Serial communication problem with 6 linear axes (no replies)

HI .
I'm using marlin 2.0 , bugfix version for my 6 axes robot (not a 3D printer ) + MKS GEN v1.4 motherboard + MKS TFT32 . TFT firmware is open source Bigtreetech . Serial port is 0 with 115200 baudrate and I'm using AUX 1 of MKS GEN v1.4 motherboard for TFT32. of course Serial communication in pronterface program established without problems in all cases .
when I define LINEAR_AXES 3 , everything is ok but when I define LINEAR_AXES 6 and set depended configs , mks tft 32 will disconnect and show "NO printer attached" message .
What tests did I take ?
Upload marlin with original configurations ( just change motherboard and baudrate) ==> everything is OK
Several baudrates and serial ports ==> "NO printer attached" message
Several pins definition for 6th axis ==> "NO printer attached" message

I think the main problem is additional axes and cause disconnection .

can anyone help me with this issue ?
thanks and sorry for my English !!