When does command 108 change the BACK message?

When in burst mode a BACK must immediately follow every ACK from the burst-mode device.  

Command 108 response(ACK) must indicate the new Command number specified to be burst.  So from a strict application of Protocol requirements, the BACK immediately after the Command 108 ACK must contain the new Command.

STX cmd 108 to change burst command

ACK cmd 108 confirms burst command changes

BACK of the command number from the STX/ACK of cmd 108

However, the test specification recognizes that a burst message may already be queued in the field device.  Consequently the test allows one additional BACK to not contain the new Command response.  i.e., The tests recognize that the device, from a practical point of view, may not have the processing power to immediately generate and burst the new Command number.  However the new command number must be in all subsequent BACKs.

BACK of command X

STX cmd 108 to change burst command to Y

ACK cmd 108 confirms burst command changes to Y

BACK of command X

BACK of the command Y from the STX/ACK of cmd 108

Most devices are able to burst the new Command in the BACK that immediately follows the Command 108 ACK.