Must press READ button on CNC twice to download a file?
1) Many CNCs read a percent sign (%) as the END of a file. On Fanuc, Yasnac, and many others, if your files have a “%” at the beginning, you might not use a “leader” when you configure your device. On these CNCs, a line-feed (LF) starts the read process, and a “%” ends the read process. Also, anything the CNC receives before the first LF is ignored (this is the “Label Skip” function).

First block of the program is lost when loading into the CNC
1) You may need a LF (line-feed) sent as the “leader” (see item above). CNCs with a “label-skip” function (Fanuc, Yasnac) must see a LF character before the first block of the program. In Settings add <10> to Send Extra Characters Before File.

Can receive data from CNC, but can’t send to CNC
1) Your CNC may require that you begin sending data with some special character. Some CNCs require a percent sign (%) to be the first character sent. If this is so, Either you must have this character in your file, OR you must have this character in Send Extra Characters. If the character is in your file, you can leave the box blank.
2) You may also be configured to send the wrong tape code. If your CNC requires ASCII even parity, and you are sending EIA code, the CNC may ignore you when you send to it. 

When receiving data from CNC, some characters are missing (or blocks run together)
1) This is a handshaking problem. On most CNCs (Fanuc, Yasnac, etc.) you MUST connect pin #5 on the CNCs serial port to the correct pin on the PC (see cable diagram, page 34). If this pin is “jumpered-out”, then the CNC won’t be able to handshake properly when it SENDS.
2) On CNCs that use Xon/Xoff handshaking, be sure the CNCs parameters are set to enable this function. Note: Xon/Xoff handshaking is sometimes referred to as “Device Control” or “DC” codes. 

When sending data to the CNC, some characters are missing (or blocks run together)
1) Be sure the handshaking setting matches the type of handshaking used by the CNC. Most CNCs use Xon/Xoff (sometimes referred to as “DC codes”) when receiving data. 
2) If you are also having trouble receiving data from the CNC, you may have a bad wire in your cable, or you may also have a “FIFO” buffer problem that prevents the PC from receiving the CNCs Xoff handshake signals.