Once you have gotten thru the handshake, the INIT messages are asynchronous, so it possible for both master and slave to send the first INIT message at the same time. It just that both have to wait for the INITA message before they can send the INITB and so on. The following are the messages I get when talking to a ATT 3B2 system. who message control byte values send $10 $09 $71 $aa $39 $eb 0,7,1 get $10 $09 $6f $2a $3b $77 0,7,3 get $10 $09 $79 $aa $31 $6b 0,6,1 send $10 $09 $79 $2a $31 $eb 0,6,1 get $10 $09 $7f $2a $2b $77 0,5,3 send $10 $09 $81 $aa $29 $0b 0,5,1 The message format is byte 0 - synch byte always $10 byte 1 - buffer size, with protocol g, its either 2 or 9 (control) bytes 2 and 3 - are checksum byte 4 - control byte byte 5 - xor of bytes 1,2,3,4 The format of the control byte is bits 0 and 1 - packet type, 0 - CTRL, 2 - LNGDAT, 3 - SHTDAT bits 2, 3 and 4 - for CTRL packets, the type of CTRL packet 1 - CLOSE 2 - RJ 3 - SRJ 4 - RR 5 - INITC 6 - INITB 7 - INITA bits 5, 6 and 7 - for INIT type, size of INIT, for RJ, SRJ and RR, seqnum of message. So, the message you are getting is right (but not the checksums). It is a CTRL packet with a INITA, value of 3 (remember that the value is the last three bits, not four). Carl Fosler Intermetrics, Inc. Bethesda, MD. 301-657-3775. fosler@inmet.inmet.com