PIC 16F877a USART problem

Good morning Ladies & Gents,

I have a slight issue that has been troubling me for a few days. I have a PIC 16f877a which is part of my data logging system, this records data and sends it to a RPi. Currently the entire system is powered by a voltage regulator circuit, this has proven capable of supplying the PIC and associated circuits with the correct ammount of power. The only problem is the transmission works when the PIC is supplied by USB but not the battery. Everything else on the data logger works when on battery power except the transmit function. I can't have a split power supply as the potential difference between the two circuits causes problems.

Can anyone shed some light on this. I am using the development board from hobbycomponents.com

Thanks in advance

Are you saying you use a serial to USB converter module or you are just using the power available at the USB socket to power a direct serial link?
Without more information I'm guessing the problem is the module uses a supply derived from the USB port which you are not replicating when on battery power.

Brian.

The development board has several connectors. I am just using the USB as a means to power it. I have a ICSP connector for connecting a PICKIT and the battery is plugged into the 5mm DC socket. I can't think what the difference between the USB and DC socket are? They are both supplying the same voltage.

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I hope this helps

I looked at their web site but they don't show a schematic. I have purchased from them before and they are very good at shipping and their prices are excellent but they have no technical support.

Another question: the board has a USB interface and a standard serial interface, is the USB socket used only as a power input (D+/D- unconected) or does it have a USB serial interface on-board as well? I'm guessing the small IC a MAX232 or MAX3232, am I correct?

Brian.

This is the circuit schematic. Shows both the 5mm and USB at bottom left of the diagram

D-D+ both disconnected

Ive just changed the voltage regulator from a LF50CP to a 7805 and still no change

They are directly joined so electrically both sources are identical and should work equally. I assume SW1 just links VCC to the fuse and doesn't connect anywhere else.

It's experimenting time!
Can you measure the voltages on U2 (the MAX232) pins 2, 6, 13, 14 and 16 then report them to me. I'm trying to establish whether the PIC or the interface is responsible for the transmission failure.

Brian.

Pins:
2 6.49V
6 -6.88V
13 0V
14 -6.88V
16 4.95V

I have tried using another PIC but I get the same results. One thing I have just noticed (was too bright in my office earlier) With the battery connected, but the board turned off. L9 flashes in a periodic fashion.

Thanks for your help on this so far, its very much appreciated.