I’m currently working on a project that requires communication to be established from SIEMENS S7-1200 PLC to the Distributed Control System (DCS) via MODBUS RTU protocol. The PLC is configured as a MASTER in the PLC program [see attached link for screenshots] and the DCS has to be configured as a SLAVE. I have started by first trying to connect my laptop directly with the CM 1241 communication module to figure out the holding register mapping. The pin configurations on both the PLC (CM 1241) and laptop end are as follows:

MODBUS Slave is the software utility that is being used. The settings configured in the software matches those that exist in the code are as follows:

Upon establishing the connection successfully, the holding registers show no valves even though the data traffic is active. Also, at the PLC end, the status LEDs of Tx and Rx is as such as that Rx is consistently AMBER while Tx LED does not light up. Though in the PLC program, the ‘LOAD.DONE’ bit is HIGH in the ‘MB_COMM_LOAD’ block and there is no error.

1. Does this mean that the request sent by the PLC (MASTER) is not being received by the SLAVE device?
2. And, is there a step that is missed along the way or something that needs to be rectified in the existing settings?
3. Also, at the DCS end, it requires an ‘offset’ address that needs to be configured in the settings. What does this ‘offset’ address mean and where would this address come from? The PLC end?

The screenshots of the entire MODBUS communication settings have been attached for reference. Any assistance in this matter would be highly appreciable.

https://www.dropbox.com/s/k1q3ee3pus367js/MODBUS%20Query.rar?dl=0

Hello,

first thing what had get to my eyes is on MB_COMM_LOAD port address, simply hex 11 is looking strange, I expect another value. Look to PLC HW cfg and check that address of CM1241 module, or click on that PORT area on MB_COMM_LOAD and select right value from list. This is probably problem. I know that you posted that all goes ok with establish of comm_load but can you post Picture of that hw cfg address?

>Upon establishing the connection successfully, the holding registers show no valves even though the data traffic is active.

1)  Most generic Modbus slave simulator programs require you to put some data into the slave registers; they do not self-populate with data.  

Have you put some known non-zero data into the slave registers?

2)  The lack of a blinking Tx LED on the PLC indicates that it is not polling/sending requests.

I'm a Modbus guy, not a Siemens guy, so I can't tell from your screen shots; what logic creates a continuing polling action (say, once every second or two seconds) by the Master, so that it is not just a single, one-shot request?

Hello Fahad,

in case of not light TX but light RX absolutely, pls cross wires Data+, Data-. This is also common problem of not functional communication.

FahadAmir

I’m currently working on a project that requires communication to be established from SIEMENS S7-1200 PLC to the Distributed Control System (DCS) via MODBUS RTU protocol. The PLC is configured as a MASTER in the PLC program [see attached link for screenshots] and the DCS has to be configured as a SLAVE. I have started by first trying to connect my laptop directly with the CM 1241 communication module to figure out the holding register mapping. The pin configurations on both the PLC (CM 1241) and laptop end are as follows:

Pin	CM 1241 (via DB9 connector)	Laptop (via RS485 to USB converter)
D+	3	2
D-	8	1
GND	5	5

MODBUS Slave is the software utility that is being used. The settings configured in the software matches those that exist in the code are as follows:

Protocol	RS-485 ( 2-wire half-duplex)
Baud Rate	9600
Data Bits	8
Stop Bit	1
Parity	None
PLC ID	2
Starting Address	40000

Upon establishing the connection successfully, the holding registers show no valves even though the data traffic is active. Also, at the PLC end, the status LEDs of Tx and Rx is as such as that Rx is consistently AMBER while Tx LED does not light up. Though in the PLC program, the ‘LOAD.DONE’ bit is HIGH in the ‘MB_COMM_LOAD’ block and there is no error.

1. Does this mean that the request sent by the PLC (MASTER) is not being received by the SLAVE device?
2. And, is there a step that is missed along the way or something that needs to be rectified in the existing settings?
3. Also, at the DCS end, it requires an ‘offset’ address that needs to be configured in the settings. What does this ‘offset’ address mean and where would this address come from? The PLC end?

The screenshots of the entire MODBUS communication settings have been attached for reference. Any assistance in this matter would be highly appreciable.

https://www.dropbox.com/s/k1q3ee3pus367js/MODBUS%20Query.rar?dl=0

Hello,

Sorry I have the same problem that you had communication with the module CM1241 of siemens, did you manage to solve it? Could you tell me how? 

Thank you very much good day, greetings

Hello,

pls explain your problem clearly. What did you already, what kind of HW, SW?

Hello, 

Currently my problem is hardware (attached image), the CM 1241 does not blink the LEDs although in the software developed no error is detected.

rojasalmut2

Hello, 

Currently my problem is hardware (attached image), the CM 1241 does not blink the LEDs although in the software developed no error is detected.

 There are two unobvious things you should to know, when working with CM 1241 via RS 485 via modbus:

1. Port parameters from HW configuration of CM 1241 have no matter.. because when you call Modbus_Comm_Load - parameters from its instance appling to CM 1241.

2. You have to change mode of communication to RS 485 in the instance DB of Modbus_Comm_Load! Variable MODE in static area should be set to 4. If you will not do that - it will not work at all (no blinking)