Understanding Scrambled Strings Between CJ and Sysmac Series PLCs
Written by Arlo D'Cruz
Updated at November 6th, 2025
When sending string data between a CJ-series PLC and a Sysmac-series (NJ/NX) PLC via EtherNet/IP, users may notice that the received string appears "scrambled".
For example, if a CJ PLC stores the string "ABCD" in words D0 and D1, the Sysmac PLC may decode the same data as "BADC". This occurs because of differences in how strings are stored in memory between the two PLC families.
How Strings Are Stored in the CJ-Series PLC
In the CJ series, each word (16 bits) holds two ASCII characters, but the byte order is reversed compared to Sysmac PLCs.
For instance, consider the string "ABCD" stored as follows:
| CJ Word | Content | Binary Representation | Description |
|---|---|---|---|
| D0 | 41 42 |
01000001 01000010 |
“A” is stored in the upper byte (bits 8–15), “B” in the lower byte (bits 0–7) |
| D1 | 43 44 |
01000011 01000100 |
“C” is stored in the upper byte, “D” in the lower byte |
How Sysmac Reads the Same Memory Area
When this data is read via EtherNet/IP, the Sysmac PLC interprets the data byte by byte, without accounting for the CJ byte order.
The mapping below shows how bytes are interpreted on the Sysmac side:
So in memory, D0 and D1 contain:
D0 = 4142h
D1 = 4344h| CJ Memory Location | Character | Sysmac Byte Index | Notes |
|---|---|---|---|
| D0, byte 0 (bits 0–7) | B | Byte 1 | Lower byte of D0 |
| D0, byte 1 (bits 8–15) | A | Byte 2 | Upper byte of D0 |
| D1, byte 0 (bits 0–7) | D | Byte 3 | Lower byte of D1 |
| D1, byte 1 (bits 8–15) | C | Byte 4 | Upper byte of D1 |
Thus, when the Sysmac PLC converts these bytes into a string, the result is:
When read as a contiguous byte array, the original string "ABCD" is stored in the STRING variable as "BADC".
How to Correct the Byte Order
To ensure strings are read correctly, the byte order must be swapped. This can be done either on the CJ or the Sysmac side.
Option 1: Swap Bytes in the CJ PLC
Use the SWAP(637) instruction to reverse the bytes within each word before transmission.
Example:
The resulting data will appear correctly as "ABCD" when read by the Sysmac PLC.
Option 2: Swap Bytes in the Sysmac PLC
On the Sysmac side, it’s recommended to read the data as a BYTE array instead of a STRING.
You can then use the Exchange() function to swap each byte pair before converting back to a string.
Example Structured Text:
// Swap each pair of bytes
Exchange(str_As_Byte_Ary[0], str_As_Byte_Ary[1]);
Exchange(str_As_Byte_Ary[2], str_As_Byte_Ary[3]);
// Convert to string
OutputString := AryToString(str_As_Byte_Ary, 4);