Are the racers racing each other, or the clock?
more than one rider at a time may reach the finish line. Lining to to scan RFID tags to stop the clock hardly seems like a viable solution.
How is sending the switch state going to prove useful? What you need to send is the RFID tag and the time that the switch was triggered
The end time will be defined when the rider scans his/her tag again
Getting them to not pass each other on the course, and to line up neatly at the finish line will be the biggest challenge.
Can you tell me if it is possible to connect a real time clock and an Xbee to an Arduino uno at the same time as they would both require the serial connection?
Sure it is, and no they don't. I'm not aware of any RTC with a serial interface.
If I have a real time clock module in the start Arduino and a real time clock module in the finish Arduino can I get them to automatically sync time with each other?
No. On the other hand, you don't need to. Scan a tag. Send the data. The receiver time stamps the receipt of the tag, defining the start time. Scan a tag on the other end to record the end time for that tag.
For simplicity's sake would it be ok to assume that there is negligible delay from when the start gate is triggered and when the receiving Arduino is actually aware that it was triggered?
At the very least would this delay be consistent for every rider throughout the day?
What constraints are you trying to meet?
What sort of distance will there be between the start/end points?
How tolerant will you be of false/missing results?
Does it need to be tamper-proof?
Can you get away with simply banning overtaking?
Is there anything stopping you having a human element involved in the normal operation of this?
Depends on how you actually write the code. The delay can be anywhere from a few hundred nanoseconds to days.The delay can be small enough to be considered insignificant. If the delay is on the order of microseconds or less, I doubt that the delay will be observable in the measured time with respect to the actual time. Certainly the displayed value of the measured time could have more variation (between the measured time and the displayed time) than the actual delay values.