Melzi board question

Hi,

I have a tricolour mendel using the two typical master-slave Melzi boards, and was thinking on adding a lcd to it. I've searched all around and seems to be quite difficult to achieve this because of the lack of aditional gpio for the task on the Melzi. I was wondering if I could use the serial interface that normally drives the usb on the slave melzi, because the usb its not used there. I can work around firmware dev but I have little to none experience on soldering .

Can anyone help me with this questions:
- is this actually possible?
- I imagine that is not as easy as getting rid of miniusb and use DM/DP as rx/tx is it? Can anyone give me just some orientation on what should I alter on the slave board?

EDIT:
Looking at schematics I realized how retarded my DM/DP question must sound .... think its the ATMEGA1284P AOFI and AIFO pins I'm looking for, no?

ps: sorry my english

Thanks and best regards,
MSB

Edited 3 time(s). Last edit at 10/18/2013 08:30AM by msb.

Taking in account what I need to connect the lcd, this is what I was thinking (with my limited knowledge on electronics ):


Any sugestions/warnings before I totally mess up my board?

Thanks

Edited 1 time(s). Last edit at 10/18/2013 08:42AM by msb.

I found this in my notes:

Melzi + Raspberry Pi combo (LCD)
[forums.reprap.org]

Thank you A2 for the link.... search is a never ending story

RPi is an interesting approach, I've already thought going that way. But before that, I was trying to simplify setup, using the Melzis I already have... not yet sure if going to a dead end.

I can not think of any reason why what you suggested won't work. I mean you will probably need to disconnect FTDI's TX connection to uC's RX, if you need to send data from LCD module back, since FTDI TX will be pulling the line up, what you connect there could destroy TX on FTDI or on your LCD module. If you are using the marlin master/slave firmware's like myself, you will be doing this modification on the slave board right? So you will have to transfer what data you want to put up on the LCD from master marlin to the slave one. Then push that info to LCD module on the slave right? In that case, why not tap into the master to slave cable and listen for your own codes on LCD module, and modify the master firmware to send them through the slave connection, and modify the slave firmware to ignore those unknown LCD commands (if it's not doing that already), and do no soldering at all?

What do you plan to show on the LCD?

I'm planning to make the printer as much independent as possible from the pc connection, but without loosing the possibility of connecting to a PC if I want to. So yes, I was planning to do this on slave, allowing the master to connect to pc as it is. Implement commands like move axys, change/check temp, sdcard print...

Thanks for the advice on FTDI, I was forgetting that ... and your suggestion about tapping the master-slave connection already came to my mind but I thought that it would interfere with the master-slave communication (signal lost for example). It is much simpler indeed, do you think it would work hardware wise?
Changing the firmware would be simpler, I could had some kind of tags to differentiate the slave<->master vs lcd<->master. Actually it would probably be simpler than changing the firmware to make the master receive commands from the slave (talk about schizophrenia ).

Thank you very much ozgur.

regards

I didn't have the time to get to the firmware and the electronics just yet, my setup have been up and running just 2 weeks now. So i'm not experienced with Melzi boards, interface specs, but in general i think it will work hardware wise, i mean, if you happen to have some problems you could shorten the connection wires, lower the baud rates and get away with it. I mean, that's what i'd try first.

Without building a lot, or modifying a lot, you could easily get a Y connection from master to slave TX line, to some other listening interface, say an RS232 on a PC, or another board that you can monitor the transmission without a hassle. Patch the Melzi master firmware to send stuff to slave, so that you can check if you can find a nice spot in the firmware to do that, and see if you can sniff the TX line over the PC without disturbing the slave's operation. If all is fine then you can do a permanent connection to the board or implement the LCD comm protocol and stuff like that.

If not, then i'd change the master->slave connection to, master->lcd_module->slave. On the lcd module, beef up the TX signals with a line driver or something before passing them along. Also sniff the master to slave TX line with your controller on the lcd_module for your own commands. This way is has to work hardware wise, but there is a little bit more to do in circuits then.. Actually in this case, you can also do it like this. Get the RX-TX from master to your module, get the RX-TX from slave to your module. This way master will not be connected electrically to the slave. In your lcd uC, you can consume LCD commands from master and slave, and pass through the remaining data back and forth. You will then have an un touched slave firmware, better electrical connection. Very little lag on packets between master<->slave but i don't think that is an issue. A little bit more work on the lcd module firmware for sure..

I would want to keep my usb on the slave board working for firmware updates and troubleshooting.

And good thing about minimal firmware modification is that when a new firmware comes out, you can transfer your firmware patches to that easily.

by the way, this is off the top of my head, i haven't thought about it too much yet, there might be some unseen factors here, as i said i'm not very experienced on Melzi board yet. What do you think?

I think that's a very good idea ozgur, I'll take a look at the LCD side if I can accomplish that connection, but don' see why it wouldn't work. Just have to make sure that LCD gives priority to master-slave connection and I think it will have none to little interference on it.

Thank you again,
best regards
MB