for port A, apparently as a compatibility thing for SC110. I don't have a SC110 (I'm using the later RCBus-design), so not sure what the problem was/is about.
However, commenting out this line in the that file, assembling, and reflashing the chip works:
The file says that this line was needed for SC110 port A to work correctly, but on my later board, it works now using the CTC.
Anyway, nothing to see here. Just a note to a future person to save them some hardware debugging.
Problem: my SCC CTC+SIO card didn't work when jumpered to use-CTC-for-port A.
It worked fine when port A directly used a clock, but I wanted to have software-adjustable baud rates (I have a real terminal that requires specific rates).
Exploring, I found that it worked fine for CTC-for-port-B ("baud $2 $96", "console 2" switched to using port B at 9600 baud).
I assumed this was a hardware problem and scoured the board for bad soldering and swapped out a CTC and SIO/2 for known good ones; none of this changed anything.
Digging into the software (SCMonitor/Source/Config/config_S8_SC150_151.asm), I found that it disallows SIO+CTC for port A, apparently as a compatibility thing for SC110. I don't have a SC110 (I'm using the later RCBus-design), so not sure what the problem was/is about.
However, commenting out this line in the that file, assembling, and reflashing the chip works:
kSIO1ACTC: .SET 0 ;Port A's CTC register (0 if n/a)
The file says that this line was needed for SC110 port A to work correctly, but on my later board, it works now using the CTC.
Anyway, nothing to see here. Just a note to a future person to save them some hardware debugging.
Joel Burton
Jan 8, 2026, 7:35:24 PM (3 days ago) Jan 8
to retro-comp
p.s. I've been avoiding digging into the assembly for the monitor/firmware for these boards since I'm not a Windows user and the Workshop software is Windows-only. However, necessity here got me over that, and I'm happy to say that it works fine running under WINE with desktop Linux.
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Steve Cousins
Jan 10, 2026, 3:51:47 PM (yesterday) Jan 10
to retro-comp
You are right in determining this is due to compatibility issues.
When I developed the earlier SC110 module I hardwired the port A clock source to 7.3728MHz. This was done to ensure the default comms port (A) of the module would work with all 'standard' RC2014 firmware and software. SCM also assumes the port A clock source is 7.3728MHz so that it works with the RC2014 SIO serial module.
The later SC725 module provides a jumper for the port A clock source. However, the default assumption is that the clock source is 7.3728MHz, again for software compatibility. Again, SCM also assumes the port A clock source is 7.3728MHz and thus works with the RC2014 SIO serial module.
SCM could probably be smarter and work out if the CTC is generating the port A clock, but currently this is not the case. To generate 115200 baud serial with a CTC in the loop, the SIO has to be programmed differently to when there is no CTC in the loop.
The SC725 page on my website attempts to explain the limitations of the various configurations of SCM.
CP/M and other software would also have to be CTC aware to handle this issue. The easy solution for maximum compatibility is to assume the port A clock source is 7.3728MHz. Thus the recommended jumper setting.
Port B use is generally optional so the same assumption is not essential.
Steve
When I developed the earlier SC110 module I hardwired the port A clock source to 7.3728MHz. This was done to ensure the default comms port (A) of the module would work with all 'standard' RC2014 firmware and software. SCM also assumes the port A clock source is 7.3728MHz so that it works with the RC2014 SIO serial module.
The later SC725 module provides a jumper for the port A clock source. However, the default assumption is that the clock source is 7.3728MHz, again for software compatibility. Again, SCM also assumes the port A clock source is 7.3728MHz and thus works with the RC2014 SIO serial module.
SCM could probably be smarter and work out if the CTC is generating the port A clock, but currently this is not the case. To generate 115200 baud serial with a CTC in the loop, the SIO has to be programmed differently to when there is no CTC in the loop.
The SC725 page on my website attempts to explain the limitations of the various configurations of SCM.
CP/M and other software would also have to be CTC aware to handle this issue. The easy solution for maximum compatibility is to assume the port A clock source is 7.3728MHz. Thus the recommended jumper setting.
Port B use is generally optional so the same assumption is not essential.
Steve
Joel Burton
Jan 10, 2026, 10:23:28 PM (20 hours ago) Jan 10
to Steve Cousins, retro-comp
Thanks for the info, Steve.
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