AM3270/TR3271 Reference Manual

What are Bisync Protocols?

Bisync Protocols—Concepts and Terminology

1–2 086705 Tandem Computers Incorporated

Figure 1-1 compares bisync and async character framing.

Figure 1-1. Two Methods of Character Framing

001

1 2 3 4 5 6 7 8

Stop

Character Start

Asynchronous (Start/Stop)

Synchronization Window

Bisynchronous

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Char Buffer

SYN

Standard Bisync

Communication—

How It Works

In bisync protocols the bit and character synchronization occur at the beginning of

each message. There are five standard steps needed to synchronize the sending and

receiving devices in a bisync transmission:

1. Bit Synchronization:

The modems or other data communications devices at either end of the circuit must

achieve bit synchronization. This can be done using various clocking methods.

To be sure that the first and last characters of a transmission are sent correctly, some

older bisync devices add padding (or pad) characters before the first SYN character

and after every BCC, EOT, and ACK. Two pad characters, sent before each message,

allow the devices at either end to achieve bit synchronization. Pad characters are

usually either hex AA or hex 55, but can be any character other than SYN.

AM3270 and TR3271 send trailing pads.

2. Character Synchronization:

A sequence of at least two SYN characters are then sent to synchronize characters

in the DTE link interface equipment. The bit stream is searched for the pattern of

bits that identifies the SYN character. To avoid identifying a false SYN, most

systems require transmission of two SYNs in a row, called a SYN sequence.

In EBCDIC the SYN character is hex 32 (or b’0011 0010’), and in ASCII it is hex 16

(or b’0001 0110’).