This will become quite complicated. There are many rules for the uses or SSR. Most SSRs found at many airfields across the UK that provide a Lower Airspace Radar Service generally has a 10-12RPM. Its slower than PSR and its uses are slightly different.
Sticking with SSR though. For changing the code its not really going matter if an emergency code flicks on for a revolution then off to the code requested. It happens all of the time. During the Olympics you wont get any visits from helicopters or fighter aeroplanes after such a very short period, but do double check that once set it is not set to one of the emergency codes.
I don't change to stby mode before changing as it is so quick, but it really doesn't matter for two reasons. Providing PSR is available, radar identification can be maintained so no need for either SSR number. But firstly, if you are requesting a service that requires radar identification then this should be done in good time: before entering the restricted zone or CAS or whatever. Mode 3A still needs to be validated, and mode C verified or switched off.
Second, if remaining on the 7000 you're mode C may not be verified at all by the ground unit. It's not required unless they issue a squawk in which case every attempt should be made to validate the 3A and verify C (or switch it off if out of limits). There are airfield specific conspicuity codes out there used daily, possibly everyone here has been issued a common Basic Service code (?). Other units will recognise these codes (they are all published somewhere in the AIP I think) but will treat them as unknown aircraft (i.e. 7000 squawks) with the benefit of knowing who to telephone to arrange a deal regarding reducing standard separation.
If you are asked to change squawks during a handover it is accepted that for the next few SSR revolutions mode C may not be visible. The first unit that issued you the code has verified your mode C and its not about to read incorrectly when you change the 3A. But then also you might be on a 'cleared' course/alt, or in reciept of a service that requires you to tell the ground unit you are doing something different, or request a change to what you are doing. Thence he drop out of mode C doesn't matter after the initial verification on identification (unless it fails, which will be very obvious on the ground, and is then switched off - a time when it is utmost important to tell ATC what you are doing vertically!).
Of course some of this changes if PSR isn't available... in which case switching to Stby to change code means being lost on the screen for 15 seconds - not nice in a busy place.
Last edited by GeeWhizz; 3rd Jan 2012 at 23:03.