iso14443 atqa

Hi,
I'm trying to get the uid of a mifare S50 tag with my S6700 transceiver.
I send REQA command and I get the right ATQA (and matches with Mifare 1k card).
But when I send the first SEL for anticollision

S1
0x4E // command for S6700
0x93 // select
(1) // parity bit
0x20 // 0-bit length
(0) // parity bit
ES1
tran1
....wait data....
tran2

I get no answer.... Somebody has found this problem before? What I'm doing wrong?

Thanks for any hint...