Raspberry Pi Forums

Hi everyone,

A thermal printer I had connected to serial TXD and GND stopped working all of a sudden, so after a few hours of pulling my hair out I spotted this on the back of the PCB (rev1 board):
I'm not too knowledgeable with the electrical aspect of things, but I spotted this on page 3 of the schematics document:
So if I understand this correctly this is simply a resistor going to an Ethernet LED and it just means no more light shows, but could this have any other effects? Or perhaps something else blew and I can't see it..

It is worth noting the printer is fine, I swapped this Raspberry Pi out with another one I have, used the same connections, same SD card ...etc and it worked as expected. Also, the Raspberry Pis both sit comfortably inside cases so this couldn't have been physically damaged by accident.

This damaged one still boots, runs XBMC, plays media.

Maybe the resistor and the serial connection issue aren't related, I have no clue. Any thoughts?

I would agree with you; you wont get a 100M LED any more, but apart from that there should be no effects.

Unless the resistor is shorting to the case, I'd say you need to look elsewhere. Either the board got very hot recently, or that resistor has always been standing up. It's known as "tomb-stoning" and is a well-known problem during manufacture, but it is highly unlikely to happen at any other time. It is caused by the surface tension of the solder, and wont occur unless the solder is molten.

Thanks for the info. The resistor does look slightly warped, perhaps that confirms heat was an issue. I tried going over the schematics to understand where the serial TXD GPIO pin goes to in an effort to see if there is any physical damage (at least visually) but that is a bit beyond my understanding.

Besides running code or say connecting an LED to TXD while sending data (if that is a sensible way of testing), what other methods are recommended to do an all encompassing diagnostic of the RPi?

Cheers

rurwin wrote:I would agree with you; you wont get a 100M LED any more, but apart from that there should be no effects.

Unless the resistor is shorting to the case, I'd say you need to look elsewhere. Either the board got very hot recently, or that resistor has always been standing up. It's known as "tomb-stoning" and is a well-known problem during manufacture, but it is highly unlikely to happen at any other time. It is caused by the surface tension of the solder, and wont occur unless the solder is molten.

An easy test would be connecting TXD 2 RXD. If you open a terminal program, everything you type should appear on the screen as the characters sent out are also coming in again. Some things like correct baudrate and data format settings aren't checked like that.
What interface did you use to connect your Thermal printer to the TXD line? Is the printer interface V24 or TTL? If it's TTL, are you sure it's 3V3 tolerant? Most older designs expect 5V.

Hi obcd,

The printer is the following from Adafruit: http://adafruit.com/products/597. Quote from the product page: "you simply need a 3.3V-5V TTL serial output from your microcontroller".

I reverted settings the instructions had called for to free up the serial port for software use (editing inittab and cmdline.txt) and connected the pins, unfortunately I don't see what you describe. Note this was over SSH but that shouldn't matter correct?

Thanks

obcd wrote:An easy test would be connecting TXD 2 RXD. If you open a terminal program, everything you type should appear on the screen as the characters sent out are also coming in again. Some things like correct baudrate and data format settings aren't checked like that.
What interface did you use to connect your Thermal printer to the TXD line? Is the printer interface V24 or TTL? If it's TTL, are you sure it's 3V3 tolerant? Most older designs expect 5V.

You shouldn't revert the settings. You should leave the serial port as general purpose serial port.
If you use a terminal program like minicomm, every character you type is sent out on the serial port, and every character that is received on the serial port is printed on the terminal window. So, if you connect TXD and RXD, a key is sent out on TXD and received at the same time on RXD. So basically, what you type should appear on the screen.

I've tested what you mention under both options above (original and modified files) and unfortunately it doesn't look like anything serial is working

I'm all out of thoughts on this one to be honest..

I can't figure out the schematics either to understand where the serial send/receive pins go on the board to take a closer look.

Thanks for your help so far!

The TXD and RXD lines are gpio lines.
They simply run to the Broadcom SOC which is a micro bga chip.
There isn't much you can do hardware wise.
You could measure the TXD voltage. It should be approx. 3V or higher.
If you dump a file to that serial port, you should see it lowering 0.1 - 0.5V as an indication that data is being transmitted. For better analyses, you need a scope.
Are you sure you have all handshaking turned off?
Hardware handshaking can prevent the system to start outputting it's data.