works to start the models, though FIFO status light is still red.  There is also still an offset on DAC outputs, but not the same as the previously noted offset (-740 mV, down from -2.2 V).

The RCG runtime diagnostics guide (p18) suggests this bit indicates either FIFO is empty (DAC module clocking more often than it should, or IOP code running too long), or FIFO contains too many samples (DC not clocking properly or at all).  It also suggests this error should manifest as an "errant, noisy, DAC output or... complete loss of DAC output signal," which doesn't sound like the fixed offset I'm seeing. Nonetheless, the IOP shouldn't be running long (we've run these models together before), so I check the timing cables to DAC and ADC.

The ADC (but not DAC) haphazardly connected (attachment 1 is the ADC cable). After reseating the DAC cable and rebooting cymac1, the FIFO error is resolved but I no longer get any signal when driving the DAC. I don't see any change in status if I unplug the DAC, so perhaps there just isn't an indicator for 'DAC is unplugged.'

Reseating the ADC cables caused the FE to crash (predictably), but restarting the FE with rtcds restart --all (or stop followed by start) left timing errors. I tried to rmmod the x1iop kernel after stopping the models, but rmmod was not a recognized command. I installed kmod (the package containing rmmod), but the command was still not recognized; when I uninstalled kmod, several rts packages were also uninstalled (odd, since kmod couldn't have been a dependency before it was installed). Fortunately, simply reinstalling advligorts-cymac recovered the state with FIFO status light red. Not sure why the FIFO error returned, but I re-reseated the cables through this so maybe it is just flaky connectors. 

In the end, I left the system as it was before, but mounted the ADC+timing breakout board inside cymac1 next to the DAC+timing board. (attachment 2).

I also checked that the timing signals looked OK (attachment 3 shows the DAC clock on trace 1, ADC clock on trace 2; attachment 4 is zoomed in near 0V, where you can see a slight difference in decay to 0 on the two clocks).