11/45 progress

Or alternatively you've got address bus problems too... My aim (in case you've not guessed) is to try and see if you can write even a single bit to a device (or memory) over the Unibus. That sounds like a current loop cable. Do you have a terminal with a 20mA loop input? If you look at the prints for the M7800, you'll see the transmit output of the UART (normally labelled SO -- serial output. I forget the pin number, but it's somewhere in the low 20's) goes to the current loop driver. It also goes to one section of a 1488 RS232 buffer. The RS232 buffer (and the companion 1489 receiver) were not fitted on all versions of the DL11. It's worth checking if your card as them, if not, they can always be fitted (I think all PCBs have the pads and tracks for them), but you may need to fit some extra passives too. If you need to do this. I'll dig out my prints and tell you just what to do. If you do have the RS232 buffer, then you can connect the output of that section (which is routed to a pin on the BERG connector) to the RS232 receive input of a terminal. Right :-) There is one common ground (0V) rail for the entire machine. It is carried over the unibus cables (unlike the power voltages, which are separately supplied to each backplane and device). It's also connected to the chassis metalwork of the machine For high-frequency work, you should pick a ground close to the point you're measuring since the ground wires do have some impedance. But for low frequency tests which this is, you can use any ground in the machine. The best one is the ground pin of the chip you're working on, or a chip near it on the same board, but I suspect just clipping the ground lead of your counter onto the CPU chassis will do. My memory says pin 3, but _please check that_. OK, good. I hope you don't think I was stating the obvious here, it's just that 'silly faults' like that can be a right pain to trace. -tony