Philip BraggJun 3
The third item of test equipment from the rally was this Farnell 30V 2A GPIB programmable PSU for a fiver. Earlier tests showed the output was hunting around a bit when lightly loaded, this turned out to be a missing sense line shorting strap on the back panel. Just needs feet and to figure out GPIB commands. Do you know them?
Show threadPhilip BraggJun 3
Send it any character and the GPIB responds with lots of ASCII 1s and 0s. The pattern is different if the output switch on the front panel is switched between On and Off. I haven't managed to guess the commands to set a voltage or current limit yet. I've tried the obvious characters already.
Show threadPhilip BraggJun 4
Update:
The IC that I'd assumed was a microcontroller is a dedicated IEEE488 controller from Philips, HEF4738VP. It deals with the GPIB bus prototol can delivers (or consumes) a series of bytes from a simple 8 bit bus in the instrument. This takes the form of a counter, some gates and a 1-of-16 decoder, some more gates, 6 4-bit latches and two 3.5 digit BCD programmable potentiometers that have their MSD tied to a logic 1, so they're only 3 digit. All I need to do is figure out how the logic mangles an incoming stream of bytes in order to fill all the latches. There is no readback - the section of the PCB where this could be implemented is entirely unpopulated, so it's write only - which is curious as it's emitting 1s and 0s randomly when addressed on the GPIB. Every IC is in a cheap socket, which might make it easier…
Show threadPhilip BraggJun 4
It is one of those pretty translucent boards
Show threadPhilip BraggJun 4
There is, in fact, one bit of readback. I'm not sure what it means yet but most likely it is an indication if the PSU is operating in CC or CV mode.
Show threadPhilip Bragg
The logic was giving me a headache so I identified the key parts of the state machine, and how the digits were clocked through to the DACs which then enabled me to monitor a specific line while typing letters and numbers at it from a terminal session. The programming string for voltage is VxxxP where xxx is a between 000 and 999. 999 sets the output to 30 volts.
Jun 5 at 11:18amWeb
Show threadPhilip BraggJun 5
Programming current is similar, IxxxP where 000 is 0mA and 999 is the full current capability, a little over 2A
Show threadPhilip BraggJun 5
Typically I've been distracted and now though I know those two commands I also now know that there must be another command because from a cold start I can't get it to do anything!
Show threadPhilip BraggJun 5
Scrollback to the rescue, sort of, I pasted the nonsense I'd been typing to brute force it and now it's working again. One of the characters or perhaps more than one, in that mess is the key!
Show threadPhilip BraggJun 5
Readback bit is 0 for constant voltage and 1 for constant current mode.
Show threadPhilip BraggJun 5
Looks like I have made an error and the P isn't required, but I know P causes the state machine to reset so it must be important. Hmmm
Show threadPhilip BraggJun 5
The missing link is C. I still don't know why. P does a reset, so it's best to start with that, I suppose. A complete string would be PV999I999C
That programs the supply from cold (it has no NVRAM so it doesn't remember settings)
Show threadPhilip BraggJun 5
As well as B there is C, which is the opposite of B. Maybe break/connect? Unsure, anyway, C turns on and B turns off. Neither really does a disconnect as there doesn't seem to be a relay in there, it just drops to 0V
Show threadPhilip BraggJun 5
S and R have a similar effect, set and reset? Probably just the same LSBs.