This is correct, the uplink message generated by the TCAS transponder is in mode C format, so it transmits on 1030 Mhz with a 21 microsecond delay between the P1 and P3 pulses. The mode A uplink format (only by ground stations in the civil world) is on the same frequency, however it has a 8 microsecond space in the between the P1 and P3 pulses. The reply from the transponder to the uplink message is on 1090 Mhz. Ground stations transmit alternate Mode A and Mode C uplink messages, TCAS uses a mode C uplink.
The format of the packet in response by the transponder to an uplink message is exactly the same regardless if it was a Mode A or Mode C uplink message.
So if the transponder received a Mode C transponder uplink message and pulses in the reply was
A1 A2 A4 B1 B2 B4 C1 C2 C4 D1 D2 D4
0 1 1 1 0 1 0 1 0 0 0 0
That would be decoded as 10,000 ft.
However if it was a Mode A uplink, and the transponder reply was
0 1 1 1 0 1 0 1 0 0 0 0
That means squawk code of 6520, exact same pulse format as 10,000 ft.
The transponder replies are the same, how the packet is decoded depends on what the ground station was expecting. In the civil aviation world, only ground stations use mode A uplink messages, so only ground stations expect squawk codes and decode the packets accordingly.
On the military side, there are lot more modes. Some military aircraft can uplink and decode squawk codes, this is known as military mode 3.
Thanks SWH that's a very good explanation of the technical aspects of transponder replies.
So, in pilot terminology, when a TCAS II mode S transponder shouts 'is anybody there' an obsolete mode A only transponder won't hear the call so won't respond whereas a nearly obsolete Mode A/C capable transponder with no altitude encoder will say 'I'm here but I've no idea of what my altitude is'?