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Old January 8th, 2018, 12:49 PM   #9
Ken Roach
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Join Date: Apr 2002
Location: Seattle, WA
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And a bit of sorta-trivia: why are the commands called "Typed Data Table Read/Write" ? That always gave me a picture of somebody tapping on a mechanical typewriter.

The reason is that those commands specify the data type with a byte inside the command frame. There's a different value for Integer (N) files, or Real (F) files, or String (ST) files or Binary (B) files.

This was baked into the A-B protocols in part because the most common competitive protocol at the time was Modbus, which is notorious for having no explicit way to determine the data type or data element size once you exceeded the default (0,1,3,4) data types.

The other reason is that PLC-2 commands are flat-file memory access commands that have even less data organization information than Modbus does.
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