Hi everyone

I am using S7200-CPU226 acting as a master to read Modbus registers from a GC (registers 40103 -40117)

• GC data frame is Real (Floating Point 32 bit)
• Baud rate = 19200
• Slave address = 1
• I managed to get some data on registers VB 0 ~ VB 16 ... However ,It does not seem correct or I dont know how to read the values. What doesnt make sense is the received data does not change when I change the number of reading registers in Modbus parameters ,eg : when i set :
  addr= 40103
  Count=1
  in order to read 1 register ,instead of 4 VB (4 x 8 =32 )registers there are 8 ~9 VB registers filled with some data

Can it be a problem with out wiring and the cable ?Is there a problem with Port 1 of CPU or it has to be PORT 0 ?

I attach the code
I am really stucked with this problem , appreciate if someone cna help me with this

The wiring we used is not profibus, I give you the detailed wiring here:

• A - (from GC controller,RS485) =======>>>> pin No.8
• B + (from GC controller,RS485)=======>>>> pin No.3
• Shield (From GC controller,RS485)=====>>>> pin No.1
• +24 V (From GC micro controller,RS485)=>>>> pin No.2
  
  I am not very sure about the last 2 wires though !

I also attach the project we used

Regards

from what i know:

CPU 226/ RS485(GC)

Pin 3 --------( + )

Pin 5 --------( ground )

Pin 8 --------( - )

I dont't kno about 24V.

Anyway, you should try switching + with - and be sure you configured the right port(0 or 1).

Else:

- Check if your cable is connected physically to port0 or 1 and correct in your program to match the physical layout.

- Your device is set accordingly to the settings made in your program? Adress/BaudRate/Parity

- Also you could try lowering the baud rate.

Thanks I am going to test this and will let you know if it works ,

Is there anyway I can test Modbus wiring when I have only a PLC,S7 200 and a PCwith a serial port?
I was thinking about writing a code to read values from VB memory and write them toModbus registers (eg. 40001~40004) .Do you think it is possible to use Modscan to read the sent values on the PC ?

I attach the code

Hello,
ModScan acts as Modbus master. As the S7 is master too, you can't use ModScan for testing.

Regards,
Kaulquappe

other ideas:

what error code does MBUS_MSG_P1 return?

registers are 16bit long and if the data is 32bit it's normal to be split between 2 registers. That's why you need to read 2 registers for a floating point value.

for example you could read 80 register in 1 message, just replace 2 with 80 in register COUNT.

just remember to reserve 80*4=320 VB memory locations(not to be altered by other functions in your program). I don't know if it's a limit to the number of register read/write in a message.

I've reread your fist post and for reading real value from address 40103 you should put 2 in register COUNT.If you want to read more just change count value to a multiple of 2 (for floating point or dword values). And you can view your data in status chart, if your data pointer is starting &VB0, define VD0 as floating point and view your data.

No error ! error value is 0 and data is apparently sent correctly .However ,I cant read the values .

Strange thing is ,once I trigger the receive block ,I can see the data stored in VB0~VB34 which does not make sense because it is supposed to read only 4 registers ( 2 x 32 bit ) and store them into VB0 ~ VB 8 ...

Could it be a problem with wirings?