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Old 08-04-2020, 09:50 PM   #1
PinGuy
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Default Bench testing this CPU board

Howdy guys, I am trying to diagnose a Lunar DPX-SF bone densitometer.
The original issue was that the Reset button was pushed by mistake while the machine was doing its "calibration" and from that point on, the software is not connecting with the machine anymore.
There is an executable which should show the firmware version of the equipment, when run it shows "Invalid response from the SBC-03". If the serial cable is disconnected it takes a while and then says "No response from the SBC-03". So something got corrupted, I don't know if the 6264 RAM could be damaged, or this is a serial comm interface problem. The PC's serial port was tested ok, just in case (tried another PC as well).
I got the CPU board out of the machine and took it to my bench, so I don't have the full machine here, cable harness, nor schematics unfortunately.
I supplied +5, +12 and -12V to power the board, at their respective test points. The problem is that I always get "No response from the SBC-03" on the PC, so I can't continue diagnosing the original issue.
I see a MAX250 and MAX251 combo for serial comms on the mobo. I see ISO +V, ISO -V and ISO GND points in the area, and they go to pins 2, 13 and 14 of the MAX251 respectively. I think I am missing that voltage because of some missing connectors from the main equipment. The question is: is it OK to plug in another 5V PSU in there, to have the serial comms working?
I attach a pic for reference. TIA
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File Type: jpg CPU.jpg (968.0 KB, 16 views)
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Old 08-05-2020, 06:43 AM   #2
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Default Re: Bench testing this CPU board

first move is to dump the eprom and eeprom, and put a sticker over the eprom window.
as for the isolated port, you really need a wiring diagram of the machine to be doing this type of work.
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Old 08-05-2020, 09:58 AM   #3
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Default Re: Bench testing this CPU board

Hey stj, thanks!
Got some progress, my 5V supply was falling unexpectedly
So having that solved, I got the "illegal response" message so I'm back to the original issue.
I started by dumping the EPROM (has the sticker, it just unglued before taking the photo hehe), and also the E2PROM, the contents are fine, no corruption there. Changed the socketed 6264, no difference. There's still another 6264 non-socketed...
Now I can't find much info about this MAX250 & 251 configuration. How to diagnose such circuit? And if failed, what can be a good approach to replace it?
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Old 08-05-2020, 10:37 AM   #4
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Default Re: Bench testing this CPU board

the serial port(s) are clearly opto-isolated from the board,
so the serial cable has to be powering the interface.
if you arent powering that circuitry then it wont work.
also, it's obviously a custom cable with a psu and the other end
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Old 08-05-2020, 11:18 AM   #5
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Default Re: Bench testing this CPU board

Quote:
Originally Posted by stj View Post
so the serial cable has to be powering the interface.
The serial cable? The other end goes to the PC, so you say the PC is powering the MAX250/251 arrangement?
I started scoping it, and clearly as it is, the board is not booting, I have no movement in the address/data lines of the uP at all.
I'm wonder if the missing J4/J5 connections from the machine could prevent the CPU from booting..
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Old 08-05-2020, 01:01 PM   #6
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Default Re: Bench testing this CPU board

Erm, no. The J4/J5 are optic sensors for motor positioning, nothing to do with uP startup.
The clock signal is there, but there's definitely no data line "movement". The reset line goes high properly, so this is not a "stuck reset" situation.
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Old 08-05-2020, 09:37 PM   #7
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Default Re: Bench testing this CPU board

My last statement was wrong, there IS a reset problem. Reset is active high for the uP, and I'm getting 3.8V on it right from power on. While pressing the reset button, it goes to 5V. So something is keeping the reset line high at 3.8V. I then checked the reset IC (DS1232) and to my surprise, pin 6 (!RST) is always low, and pin 5 (RST) blinks on and off continually (0-5V), about once per second.
Does this mean this IC is bad? I understand RST and !RST should be opposite, so at least both should be blinking, right? I added a socket to it, and left pins 5 and 6 on the air: same result: just pin 5 blinks on and off, pin 6 is always 0.
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Old 08-06-2020, 02:27 AM   #8
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Default Re: Bench testing this CPU board

! means inverted,
so pins 5 & 6 should always be oposits of each other.
now you pulled the chip, try metering the resistance between holes 5&6 and the power rails - incase there is a short on the board.
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Old 08-06-2020, 09:52 PM   #9
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Talking Re: Bench testing this CPU board

Well, today I did some more testing and drawn a small schematic of the Reset section, to understand it entirely, finally.
It is meant to reset on and off constantly by design, due to the watchdog not receiving any keep-alive pulses from the MPU. The Data Terminal Ready signal from the serial port is also used as a Reset, by the OR gate at U47, so when DTR is up, the board boots and starts listening in, then again goes to the loop-reset state once the port is closed.
The problem was actually on the RXD signal, so the serial port was "deaf". The MAX251 was the culprit.


The strange thing is that the RST and !RST signals of the DS1232 are not actually opposites, at least during the reset loop !RST is always 0. That got me a bit confused. !RST is unused on this board anyway
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Old 08-07-2020, 07:59 AM   #10
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Default Re: Bench testing this CPU board

strange,
also a surprise that it has a watchdog,
i'm used to them on arcade equipment but didnt expect it on something like that!
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