IBM 3164 ASCII Manual page 91

Chapter 5. Programming considerations
Passthrough operation
5-4
The IBM 3164 has a capability to by- pass data sent from a host system to an
auxiliary device (outbound data) and to by-pass data sent from an auxiliary
device to a host system (inbound data) by using the Auxiliary Input Data
option of the Set Control 6 command. This is called a passthrough function
and to use it the word lengths on the main and the auxiliary ports should be
set to the same value. The figure below shows the concept of the outbound
and the inbound passthrough operation.
Inbound
111
Data Stream V
I
Inbound •
Data Stream
I
\ 11
Outbound
V
!
Data Stream
The IBM 3164 has a header/traiier counter for controiiing the passthrough
data stream. When the counter detects a first header (OLE.OC2) in the
outbound data stream, it deletes the header and sets the counter to 1. It then
passes the following data block which may contain headers and trailers
(OLE.OC4) to an auxiliary device, increasing the counter number by 1 each
time it detects a header and decreasing by 1 each time it detects a trailer. The
IBM 3164 stops the data transmission to an auxiliary device when the register
number becomes O. The header/trailer counter also checks for double entry
OLE characters in the outbound data stream and automatically deletes every
second OLE character it encounters. Consequently, to send a OLE character
to the first auxiliary device, two OLE characters must be sent from the host in
the passthrough data block and four OLE characters for a second auxiliary
device.
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