Help needed in interfacing plc and pic16f877a for a project

Hi friends,

i have to do a project by using PLC and PIC16F877A,
the project is to transfer the datas from PLC CONTROLLER to PIC16F877A by using MODBUS protocol in PLC,and based on that datas stepper motors have to work..

the reason for doing this project is that my actual cost for interfacing 246 stepper motor in PLC is huge as each motor occupies 4 pins so more pins needed ,so for a solution i am thinking to connect it to microcontroller and transfer the datas in to that and controlling the motors as we have 33 io pins in pic16f877a .

clarify me the below points,

1) what is modbus protocol ??
2) is it possible to convert from modbus to rs232 protocol to communicate in PIC.
3) will it be huge cost in it,or is there any other solution for doing this project ??

1) MODBUS standard: https://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf

2) You can use RS232 for a point to point communication, as long as the PLC has the interface. I suppose the PLC is the master in your setup.

3) As you have not given too much information, this is impossible to answer.

can u explain the life of a pic IC i am using pic16f877a in this project where 256 stepper motors has to be connected to the pic ICs .the product has to be stay long ,please guide in to choose best pic for that.or shall i choose 8051 ic ??

suggest me the driver for stepper motor ,i checked uln2003 driver where only 7 pins are there so that 1 motor can be connected ,suggest me another driver to connect more stepper motors for 12v output.

How many pins do you need for each motor? I would suspect 4, so if you change to ULN2803 you'll have 8 outputs, or 2 motors. It all depends on the current you need for each coil. The ULN2x03a can only supply abs. max 500mA on one output, and a total current for the IC of 2.5A.

If you want to run many PIC processors on one MODBUS channel, you need to use a multidrop solution, like RS485. The MODBUS protocol is a 2way master slave protocol, so you'll need to implement a half duplex, multidrop solution.

Thanks a lot u really helped me .

can u say about the life of PIC ic which one to choose if possible ??

and whether the distance between the pic controller and motor affects the voltage inputs of motor ???

There is too little information about your project to select a microcontroller from. If you think of the life-before-obsolence of the PIC microcontroller, I don't think that is to big a problem if you select a type and package that are 'active'.
Yes, the voltage you get to the motor will be affected by the lenght and size(area) of the cable. You may also have an inductive effect of long cables, depending on the frequence you run your motors with, how fast you are stepping. It will normally be better to have a long communication cable than a long power cable.

rangerskm maybe you can use drv8811 as stepper drivers they only need 3 pins to control the motor (step, direction and enable) so you can drive more steppers by PIC.
I prefer Atmel Atmega intead of PIcs but is your choice I'm a little rusty in Pics....:wink: