KA630 SLU details?

It's a bit of a long shot, I know, but is there anyone here who knows details of the KA630 (MicroVAX-II CPU) console serial line hardware? My emulator is failing selftest 2 and it appears to be because it's expecting the console serial line to act in some way mine does't. But it's not clear from the code what it's expecting, just that it's obviously different from what I've implemented. In particular, what I see in my instruction trace is: - set stuff up, including putting SLU in loopback - drop IPL - take transmitter-ready interrupt - tell it to transmit a NUL - REI - enter wait loop (with countdown timeout) - take transmitter-ready interrupt, long before countdown expires - do nothing that touches the SLU - REI - take transmitter-ready interrupt again immediately - fail the test But I thought real hardware acted this way: don't clear an interrupt condition and it fires immediately upon REI. So there's clearly something I don't understand going on, hence my question.... /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B