Tuesday, June 19, 2018

Cleaning up board, identifying bad USB module


To rule out a problem with my USB daughtercard I ordered another and expect to have it by Thursday morning. I will make sure that the software link to the PC works before I make any changes or insert it on my main board, thus ensuring that Windows, the board and my reconfiguration of the device to support 45 baud operation are all good.

I was going over my main board checking components before doing signal continuity tests when I discovered two diodes that were not connected in spite of the solder blobs left by the reflow. I decided to resolder all the points manually, which brought those components back to proper readings.

The board works differently now, as the small power LED does not light any more when I plug in the device. Checking other components showed me that quite a few of the capacitors were shorted underneath. I suspect that I have a few suspect soldering connections and will therefore remove each such component with my rework tool and hand solder them back after cleaning up any solder bridges on the board. This is tedious work, but important.

The green light is back on, but the USB board is still stuck with the red RX led illuminated and it won't respond to the PC in order to load drivers. By Thursday I will have a definitive answer about this board, but the first step is to remove it from the main PCB and see if it will chat by itself. I need to remove it anyway in order to install the new USB card.

Looking over the schematic, I see that the 5V from the USB link is only delivered to the main board when the power switch is on and the USB driver board has indicated it is out of suspend mode. As the board won't syn up with a PC, it is suspended. The PCB has a current limiting chip that has an enable pin hooked to the USB module, only allowing the 5V power to flow to the rest of the circuit when the USB module is not suspended.

This means I should not have the green power LED illuminated, especially if the main power switch is turned off. But it glows! I suspected that the RX lamp is lit on the USB module and is driving some path that reaches our power LED. I then checked the schematic and tested for continuity and shorts in the relevant areas.

Turns out the RX side of the USB daughter card is connected to a 1K resistor to ground and through a pushbutton to some other circuitry. Opening the pushbutton does not extinguish the power LED, nor is there a path from the RX signal over to the power LED at all. That means I have some other sneak circuit that delivers some voltage to the central power point of the board (and thus to the power LED), but not the 5V supply.

It has to be one of the other connected lines, TX, RTS/CTS,  or /SUSPEND. If TX were delivering power it would also light up the Data LED, so it is probably not the problem. Still, I checked for continuity and shorts for all the lines that might deliver power to the central bus (other than the intended path of VCC through the current limiter and on/off switch).

There is a path from the TX line to the input of an inverter, which in turn drives an enable pin on the charging control chip. If the inverter is blown out somehow it could provide a sneak path, from the input to the VCC pin, but I see infinite resistance with my VOM.

In any case I did a couple of experiments removing parts temporarily. With the 1K resistor lifted from the RX circuit and the Break button pushed, the RX LED on the USB board was extinguished. It came on with the Break button released, just as it goes on when the 1K resistor is reinserted. The voltage of this line is about 0.9V, which is what is present on the main bus and driving the green Power LED dimly.

I removed the inverter chip that might have leaked, even though the TX line is at 0V. Of course, the dim Power LED remained on. The measured voltage on the main bus is 0.9, exactly the same as the RX line. If the TX line were to rise to 1V or higher, the Data LED would illuminate, but it did not.

The path from RTS/CTS runs to the motor control relay chip pin but the other side of the input line is isolated by the on/off switch. I don't think this is the problem either.

The RX line passing through the Break pushbutton goes to the solid state relay chip (CPC1510G) U6 that will close a circuit to connect the RX pin to the main bus (+5V). This is controlled by the flow of 24V from the buck-boost chip, through the keyboard jack and a resistor. If there is enough flow, it will trigger the relay and connect RX to the main bus. This must be happening.

The 24 volts from the buck-boost chip shouldn't be on, since the main bus is off when we first plug in. The buck-boost takes the main bus voltage as input to produce the 24V output. There are two possibilities here. First, the relay chip could be bad, although my VOM shows it open in the idle state. Second, I might have a undetected short to some other line that produces current for the relay. Third, the buck-boost might be working anyway.

To eliminate the third chance, I inserted a dummy plug into the keyboard jack, opening the circuit. This should ensure that the relay doesn't close and pass RX at 0.9V to the main bus. Even with the removal of the relay U6 I didn't drop the main bus voltage, so if I don't have a hidden short then this isn't the cause.

I do have 3.3V coming from the RTS/CTS pins of the USB board, which would fire the motor control solid state relay U7 if the other side were connected through the on/off switch to the chips VCC. The VCC line goes through a 1.8V Zener diode and resistor, through the LED inside the photorelay and back to the RTS/CTS line..

That should cause the middle (Motor) LED to light in addition to the power one, since they would have the same main bus voltate in each case. It is not, which makes me suspect that the motor relay is inverted, it 'fires' when RTS/CTS is at ground level, turning on the motor control relay and LED, then when CTS/RTS goes high to 3.3V, it switches off. This doesn't make sense compared to the description of the circuit operation.

As long as the USB chip is not talking nice to PCs and not raising the /SUSPEND signal, the rest of the board won't be working. At this point, with my new USB module arriving tomorrow, I decided to remove the existing module to see if it talks to the PC when out of circuit.

To heap a bit of extra annoyance on the situation, Windows has the delightful practice of shutting down the USB ports for a device after a few Device Malfunctioned (error 43) situations, requiring a power down and reboot to clear it. After I completed this unnecessary time sucking task, I plugged in the CP2102 board by itself

Still malfunctioning and not recognized by Windows. Good thing I have the replacement USB board on its way. Thia was what I suspected but this proves the case. I need a functional USB module to make the main board work. 

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