I am sorry for my late reply and thanks for your answer. But I get another question about. What do you mean, that the spike may arrive earlier at one synapse than at another, although the first one has a greater delay? I will give a example to show my question clearly.
Here is a situation I am involved. Supposed there are two neurons named A and B, and they are connected to neuron C using stdp_synapse. During one simulation, A and B may both activate many times. Also, I suppose that A send two spikes named t1 and t2, and B send three spikes t3,t4 and t5, so my question is how you calculate the A-C weight or B-C weight according to STDP rule.? As you said, if there is a synapse, I can easily understand how the stdp_synapse work or calculate, but when there are synapses, I am confused.
Thanks for your answer again! And if I didn't make my question clearly I can explain more clearly.