- BSC supports half duplex and therefore allows point-to-point or multipoint circuits.
- all transmission is between the control station and one of the tributary stations.
- the 2780/3780 version of Bisync only allows point-to-point operation and in this case either end can 'bid' for the line by sending a poll, collisions are delt with by having different timeouts at each end.
- In the 3270 version of Bisync the tributary stations cannot initiate transmission until polled.
- There are 2 types of poll:
Polling = invitation to send
Selection = request to receive
ie, if the control station has somthing to send, it will first send a
'select' otherwise it will keep sending polls.
In the most general case there could be several 3270 control units on a line
each with several screens, so the polling message includes both the address
of the CU (control unit) and the address of the screen on that CU.
Both the poll and the select consists of the CU address followed by the device address followed by ENQ, both these addresses are sent twice because polling and selection is not error protected. When the control station has nothing to send it might poll with a general poll - 7F(EBCDIC) or 22(ascii) is used in place of the device address to invite any device to transmit.
Polling and selection are discriminated by using different CU addresses -
see CU and device addressing table.
so a 3270 poll is as follows,
3274 poll address
3274 poll address
14 - Polling
Host invites terminal to send
15 - Selection
The Select is the same as the poll in every respect except the address used,
see address table.
Host requests to send
CU and device addressing table
|Addresses for: device selection|
|CU poll||CU selection|
|Fixed return||Test Request|
|device or control unit No||char (hex)||EBCDIC (hex)||ascii||char (hex)||EBCDIC||ASCII (hex)|
goto next document - practical IBM1