Electronics Forum

by **JoshMac** » Sun Dec 23, 2012 5:36 am

Hello everybody,

First of all, I apologise if this topic has been already been discussed, I have searched, however, I haven't found anywhere that gives an answer, most problems were MCU code based. Also thank you in advance for any help given. I have been designing electronic circuits for many years now, however, my knowledge is primarily digitally based, so it doesn't really help me in this situation.

The Circuit:
I am using a Tronix DACIO 300 (fantastic little device) with 2x Parallax potentiometer 2-axis joysticks, they are 10K each axis. The DACIO is feeding the RS232 through a COM port I have them interfaced individually with no problem at all. I haven't written a small VB application to display the information on progress bars on screen. That part was simple and has given me no problems. The joysticks are connected to the DACIO with a 6" piece of cable to ensure there is almost no loss or floating resistance in the cable.

The Problem:
I believe the potentiometers are interfering with each other by creating a load on the power . When Joystick 1 is for example fully forward (0 ohms to the DACIO), it basically nulls the Joystick 2 and visa versa, however individually they work with no problems at all. I think this may be because they are sharing the same 5v rail and sharing the same 0v rail. I am presuming a capacitor across the pot may help? or perhaps a resistor inline with the POT to make it more sensitive? or a transistor with the base controlled by the pot to attempt to isolate the 2 pots?

Thank you in advance and any help is appreciated.
Josh

by **pebe** » Sun Dec 23, 2012 2:50 pm

Hi Josh,

I would be surprised if it were the loading on the pots that is causing the problem. I have looked up a data sheet for the Tronix DAC and it looks like the 5V supply is well regulated and has ample current capability. The two pots constitute a load of 5K so only draw 1mA.

I was surprised to see no spec for the analogue ports, but the input resistance of DACs is usually quite high, usually drawing no more than a few picoamps. If moving the slider of one pot affect the stabilised 5V supply, then it would appear that the inputs are not such a high impedance, after all.

You could insert a microammeter in series with the lead from one of the wipers to see if that is your problem.

by **JoshMac** » Wed Dec 26, 2012 3:10 pm

Hi Pebe,

Thank you for your reply.

I believe the MCU is a PIC16F877A or equ, so I have been using the specs from Microchip for the details on the analogue port.

I have ordered a new MCU from TroniSoft, along with a few spares incase the unit has had some ESD damage, as now the Dacio300 does not power up at all, I get the feeling the unstable analogue input may have been a sign of the MCU kicking the bucket. After changing boards, MAX232, rs232 cable and rs232 port, I still had no luck. ESD is a very real possibility, as careful as I am with ESD, this same board has been used in several products so it may have been affected while being moved around.

The new boards along with a few extra goodies should be arriving soon. So I would like to leave the topic open, come back and update when they have arrived.

If anybody has any further ideas, I would love to hear your opinion and ideas.

Thank you,
Josh

by **JoshMac** » Thu Jan 17, 2013 4:54 am

Hi again,

I have been in contact with TroniSoft, they have recommended that I add a decoupling cap for the pots. I have tried this but sadly, no luck.

I also tried switching the inputs around so to use the MSB and LSB on portA. No change
I have added a decoupling capacitor as you recommended. No change
I have added a 10k resistor in series with both the pots to try and lessen mAh draw. No change
I have replaced the 10k resistors with diodes in an attempt to isolate the pots. No change
I have tested the PCB the joysticks are on seperatly and they are outputing fine.

Could the remaining non-used pins effect the 2 pins I am using?

Any other ideas?

Thanks,
Josh

by **pebe** » Fri Jan 18, 2013 7:47 pm

You are making changes and looking at the result after the DAC10300, the RS232 to USB converter, and your programming for the PC. You need to try to isolate which section has the problem.

Go back and wire the inputs as per your original circuit. Now start with the pots and their input to the DAC (that's what you first suspected). With a digital multimeter, measure the voltage at A0 and set the pot so it measures 2.5V relative to ground. Now vary the other pot from end to end and see if it affects A0 voltage. Now repeat the test but this time measuring changes to A1 instead of A0.

If both tests show no change when you change the pot positions (OK), then you can safely assume the fault is elsewhere.

If OK then you might look at your programming of the DAC. As you are not using the digital ports, then it's best to set them as inputs with pullups. If you set them as outputs you may accidentally short a pin to ground or V+ and damage the port. You have 8 input pins on port A, so it's always good practice to ground unused inputs.

If all looks OK so far then it could be the RS232 to USB converter. Could you bypass it and write a program for the PC to read RS232 direct?

I hope you may find these to be useful pointers.

by **JoshMac** » Tue Jan 22, 2013 1:18 pm

Hi Pepe,

The problem has been established eventually.

I have been speaking to the tech dept at Tronicsoft and after trying to repilicate my problem, they have found there was an issue with the latest release of firmware (v1.4) which when it was recently recompiled for some reason the program has skipped a delay routine between changing inputs in the mux, causing 1 analog input to interfere with the other inputs.

Thanks for all you help getting to this point. I really appriciate the advice,

Thanks,
Josh