CP-341 Modbus ASCII protocal

PlUg & PlAy

 

danw

Some masters have a problem with 2 stop bits.   You might ask Siemens specifically if its Modbus ASCII serial supports 2 stop bits (any and all serial ports support one stop bit).  

Since Modbus ASCII is a 7 bit word, it is not unusual to find Modbus ASCII using 2 stop bits to make a 10 bit data word (1 start, 7 data, 2 stop), but then again Modbus ASCII is relatively rare nowadays compared to Modbus RTU. 

Dan

 HI,

If we are using 8data, 1stop that time to be 9bit data word and 1 start bit also. totally we have 10 bit word. . i am not familiar with modbus so i am asking..

1)   No, you can not setup Modbus ASCII as 8 data bits. 

The Modbus standards define Modbus ASCII as a 7 bit data word (plus start and stop bits) and Modbus RTU as an 8 bit data word (plus start and stop bits).   The interpretation of the data at both the serial bit level and the firmware operational level MUST comply with the data word size. Modbus ASCII is a 7 bit data word, period.

2)  If the serial UART hardware and its firmware interpretation supports it, you might get Modbus ASCII to run at 1 start bit, 7 data bits and 1 stop bit, maybe.  Just maybe.   I would not hold my breath.   As I said before most masters have a problem with Modbus ASCII.

3)  The discussion above reveals how much of a problem Modbus ASCII is to the Siemens product line.

Modbus ASCII is typically used by small, cheap single loop temperature controllers because Modbus ASCII has less restrictive timing requirements, that allows for successful Modbus ASCII communications that would fail under Modbus RTU timing requirements when executed by the low powered microprocessors used by these types of products. 

Thermon is big in heat trace and has a number of controllers.  If the Thermon controller you're using is a single loop controller it is really no different that any of dozens of temperature controllers that have an RTD input and a relay or solid state output.    

With all the discussion about different licenses, firmware versions, CPUs and whatnot, it might be that the least expensive solution is to either do the control that the Thermon is doing in the PLC or to replace the Thermon controller with a competitive unit talks Modbus RTU or Profibus.

danw

 

1)   No, you can not setup Modbus ASCII as 8 data bits. 

The Modbus standards define Modbus ASCII as a 7 bit data word (plus start and stop bits) and Modbus RTU as an 8 bit data word (plus start and stop bits).   The interpretation of the data at both the serial bit level and the firmware operational level MUST comply with the data word size. Modbus ASCII is a 7 bit data word, period.

2)  If the serial UART hardware and its firmware interpretation supports it, you might get Modbus ASCII to run at 1 start bit, 7 data bits and 1 stop bit, maybe.  Just maybe.   I would not hold my breath.   As I said before most masters have a problem with Modbus ASCII.

3)  The discussion above reveals how much of a problem Modbus ASCII is to the Siemens product line.

Modbus ASCII is typically used by small, cheap single loop temperature controllers because Modbus ASCII has less restrictive timing requirements, that allows for successful Modbus ASCII communications that would fail under Modbus RTU timing requirements when executed by the low powered microprocessors used by these types of products. 

Thermon is big in heat trace and has a number of controllers.  If the Thermon controller you're using is a single loop controller it is really no different that any of dozens of temperature controllers that have an RTD input and a relay or solid state output.    

With all the discussion about different licenses, firmware versions, CPUs and whatnot, it might be that the least expensive solution is to either do the control that the Thermon is doing in the PLC or to replace the Thermon controller with a competitive unit talks Modbus RTU or Profibus.

 Hi,

ok i agree your point about the data bits. i saw in the hardware cofiguration.

i cannot to say client about changing thermon model. because they have installed already 32 panels.

now i need to read the datas from the existing controller. we have already provided all material to customer. try to understand my situtation. please help to solve this issue with this configuration.

Hello,

there can be 3 solutions in my opinion:

1. implement your messages with ASCII standard driver. If you're skilled can be solved without problems. If you have many requests to registers, big work for you but can be solved.

2. second, maybe best solution can be buy good gateway that can convert physically some protocol to another needed protocol. I got one that is on one side MB TCP (Server/Client) and on another side MB RTU or MB ASCII Master/Slave. With this solution you can win over your problem. You need find probably gateway that convert MB RTU to MB ASCII, I expect you have MB RTU licence and dongle available so this will be solution for you.

3. My mentioned Gateway is good in case of S7-1200 or S7-1500 CPUs. Look, also this can be not big money investment (your side make trouble), one S7-1200 is relatively cheap piece of HW so with suitable Gateway you can communicate over MB TCP to MB ASCII and to Slaves. I expect price about 500 euros. Then you can maybe exchange data between both systems via some interface, S7-300 with S7-1x00.

Keep in mind that Gateways change protocols physically, so you prepare communication to first point of Gateway interface type, then all is configured in Gateway as you need.

Unless someone here (or through formal Siemens support, this is only a user forum) can tell exactly what hardware/firmware/licensing combination will do Modbus ASCII communications, then I agree with Duris and his recommendation to use a gateway that is compatible with your PLC on one side (Modbus TCP or Modbus RTU) and Modbus ASCII on the Thermon side.

I'm a Modbus guy, not a Siemens PLC guy, so I can only bring general observations about Modbus to the table, not specific solutions for Siemens PLCs.

Dear PlUg & PlAy,

when it comes to Modbus a S7-300 is not really a good choice.

The CP341 + license dongle is by far the most expensive solution.

Anyway, these are your options now:

1. Write your own Modbus ASCII application (not recommended for beginners).
2. Exchange or buy another dongle for Modbus ASCII.
3. Use the Modbus RTU protocol in combination with a Modbus RTU/Modbus ASCII gateway

Best Regards.

TIBI68

Hello TIBI,

thnx for enter this conversation and giving sugestions, everytime appreciated this. One problem is with already purchased CP341 with order number not supported MB ASCII protocol. Also solution with S7-1x00 can't be solved because of only RTU protocol supported, so again sugested Gateway must be used. Simply for first is important that Slaves only support MB ASCII protocol.

I totally agree with you, he is in deep sh..

Therefore I have provided some options but the choice is on him.

Best Regards.

TIBI68

Yes,

from my side can be realized with all 3 my sugested solutions, but we still waiting for his news.

Hi TIB & Duris,

i am confused. I am interested to written in my own application. C can you please help how to. This.

i have framed the message based that modbus ascii frame structure . : 02 04 00 65 00 01 94 0D 0A and I have selected as standard ASCII 

after that the tx blinking but I did not received status in done error and status.. can you please explain how can I send this information..

First you should study on Modbus RTU and Modbus ASCII.

Then, once you understand you can start writing you code.

Check on the internet, there is all information available.

Here is an example site:

http://www.simplymodbus.ca/ASCII.htm

Best Regards.

TIBI68