MKVIe Software Block Question

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brownsugar

There is a block of software in our system used to calculate the output to a control valve (actually the SRV). Inside the block is a multiplier that is labeled as "scan time". Does anyone know how this scan time is determined?

Also, internal to the block is another calculation that is represented by a square with a "-1" and a "Z" inside the square. I believe this is a shift function from my old MKV knowledge, but I just don't have a clear understanding. If someone can help, I'd appreciate it.

Thanks!
 
brownsugar,

Some blocks need to know what the application code scan rate is--for many Mark VIe heavy duty gas turbines run the application code at 40 ms, or 25 Hz. The scan rate is specified in one of the main application code configuration pages--by GE or the packager of the turbine. The block automatically knows what the scan rate is; it's not something the programmer has to provide for the block to work. This is helpful when some function needs to "divide" some function evenly across a number of scans (as it was explained to me), and the only reason it's shown is that there are some blocks which function very similarly--but can't split the function evenly across a number of scans. It's basically just to let advanced programmers know the complete functionality of the block, if advanced capabilities are required.

The Mark VI and the Mark VIe use floating point math and don't need shifts like the Mark IV and the Mark V did, which used fixed point math. This is a great help to the programmers--but didn't really mean much to the technicians, unless the programmers chose the wrong shifts (which did, occasionally and usually very untimely, did occur). For someone reading or analyzing sequencing which has been running fine for weeks, months, years and decades--shifts were just a confusion factor. It's great that the Speedtronic Mark VI and -Mark VIe use floating point math, and don't require shifts.

If you have your old Mark V Application Guide, GEH-6195, the generic BBL information in the appendix in the back of the manual has a very good description of all of the block elements. One of the really nice things about GE Speedtronic turbine control systems (or, since another company trademarked the name Speedtronic, as GE now likes to call the Speedtronic, the 'Mark*' control system) is that they tended to use the same descriptions and depictions from Mark IV, to Mark V, to Mark VI to Mark VIe. The Z-1 means 'the value in the previous scan'.

Hope this helps!
 
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