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Fanucs use the ISO 840 character set, and any non-ISO character can cause a TH alarm. ISO 840 is basically a "subset" of ASCII Even parity code. ASCII contains all the letters of the alphabet (upper and lower case), plus the numbers and the punctuation marks that you see on your computer keyboard. ISO, on the other hand, only has UPPER CASE letters, numbers, and a few selected punctuation marks. Most DNC systems are set up to send ASCII Even parity, so any non-ISO character in your program can cause the Fanuc to give the TH alarm. Characters like these:
Any lower case letter.
These punctuation marks: !@,$^&_{}|\;"<>? and '
Notice that some punctuation marks are OK, like the forward slash (block delete), the asterisk (multiplication in a macro), and the decimal point are allowed. Others, like the double-quotation marks are not allowed, so any comment in the program like one of these will cause a TH alarm:
N001 (TOOL 2 - 1/2" DRILL) - the double quote mark is illegal
N001 (tool 2 = 1/2 inch drill) - the lower case letters are illegal
N001 G90 G00 X1,2345 - the comma character is illegal
If your program is all ISO 840 compatible, then you may have another communication problem, like a mis-match of the "stop-bits" setting. There is a parameter in all Fanuc controls for baudrate and "stop-bits". If the baudrate doesn't match the DNC system, then you won't have ANY communications at all. If the stop-bits setting doesn't match the DNC system, you might get occasional TH alarms even if the program is otherwise OK. If the stop-bits setting is mismatched, then you'll get TH alarms kind of randomly at different points in the program each time.
Originally Posted by mhykel 
PARITY 
