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Microchip mx795 based Industrial PLC

Created Sat, 05 Jan 2013 21:08:33 +0000 by i.merello


Sat, 05 Jan 2013 21:08:33 +0000

Hello I have designed an Industrial PLC based on the same PIC32 as the Cerebot MX7ck and programable vía Pickit3 and MPLAB. It has 12 inputs (some of them analog) 8 digital or PWM (with some restrictions) 5A 24V capable outputs and 4 ports configurable as input or output 5A 24V. In the communications part, it has CAN, RS485 (For Modbus), RS232 and Ethernet. The Ethernet design is based 100% on the Cerebot Mx7ck. but I didnt use the resistances between the Phy and the PIC32. Are them important? Im new in ethernet and I want to verify that the design is correct and I dont know how to start. I just want to know that the Ethernet part is OK to manufacture the second and final design becacuse all the rest is proven. As I have the Cerebot I have made some investigations with it and the TCP/IP stack (microchip) but I have had no success. I have used the Chipkit web server example whith the MPIDE and then extract the .hex with the MPLABX. And programed my PLC but although the leds of the ethernet connector flash they do in a diferent way and the web server doesnt run. Can anyone give me a clue? Could it be because of the resistances mentioned? Regards and Happy New year to everyone.


Sun, 06 Jan 2013 01:17:43 +0000

Typically communication buses require the proper termination resistance to control signal reflections on the bus.



Sun, 06 Jan 2013 19:22:27 +0000

Thank you james. The resistances Im talking about arent the ones of CAN or RS485 (termination resistances wich are implemented) but the ones wich the Cerebot use to comunicate the PHY and the PIC32 in the Ethernet part (for example R22, R21... of the Cerebot MX7ck). Thank you for your interest!


Mon, 07 Jan 2013 19:51:03 +0000

I attach a image of the board in case it can help anyone helping me ;)


Wed, 09 Jan 2013 11:18:28 +0000

Can anyone help me? I attach a drawing with the Ethernet scheme and the resistors I´m talking about. Thanks

Jacob Christ

Fri, 22 Feb 2013 08:09:06 +0000

I don't think the missing 50 ohm resistors between the uC and the PHY are needed.

How did you transfer the hex file from your Cerabot to your custom board? Did you get its bootloader too? I've never used a Cerabot board, does it even have a bootloader?

The pinout for the magnetics in the posted schematics looks different that what is physically mounted on your board (the schematic looks like a dual port). Is pinout of your magnetics correct part.

Does the switch you connect the board to detect its carrier? (Does the link lite come on?)

The portion of the schematic you posted looks correct. Can you post the entire schematic so that I can check connections to the uC?