IBM 3164 ASCII Manual page 92

When the IBM 3164 receives an inbound data stream from an auxiliary device,
the header/trailer counter does exactly the opposite of the above; it adds a
header and a trailer before sending the data block to the host and also adds a
DLE character after every DLE character.
The figure below shows the concept of an outbound passthrough operation.
Legend:
H: header (DLE.DC2)
T: trailer (DLEDC4)
3164
A data
C data
D data*
E data
Auxiliary
Device
Note:
If the auxiliary device has a passthrough capability, the D data is
passed to a lower auxiliary device.
Text format (main port)
The IBM 3164 sends either of the following fields to the host:
• A modified field with MDT on
• An unprotected field
• All fields.
Which type of field will be transmitted to the host depends on the "field data
transmission mode" set by the host command. At power-on reset (POR) this
mode is set to the default mode which is the "modified field data
transmission" mode. Then the fields with the MDT bit set to on will be sent
to the host. If this mode is changed to the "unprotected field data
transmission" mode by the Enable Unprotected Field Data Transmission
command issued by the host, then all the unprotected fields will be sent. The
Disable Unprotected Field Data Transmission command must be issued by the
host system to set the IBM 3164 to the default (modified field data
transmission) mode again. The Read All command issued by the host sends
all fields to the host.
If the transmission of character attributes is enabled, the Set Character
Attribute response is sent to the host prior to the affected character whenever
a character attribute is changed.
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