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# Position Field Instructions

Most I/O PLEXER instructions have a position field following the function code. The contents of this field determine which modules are affected by the instruction. Some instructions affect all modules, in these, the position field determines what the effect is for each module. In either case, the construction of the position field follows the same reules.

The position field is a 4 hex digit representation of a 16 digit number. The position field is a 16-digit number (one digit for each possible module) but each digit can only be a 1 or a 0 (these digits are called bits).

To fill the position field, perform the following steps:

1. Make a list of the modules that the instruction is to affect, for example: 0, 3, 8, 12 and 15

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2. Make a 16 digit number with a 1 in each position listed in Step 1, and a 0 in all the other positions. The left most digit of the number is module 15, the right most is module 0. The number for our example is: .

3. Divide the number built in Step 2 info four 4-digit numbers. Our example appears as 1001 0001 0000 1001.

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4. Using the table below, look up each of the 4 digit numbers in Step 3 and replace it with the corresponding hex number or letter. Our example becomes 9109H. This is the value that should be put in the position field of the instruction. This table is in the Protocol Instruction Set (Quick Reference Guide) for easy access.
 Module # 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Hex Digit 1st Digit 2nd Digit 3rd Digit 4th Digit Binary 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 Hex 0 1 2 3 4 5 6 7 8 9 A B C D E F

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5. It is always acceptable to use a 4-digit value in an instruction which requires a position field. Some instructions also accept an abbreviated version which has had the leading hex zeroes omitted. In some instructions the position field is optional, and assumes a value of FFFFH (all modules affected) if it is omitted.

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