It is not possible to send VLAN tagged frames with a length of only 64 bytes (including FCS)

It is not possible to send VLAN tagged frames (IEEE 801.2q) with a length of only 64 bytes (including FCS). The 2-port-switch adapter currently always adds additional 4 bytes to the frame length in case the frame uses the ether-type 0x8100.

According to the IEEE 801.2q specification (C.4.4 Padding and frame size considerations) it depends on the application whether tagged frames are sent with size 68 or 64. Basically both ways are correct.

However, Ethernet/IP defines its DLR frames to be VLAN tagged frames with a size of 64 bytes (including FCS). Therefore, the 2-port-switch adapter must be able to send tagged frames with a size of 64 bytes.