Real time data exchange

Hi, I have two computers (Win XP) and must get one of this computer to send a small amount of measurement data (4k) each 20ms to the other computer. In the receiving computer there is running a real-time process in a loop (20ms, real computing time: 5ms), which needs the actual measurement data in time. There is no possibility to make both jobs on one computer. Anybody have an idea for a proper way to implement the communication? For the hardware, I think to take Firewire or a parallel cable. But how to implement the software without breaking the control process while waiting for new data. Open e network drive and write each 20ms a new file? Take a pipe? Thanks for your suggestions.


Reply to
Rainer Harthaus
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I assume this is 4 kilobytes, thus 200 kilobytes/s or about 2 Mbits/s.

What is wrong with Ethernet, even the 10 Mbit/s should barely be able to constantly keep up with that rate.

Use UDP if timing is important and you can tolerate some lost frames, the 4 KB data must be split into 3 frames.

Use TCP/IP if it is important that all data is transferred, but the timing of the delivery is not (e.g. the data has been time stamped at the sending computer).

Either synchronise the control loop with the sending cycle of the other computer or use a separate thread to receive frames while other threads do all kinds of other control activities.

By the way, disconnect both the keyboard and mouse from both computers in order to prevent any user interaction with the computers while the real time applications are running.

The Win NT series of operating systems can work with a surprisingly low jitter (within a millisecond of two with 99.9 % certainty) _provided_ that the CPU load is low (always below 50 %), the hardware is well behaving (some drivers are not well behaving and must be discarded) and there is a full control what exactly is running on the system (thus any random user interaction, such as running Word, must be prevented).

Unless these precautions are used, the peak jitter can be 50-100 ms or even more.


Reply to
Paul Keinanen

You can probably meet these requirements using TCP/IP sockets over ethernet. This depends somewhat on whether you can tolerate an occasional missed message, which would be very infrequent. Good fast ethernet adapters are readily available an inexpensive.

If your requirements are more stringent, you might look into one of the reflective memory devices. They are a lot more $$$ but can meet some serious real-time constraints.

Steve (The Duck)



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