[I believe that this is part of 87-154 Rev ? -- John Lohmeyer]

=========
place on p. 5-21 of SCSI-2 R2 in place of the existing text after Table 5-6 
=========

A pair of SYNCHRONOUS DATA TRANSFER REQUEST (SDTR) messages (Table 5-6) are 
exchanged between two devices on a bus whenever an SCSI device that can 
support synchronous data transfer recognizes that it has not communicated with 
the other SCSI device since receiving the last "hard" RESET condition or a BUS 
DEVICE RESET message.  The SCSI devices may also exchange messages to 
establish synchronous data transfer when requested to do so.   The message 
exchange establishes the permissible transfer periods and the REQ/ACK offsets 
for all logical units on the two devices.

The transfer period is the minimum time allowed between leading edges of REQ 
pulses and of successive ACK pulses to prevent device reception buffer 
overflow.

The REQ/ACK offset is the maximum number of REQ pulses allowed to be 
outstanding before its corresponding ACK pulse is received at the target.  
This value is chosen to prevent device reception buffer overflow.  A REQ/ACK 
value of zero shall indicate asynchronous mode; a value of FFh shall indicate 
unlimited offset.

The device that originates the SDTR exchange is called the originator and the 
responding device is called the respondent.  The originator sets the values in 
its SDTR message to its 'highest performance' values (minimum transfer period 
and maximum offset) that the originator can receive successfully.  The 
respondent then replies with its SDTR message specifying the larger of either 
the originator's specified transfer period or the shortest transfer period 
that the respondent can successfully receive.  The respondent's SDTR REQ/ACK 
offset is the smaller of the originator's specified offset or the respondent's 
maximum allowable offset.  Both the originator and the respondent then shall 
not either transmit at a transfer period smaller or an offset greater than 
received from the other's SDTR.  No specification is made that each device 
specifying its maximum performance values will necessarily transfer at these 
values, but only that it can receive data with those values.  Each device is 
free to transfer data at rates less and offsets less than that specified. 

Respondent SDTR Values                   Implied Agreement
---------------------------------------  --------------------------------------
(1) Non-zero REQ/ACK offset.             Each device transfers data at period 
                                         equal to or greater and offset equal
                                         or less than the values received    
                                         from the other device.
                                           
(2) REQ/ACK offset equal to zero.        Asynchronous transfer.
(3) MESSAGE REJECT                       Asynchronous transfer.

If the initiator recognizes that negotiation is required, it asserts ATN and, 
if the target implements message transfers, sends a SDTR message to begin the 
negotiation process.  After successfully completing the MESSAGE OUT phase, the 
target responds with the proper SDTR message.  If an abnormal condition 
prevents the target returning one of the responses above, the initiator and the 
target shall go to asynchronous transfer mode.

The above implied agreement for synchronous operation shall be considered to 
be negated by both devices if the initiator asserts ATN after the respondent's 
SDTR message and the first message out is either MESSAGE PARITY ERROR or 
MESSAGE REJECT and the initiator and the target shall go to asynchronous data 
transfer between the two devices.  For the MESSAGE PARITY ERROR case, the 
implied agreement shall be reinstated if a retransmittal of the respondent 
SDTR message is successfully accomplished.  If, after a target-specific-
maximum number (greater than zero) of retry attempts, the target still 
receives a MESSAGE PARITY ERROR message from the initiator after resending its 
SDTR message, the target shall terminate the retry activity either by changing 
to any other information transfer phase and transferring at least one byte, or 
if necessary, by releasing BSY (see 5.2.2 Disconnect).  The initiator shall 
accept such action as aborting the negotiation and both the initiator and the 
target shall go to asynchronous transfer mode for transfers between the two 
devices.

If the target recognizes that negotiation is required, it sends a SDTR message 
to the initiator with appropriate values.  Prior to releasing ACK on the last 
byte of the SDTR message from the target, the initiator shall assert ATN and 
shall respond its SDTR message as defined above.  If an abnormal condition 
prevents the initiator returning the responses defined above, the initiator 
and the target shall go to asynchronous transfer mode for transfers between 
the two devices.

==== the last four paragraphs on p 5-23 of SCSI-2 Revision 2 follow ====