Hello,

I have the same situation, i need to communicate (read data) with a RS485 slave modbus rtu controller using CM PtP RS422 / 485BA. Can you share with us your example for how to program the communication using freeport .

I attached the controller modbus manual.

Thanks in advance!

------------------------------------------------------------------------------------------
Split from S7-1500 and ptp communication (ascii) .

Hello,

for first maybe question how you're familiar with MB RTU message format, because expect probably another task in your side. Simply you must prepare own frame formats due your requirements (Slave address, register address, amount of needed data, CRC, ...) so I recommend for first start check good site where MB format is described:

http://www.simplymodbus.ca/

Then you can prepare your own frames via ptp instructions probably. Look, if you will have some example, without good knowledge of that message format, no luck.

Thanks for your answer, i'm not very familiar with this protocol, until now i used modbus rtu only with dedicated library instruction. I the controller modbus manual that i have attached it is described the telegram format. i will try to make some test using ptp instruction, but if someone have an example will be very useful for me.

Best regards!

Hello,

that's good manual also with needed addresses and amount of required data, no problem. If you know how to send/recv standartly, you will be lucky. Pls keep in mind, for you is very important check and also count back that CRC checksum. Without this implemented in your code after receive of message from Slave, it can be purely hazard because of wrong data possibility. Don't hesitate ask if you need.

Pls don't wait for example, put all your power to work and you'll be satisfied with yourself.

I will start to work on this, i will post here  any problem that may occur.

Regards!

Hello,

I'm back with new information and questions... I made the wiring Data + 11, Data - 4, GND  8, i made the hardware configuration according to controller settings : 9600 transmission rate, Parity none, 8 Bits, 1 Stop bits, and Send idle line 4 bit times, Receive frame Start on special condition-After detection of an idle line 4 bit times. I tried first to send and receive frame using Send_PTP, and Receive_PTP instruction, send_ptp instruction works and i gives me done true after req is 1, i can see also the TX led flashing 1 time for every send instruction. but the Receive_PTP gives me error true after the send instruction. The error is 81E2-Frame aborted: Character frame error (Check the settings for start bit, data bits, parity bit, data transmission rate, and stop bit(s).) I'm sure that settings are correct because i use the same settings with modbus pool software in order to test the communication from my pc to the controller and is working. Any idea how to resolve this ?

Best regards

Update of situation. I use one 120 ohm resistor connected between Data + and Data - on controller according with controller manual, with this resistor i have receive error, if i remove the resistor i have no receive error but i also not receiving any data.

PS When i made test with PC -Modbus Poll software connected to controller i also use this resistor and works ok.

Regards,

Hello,

for first probably check that you have selected right HW cfg - RS-485 2 wire interface. Also check right this value in instance DB, hope good remeber 4, this problem is known. Also pls give info what data you want send - Slave address, function code, register... You can better post here complete hex frame of your send message like:

01 03 30 ...

Pls check that sended message what it has inside, what bytes it has, this is important to know. You can simply check your message also with that PC application attached to your PLC if it is ok and correct.

Now is working, i modifiedSend idle time to 300 bit times and End detection of received frame to After character delay time elapses 288 bit times.

I think the problem was that my telegram it was not recognized by the controller because the idle time from the start of the telegram was to short, i also removed the resistor. Other important fact is to correct calculate the CRT, because if the CRT in wrong in send frame the slave device will not answer.

Hello,

yes that CRC is very important for both sides. Simply this topic solution (MB RTU via ASCII) can be realized quickly if you expect not so much requests/answers from Slave/s. If you expect implement Exchange more data with more different registers, function codes it can be also implemented but with more code, therefore library was created to help quick solve needs.