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Bisync Protocols—Concepts and Terminology
Bisync Procedures
For the EBCDIC code set, error checking is based on a cyclic redundancy check (CRC),
and is called CRC-16. The CRC is a division performed by both the sending and
receiving stations. The numeric binary value of the message is divided by a constant,
the quotient is discarded and the remainder is transmitted as a two-byte block check
character sequence.
For the ASCII code set, error checking is based on a vertical redundancy check (VRC)
and a longitudinal redundancy check (LRC). The VRC checks for odd parity on a
per-character basis. The LRC checks for odd parity on a line-by-line basis, bit by bit
within a block. The LRC value becomes the one-byte block check character.
Polling and Selecting
When a bisync connection is established in a multipoint link, the connection is still
bound by the standard bisync one-message-at-a-time, one-way transmission
requirements. To accomodate multi-station links, the conventions of polling and
selecting were created to organize the requests to receive and send data from stations
on the same link. The poll and select routines are sometimes considered to be a kind
of line control, with the control character sequences considered to be the line protocol.
Point-to-Point Connections
A point-to-point link connects one tributary station to a supervisory station. Such a
link may be established on a dedicated or dial-up line, and may be either half- or full-
duplex. Even if the connection is full-duplex, the bisync transmission must still be one
way at a time.
Some bisync protocols support point-to-point connections, where a supervisory station
is linked to only one tributary station. However, both AM3270 and TR3271 are
multipoint protocols, and do not use point-to-point connections.
Multipoint Connections
A multipoint link connects more than two tributary stations at a time to a supervisory
station, and therefore requires some way to control which station is using the link to
send a message at any one time. One station must be designated as the supervisory
(or control) station, and the others are designated as tributaries. These are sometimes
referred to as “master” and “slave” stations. Each station on the link has its own
unique address, and some devices such as printers may have a sub-address.
The supervisory station decides which station uses the link by a method of polling and
selecting. While two stations are sending and receiving, all other stations on the line
are in a passive receive mode.
Poll and Select Control Sequences
These are the basic control sequences for polling and selecting. Note that an ENQ
would be preceded by the usual SYNs and the four-character controller device
sequence. The polling or selection may also be preceded by an EOT, which in effect
clears the logical link between the two stations in preparation for transmitting. Refer
to Appendix A for a table of poll and select addresses.
086705 Tandem Computers Incorporated
1–9
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