My turn to ask a question; I am running some tests with serial PPI to a S7-224 over cellular (GSM/GPRS) and it doesn't appear to work. One can set a timeout under the "Set PG/PC Interface" of 1/10/100sec, but that may be just the overall timeout before a response.

The issues seems to be that PPI has the "Tsdr" requirement of 22-40 bit times during which the slave MUST return the ACK. At 9600 baud this is roughly 20-40 milliseconds; which is impossible over cellular where round-trip delays will be in the 700 to 2000 millisecond range.

Does Siemens offer some tool or setting to overcome this protocol limitation? How do people access Siemens S7-200 over radio or satellite systems where latency is higher than normal? Would a CP243 via IP allow for higher latency?

Many Thanks
- Lynn A Linse, www.digi.com

 

There is a CP modem module that is intended for remote access applications (I don't recall the model number).

The built in serial port (PPI) can also be set to free ASCII mode. There is a (rather large) Modbus slave driver that can be used in this mode. This probably works better with an S7-226 though, as you get two ports with this model and so can more easily dedicate one to free ASCII.

I am not familiar with these sorts of applications, and so cannot offer anything further than this. I believe that Siemens sells a lot of the S7-22x series for things like remote water systems, so I would be surprised if they
couldn't help you on this.