Having both SC716 & SC725 modules installed

[Jon Jones]

For reasons, I’d like to have both SC716 & SC725 modules installed in computer.

I’ve made sure the SC716 is at I/O port 0x84 and the SC725 is at its default 0x80. With either module installed, the computer boots into SCM. But with both modules installed nothing happens and the diagnostic LEDs show noting.

Below is the output of the DEVICES command from each configuration:

With SC716:

Small Computer Monitor - RC/Z80
*DEVICES
Supported devices:
= Z80 SIO (rc) @ 80
= Z80 SIO @ 80
= Z80 SIO (rc) @ 84 detected
= Z80 SIO @ 84
= Z80 CTC @ 88
= Z80 CTC @ 8C
= ACIA @ 80
= ACIA @ 40
= CF Card @ 10
= Diagnostic LEDs @ 00 detected
Console devices:
1 = Z80 SIO (rc) @ 84
2 = Z80 SIO (rc) @ 84

With SC725:

Small Computer Monitor - RC/Z80
*DEVICES
Supported devices:
= Z80 SIO (rc) @ 80 detected
= Z80 SIO @ 80
= Z80 SIO (rc) @ 84
= Z80 SIO @ 84
= Z80 CTC @ 88 detected
= Z80 CTC @ 8C
= ACIA @ 80
= ACIA @ 40
= CF Card @ 10
= Diagnostic LEDs @ 00 detected
Console devices:
1 = Z80 SIO (rc) @ 80
2 = Z80 SIO (rc) @ 80


What am I missing?

Thanks.

[Steve Cousins]

Well, I don't know the reason yet, but I've set up a similar system and I'm also having problems.

My system always shows the normal LED sequence at start up but sometimes does not show the start up message on the terminal. Other times it shows the message and a few key presses work, then it hangs. If I remove the CTC from SC725 the system works fine, with all four serial ports functioning.

Replacing SC725 with a SC603 SIO module, also works fine.

It appears my system becomes unstable when the CTC is present on SC725 and a second SIO module (eg. SC716) is present.

I don't have any jumpers fitted on SC716 to connect RX and TX to the bus. All the clock sources for SC725 are set to CLK. The clock source for SC716 is the onboard oscillator. I have not created an IEI/IEO interrupt daisy chain. There shouldn't be an address conflict.

I'll have to think on this.

Steve

[Jon Jones]

I’ve just removed the CTC chip and it now boots!

Small Computer Monitor - RC/Z80
*

*DEVICES
Supported devices:
= Z80 SIO (rc) @ 80 detected
= Z80 SIO @ 80

= Z80 SIO (rc) @ 84 detected
= Z80 SIO @ 84
= Z80 CTC @ 88
= Z80 CTC @ 8C
= ACIA @ 80
= ACIA @ 40
= CF Card @ 10
= Diagnostic LEDs @ 00 detected

Console devices: 1 = Z80 SIO (rc) @ 80
2 = Z80 SIO (rc) @ 80

3 = Z80 SIO (rc) @ 84
4 = Z80 SIO (rc) @ 84

I am seeing something “interesting” around the ordering of the modules in the backplane - but I need to do more testing before jumping to any conclusions. (It could just be a dodgy solder joint for one thing)

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[Steve Cousins]

I've had another look at this issue. I'm no longer experiencing the instability problem I described earlier. I think this might have been due to the clock module I was using to generate my system CLK and CLK2 signals, but I'm not sure.

I now have SC725 configured with JP4 set to the main bus clock CLK, JP5 set to C (which JP4 selects as CLK), JP6 set to CTC. In this configuration port A is fixed at 115200 baud from the main system clock CLK, while port B is controlled from CTC channel 1. SCM defaults to setting the CTC channel 1 and SIO port B for 115200.

SCM does not control port A with the CTC as the original SIO+CTC module (SC110) was hardwired to use the CLK signal for SIO port A for compatibility with standard RC2014 firmware that does not know anything about CTCs. Similarly, the standard configurations of RomWBW does not set up the CTC.

When a second SIO module is fitted there is a slight problem. SCM mistakenly assumes that if it detects a CTC at address 0x80 all serial ports, other than the first one, use a CTC. It therefore programs the SIO on the assumption the serial clock has a CTC as a pre-scaler. This results in a baud rate for both serial ports on the second SIO module (which does not have a CTC) being 460800.

Other than that all four serial ports are working with both SCM and RomWBW. Note with RomWBW one of the jumpers needs to be changed.

I've improved the documentation on my website for SC725.

My test systems is:
SC706 - Z80 CPU and 7.3728 MHz clock
SC608 - Memory module with both SCM and RomWBW plus 512kB RAM
SC725 - SIO+CTC module (address 0x80 and 0x88)
SC716 - SIO module (address 0x84)
SC129 - Digital I/O for diagnostic LEDs (address 0x00)

Steve

On Saturday, 4 October 2025 at 19:38:33 UTC+1 gordont...@gmail.com wrote:

[Steve Cousins]

Oops, that's SC602 not SC608