Register access via process data exchange

Bit 7 = 1_bin: Register mode

If bit 7 of the control byte is set, then the first two bytes of the user data are not used for exchanging process data, but are written into or read from the terminal's register set.

Bit 6 = 0bin: read
Bit 6 = 1bin: write

Bit 6 of the control byte specifies whether a register should be read or written. If bit 6 is not set, then a register is read out without modifying it. The value can then be taken from the input process image.

If bit 6 is set, then the user data is written into a register. As soon as the status byte has supplied an acknowledgement in the input process image, the procedure is completed (see example).

Bit 0 to 5: Address

The address of the register that is to be addressed is entered into bits 0 to 5 of the control byte.

Control byte in register mode

MSB

• REG = 0_bin: Process data exchange
• REG = 1_bin: Access to register structure
• W/R = 0_bin: Read register
• W/R = 1_bin: Write register
• A5...A0 = register address

Address bits A5 to A0 can be used to address a total of 64 registers.

The control or status byte occupies the lowest address of a logical channel. The corresponding register values are located in the following 2 data bytes. (The BK2000 is an exception: here, an unused (reserved) data byte is automatically inserted after the control or status byte, and the register value is therefore placed on a word boundary).