PLC Watchdog

Hello Experts,
I am trying to write a program in S7-300 for watchdog. The scenario is there is a third party device which communicates with S7-300 PLC in remote mode. This third party device ask for deploy, deploy reset, recover, recover reset and etc...I am not sure what this third party device is but the only thing which I can confirm is it talks through DB's and I have all the right address of these DB's to send those signals which I mentioned earlier. Now the customer is asking for every single request from his device should be acknowledged in PLC saying request received/acknowledged telling their system there is no flaw in the communication system and if there is any fault in communication/receiving then generate command acknowledge fail bit. So now I have to write a code in PLC. Can somebody suggest me the simplest way of writing this code in ladder please...
Any help in this matter will be highly appreciated.

Regards
Manoj

If this is a standard device, the customer can probably tell you how he is expecting these signals.

If not, I would generate a watchdog signal that is inverted by the slave side and sent back. If the signal received back is not inverted within a set time, you have a watchdog error.

Another option is to duplicate all signals as you receive them, perhaps with a value changed bit in it.

All in all, you should probably first determine what you want to do, after that we can help you with how to do it.

Hi,
First of all thanks for your reply and sorry for replying late as I have been sent to the site.Further to above explanation, Customer wants a "bit" signal which tells his system that request send by his system has been acknowledged and there is no communication glitch.Every time the system ask for some operation the PLC has to send the signal back acknowledging the acceptance of operation request. Now the concern is if there is real communication problem or any other hardware issue, then how the PLC would know that the system is asking for some operation.

Regards
Manoj

You won't.. Perhaps the slave diagnostics will be sufficient for you with SFC51:

CALL "RDSYSST"
REQ :=M86.0 // trigger lesing - f.eks hvert sekund, eller som her fra OB86
SZL_ID :=W#16#294 // liste-ID
INDEX :=W#16#1 // DP-mastersystem nr. Fra HW-config. Default = 1 des.
RET_VAL :=MW100
BUSY :=M102.0
SZL_HEADER:=DB505.SSL_Header
DR :=P#DB505.DBX4.0 BYTE 258 // peker til mottatt liste.

A M 86.0 // Resett triggemerker
R M 86.0

Click to expand...

If this is not sufficient for you, you will need a watchdog signal both ways. So the slave will also have to send you an alive bit .

I run a "watchdog" timer in the plc for 10 secs and every 5 secs I reset the timer register to 0 from the slave (HMI in my case).

Many Thanks Gents.... I will surely try one of this and will post the outcome of it...

Regards
Manoj