Telecommunications - Bi Sync Protocol - Polling, Selection and Addressing

 

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,

PAD

SYN

SYN

3274 poll address

3274 poll address

device address or 7F(EBCDIC) or 22(ascii)

device address or 7F(EBCDIC) or 22(ascii)

ENQ


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)
0 sp 40 20 - 60 2D
1 A C1 41 / 61 2F
2 B C2 42 S E2 53
3 C C3 43 T E3 54
4 D C4 44 U E4 55
5 E C5 45 V E5 56
6 F C6 46 W E6 57
7 G C7 47 X E7 58
8 H C8 48 Y E8 59
9 I C9 49 Z E9 5A
10 c 4A 6A
[ 5B 7C
11 . 4B 2E , 6B 2C
12 < 4C 3C % 6C 25
13 ( 4D 28 - 6D 5F
14 + 4E 2B > 6E 3E
15 4F ? 6F 3F
! 21
16 & 50 26 0 F0 30
17 J D1 4A 1 F1 31
18 K D2 4B 2 F2 32
19 L D3 4C 3 F3 33
20 M D4 4D 4 F4 34
21 N D5 4E 5 F5 35
22 O D6 4F 6 F6 36
23 P D7 50 7 F7 37
24 Q D8 51 8 F8 38
25 R D9 52 9 F9 39
26 ! 5A : 7A 3A
] 5D
27 $ 5B 24 # 7B 23
28 * 5C 2A @ 7C 40
29 ) 5D 29 ' 7D 27
30 ; 5E 3B = 7E 3D
31 - 5F "1 7F 22
^ 5E

 

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