Serial data transmission period

[sayzer]

It is my bad.
(TimeM//6) may take another 2,3,4,5 minutes depending on the minute value

You may want use "Second". Example;

SendFlag var bit

Begin:

Sendflag = 0

Main:

If (TimeM//5) = 0 then 
  if RTCsec = 0 and SendFlag = 0 then
     Hserout [hex2 RTCHour,":",hex2 RTCMin," ",DEC(TempC/10),".",#TempC dig 1,13,10]
     sendflag = 1
  ELSE
     If RTCsec > 0 then SendFlag = 0 
  ENDIF
ENDIF

or another way could be to use DIG ;

SendFlag var bit

Begin:

Sendflag = 0

Main:

SELECT CASE TimeM DIG 0

  Case 0,5    ' Every 5 minutes.

     if SendFlag = 0 then
        Hserout [hex2 RTCHour,":",hex2 RTCMin," ",DEC(TempC/10),".",#TempC dig 1,13,10]
        sendflag = 1
    ENDIF

  case 1,6    ' Reset flag 1 minute later.
    SendFlag = 0 

END SELECT

This one looks cleaner for my eyes.