# ISO 14443 Type B REQB fails to cause a response

Am I performing a reasonable REQB? What might be going wrong?

In attempting to perform my 1st ISO 14443 Type B communication with a bit-banging RF interface I do not receive any response. Same circuit works well with ISO 15693, ISO 144443 Type A. I've years of experience with these other kinds so I do not expect I've have a trivial problem like a bad tag, too far away, etc.

Outgoing command

1) 5 ms RF (no modulation)
2) SOF (12 etus 0 0000 0000 011) (0 is modulation. ETU 128/13.56 MHz or about 9.44us each)
3) 05  (10 etus 0 1010 0000 1) REQB
4) 00  (10 etus 0 0000 0000 1)
5) 00  (10 etus 0 0000 0000 1)
6) 71  (10 etus 0 1000 1110 1) CRC lsb
7) FF  (10 etus 0 1111 1111 1) CRC msb
8) EOF (10 etus 0 0000 0000 0)
9) 5 ms RF (no modulation)


Sometime during sequence #9, I'd expect to see the tag respond, but no modulation is noticed.

Tag: MAXIM MAX66040K-000AA+

RF outgoing no modulation 16.1 V p-p.
RF outgoing modulation 12.5 V p-p.
Modulation index = 13%

Maybe MAX66040K does not work with AFI of 0?
I've tried additional delay after SOF (2 to 4 etu).
I think the CRC is correct and sent in the correct byte order.

Supplied own solution.

Further research implies the modulation depth (or bit on/bit off time is just a little skewed) and that is causing the tag to respond or not. Thus the command sequence was OK, just that the modulation was not quite formed correctly.

Circuit is be a bit more challenging to work with Maxim tags than other protocols.