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titleOverview

This chapter describes a software-based architecture for mirroring data between two TSENTRY-based systems or between a TSENTRY-based system and a third-party box.  This is available as a low cost optional add-in to the standard TSENTRY system.  As opposed to a hardware-based reflective memory solution (e.g. VMIC® reflective memory cards), this design transfers data from one system to another via TCP/IP.  While a hardware-base solution provides much lower latency in terms of the communications between systems, in many cases the high-speed data transfer is unnecessary and thus the lower overhead and simpler design of a software-based solution is more appealing.

Under this architecture a client process on one machine makes a connection to the server on a second machine.  Upon connecting, the client sends a list of variable names and a requested refresh rate.  The server parses this list and responds with the types and sizes of the variables that it found and can return together with the refresh rate at which it will send updates.  After this point the server sends updates for each of the available variables at the specified interval.  A single update consists of the current value of each of the variables packaged into a single telegram so that all variables are updated simultaneously.  The updates continue indefinitely until one side or the other destroys the connection.

In this architecture data transfer is unidirectional.  Data transfer in the reverse direction can be realized by starting a server process on the second machine and configuring a client to run on the first.

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titleGsmExpSvr– Global Shared Memory Export Server

The TSENTRY GsmExpSvr process is a multithreaded TCP/IP server that allows remote (or local) clients to connect and request variables from global shared memory segments.  Upon startup it attaches to all TSENTRY global commons and begins to listen for requests.  Much like the trend server TrendSrv, each connection into the GsmExpSvr server spawns a new thread to handle all communications with that particular client.  Associated with each thread is a console window to display diagnostic information about the status of communications.

 

On systems other than Windows/TSENTRY, a GsmExpSvr server must adhere to the communications specifications laid out below, but otherwise may be designed to suit the particular system.  Such servers have been written for the QNX and Modcomp operating systems.

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titleCommunications Protocol

  1. The client establishes a connection to the server

  2. The client sends list of parameters as an ASCII string delimited by ampersands and terminated by a carriage return:

code


  1. per=<desired

  1. sampling

  1. period>&vars=<variable

  1. #1

  1. name>,<variable

  1. #2

  1. name>,...,<variable

  1. #n

  1. name>\r

 

3. The server responds with the true update rate and the size of each variable that will be returned:

Code Block
1 byte = STX 0x02
3 bytes = all zero
4 bytes = number of bytes in this message from STX to ETX, inclusive
4 bytes = message type = 0x00000000
4 bytes = true sampling period in milliseconds in long integer format
2 bytes = fundamental type for variable #1
2 bytes = both zero
4 bytes  = number of bytes for variable #1 in long integer format (a)
2 bytes = fundamental type for variable #2
2 bytes = both zero
4 bytes  = number of bytes for variable #2 in long integer format (b)
…
2 bytes = fundamental type for variable #n
2 bytes = both zero
4 bytes  = number of bytes for variable #n in long integer format (n)
1 byte = ETX 0x03

 4. The Server the begins to send periodic updates to the binary data. Each update is formatted as:

Code Block
1 byte = STX 0x02
3 bytes = all zero
4 bytes = number of bytes in this message from STX to ETX, inclusive
4 bytes = message type = 0x00000001
4 bytes = seconds timestamp for current data in long integer format
4 bytes = nanoseconds timestamp for current data in long integer format
a bytes = binary data for variable #1
x bytes = blank bytes to put following variable on 4 byte boundary
b bytes  = binary data for variable #2
x bytes = blank bytes to put following variable on 4 byte boundary
…
n bytes  = binary data for variable #n
x bytes = blank bytes to put following byte on 4 byte boundary
1 byte = ETX 0x03

5. The connection is closed

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