Serial data transmission period

[sayzer]

since you have a rtc and convert time to binary
why not

If (TimeM//5)==0 then
Hserout [dec TimeH,":",dec TimeM," ",DEC(TempC/10),".",#TempC dig 1,13,10]
ENDIF

This is one good way
It seems that when this condition gets true, then it will send data for the duration of whole 1 minute.
If you also add "Second" to this condition, it will also send the data for the duration of "1" whole Second.

So a better way is to insert a flag into it.

For example;

If (TimeM//5) = 0 and SendFlag = 0 then
  Hserout [dec TimeH,":",dec TimeM," ",DEC(TempC/10),".",#TempC dig 1,13,10]
  SendFlag = 1 
ELSE
  If (TimeM//6) = 0 then SendFlag = 0 
ENDIF