Status update MELSECA driver
Mitsubishi Melsec A-Q protocol and hardware exist in several evolution steps, and are not fully compatible:
- The first Mitsubishi PLC’s used A series CPU’s, later extended to series like AnA or AnU, with a larger protocol field for device numbers. The current CPU’s are from series Q/QnA, not fully backward compatible with A CPU variants.
- Communication options are designated C for serial (COM) port or E for ethernet (TCP) protocol. Within the Melsec A-Q protocol, several frame types are defined, frame 1 is used by A CPU’s, including variants for short and long device numbers, other frames (2..4) are compatible with Q CPU’s, most common is type 3.
- Frame 1 can also be used to communicate with Q/QnA CPU’s, but is restricted to devices found with the same names in An, AnN, AnA and AnU CPU’s, with step relay and latch relay mapped to the internal relay, while file registers are not available. Those Q CPU’s with built in ethernet port are not compatible with frame type 1, and could not be used with the earlier zenon driver.
- Melsec A-Q protocol can use ethernet in ASCII or binary mode, serial communication standard is ASCII, binary is only an option format with frame type 4.
- Mitsubishi adds compatibility information and the communication mode to the frame number, giving, for example, A compatible 1C frame or QnA compatible 3E frame, binary mode.
The zenon driver for Melsec A-Q started with serial communication for A and AnA CPU’s, that is frame 1C with short and long device numbers. Next came TCP, using frame 1E also in ASCII mode. Now we have added frame type 3E in binary mode, giving full access to the latest features for all types of Q CPU’s. Not yet covered are the ASCII modes for frame type 3E and 3C, and any kind of binary mode for serial communication.
No demand for updates to frame 1 is expected, and serial binary mode looks equally rare. But since frame type 3 is needed for full use of Q CPU’s, ASCII mode for 3C or 3E may become necessary: Serial installations have no other option, and ethernet users could find their own reasons not to switch to binary. Either more options have to be integrated into the current driver, or a split in two versions, like A/frame1 and Q/frame3, could be the better option.