IPDS for Control Units
Other Asynchronous/Synchronous Sequences in DSC
(Channel-Attached) Mode
The following charts show what can happen if the host does not issue a Read
Modified immediately after an Attention is received from the printer. These
sequences are not recommended.
Asynchronous NACK Sequences in DSC (Channel-Attached) Mode: If the printer
returns an Intervention Required (IR) or Equipment Check (EC) NACK, the NACKs
are asynchronous.
Note: In some programming environments, asynchronous NACKs may not be
desired. To force all NACKs to be returned synchronously, set the ARQ bit in the
IPDS flag byte to 1 in every command. IR or EC NACKs must be handled as if
they are asynchronous, even if the IR or EC NACKs are returned as a result of
ARQ equals 1. In DSC mode, the application must send each command as a
separate message. If the application forces synchronous NACKs, printer
processing slows.
In Figure 38, the host sends data after receiving an Attention from the printer.
184 6400 IPDS
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