Extra Parameters of MODBUS Emulation
Settings for the protocols Ethernet and RTU.
Upper register limits – the upper available register address or I/O number. Reading/writing registers/bits with higher addresses will lead to Exception response.
PDU (data portion) size – the maximum telegram length by specifying the maximum data (which excludes the message framing) length. (PDU stands for Protocol Data Unit)
Perform MOSCAD (row/table) checks – a variation of the protocol that breaks Modbus registers up into tables.
Modify coils/regs. before responding – controls the response data to a write request, because the response to some requests also return the data in the response. Normally the PLC must handle the request, and then respond with the modified (updated) contents, but some clones don’t do it this way because they are either caching the data, or there is a need to return the old values.
Disable writing of coils/regs (or read-only mode) – was added because some PLCs are just plain not writeable, in this mode, it is an error to send any command that might modify the PLC. Read-only mode can also be toggled from the "System" menu.
Use a separate set of registers for each station – is a simple table mapping which works like so:
In the above screen, an upper limit is set to only 30 registers in the PLC, what then happens if you will have 30 *255 registers, allocated in batches of 30 per station. It’s not often you need this mode, and it is not intuitive in the way it works in the user-interface, but that’s just the way it works.
You can then still see which station address you edit as below.
