IBM 3708 Description page 291

This conversion scheme results in, at worst, a two-for-one expansion. A much lower
expansion results by choosing the quadrants such that codes likely to appear
adjacent to each other in the data stream are in the same quadrant.
Protocol Enveloping Features
The 3708 monitors the data stream coming from the PC for framing and parity
errors. Checking for parity errors can be configured by the 3708's control terminal
operator. There are several possibilities:
• Parity and echoplexing configured off and the 3708 receives bad parity.
The 3708 transmits the character with the bad parity to the host. If
communication lines are relatively free of noise, this configuration generally
results in the fastest file transfers.
• Parity off and echoplexing on.
The 3708 transmits the character with the bad parity to the host and the
character with the bad parity is echoed back to the PC. Echoes can be used to
check the validity of the data received by the 3708 and can also be used to
prevent the overflow of the 3708's receive queue.
• Parity on and echoplexing off.
When parity checking is configured on, the 3708 replaces any character with bad
parity with a substitution character (defined at configuration for each port). The
substitution character is then sent to the host.
• Parity on and echoplexing on.
The 3708 transmits a substitution character to the host. The character with the
bad parity is echoed back to the Pc.
Error checking and recovery are the responsibility of the PC and host file transfer
programs.
Independent of the parity and echoplexing configurations, the 3708 also checks for
framing errors (that is, if a stop bit is expected but not received). If a framing error
occurs, the 3708 replaces the character with the parity substitution character and
sends the substitution character up to the host. If echoplexing is configured on, the
3708 echoes back the framing error to the Pc.
Again, any error recovery is exclusive between the PC and the host file transfer
programs. When retransmission of data is required, the PC must request the host to
retransmit the data. Likewise, the host needs to request that the PC retransmit any
data that the host program receives in error.
8-Bit Transparency
In protocol enveloping, the 3708 can be configured to support the transmission of
8-bit data to and from an SNA host. In this mode, the port should be configured
with no LTA characters. Data is then transmitted from the 3708 to the host after a
configurable text timeout period has expired. Although this means that the 3708
may spend time waiting for the text timeout, the advantages of configuring the 3708
this way include the ability to transfer 8-bit data (256 character sets) and to delete
any encryption requirement.
Also note that when configured for 8-bit data, the 3708 does not perform any parity
checking. Parity checking is strictly through an end-to-end protocol.
D-4
IBM 3708
Description