DF-1 RS232 protocol problem with bit 4 and 12

I'm connecting directly to a SLC 5/04 using the RSLinx DF-1 RS232 driver and going online using RSLogix 500.

I've also setup a protocol analyzer to capture the data stream going back and forth.

When I set bit 4 in the N9:2 register, I see the following mask and data values in the protocol analyzer:

Setting N9:2/4 produces:
Mask: 10 10
Data: 00 10 10 10

Setting N9:2/12 produces:
Mask: 00 10
Data: 10 00 10 10

I've tried all other remaining bits in this word and they all produce the expected results:

For example,

Setting N9:2/3 produces:
Mask: 08 00
Data: 08 00

Is there something being added to the mask and data values for bit 4 and bit 12 that I'm not aware of?

Any help would greatly be appreciated. Thank you.

Try reading an ordinary integer register with the value of 16; you should see the same kind of behavior.

It's called "DLE Escaping" and is described in the DF1 reference manual.

Because DLE (10 hex) is a special character in DF1 command frames, when it occurs in an address, data, or mask field the controller or software has to represent that the occurence is not a DLE command. It does so by repeating the 10(hex) twice.

DLE Escape occurences are frequently overlooked or mishandled by third-party DF1 drivers that I have used and troubleshot in the past. If you are writing your own DF1 driver, you need to understand and account for DLE Escaping in all cases.