Knowledge Base /
Technical Notes
PLC IO Interface for AB 1771 RIO Tieback Response Timing
By Geoff Nash
Product: PLC IO Interface

This technical note shows the IO update performance of the PLC IO Interface for AB 1771 Remote IO to a DeltaV Serial Card and also to the VIM Network Gateway.

VIM Interface Tieback timing (w/15 racks of IO)

This graph was produced using a customer database with the VIM ModbusTCP interface and PIO flash version 1.2.10. Mimic was used to simulate the DeltaV IO cards. AB 1771 RIO simulator running on a PC (this provides the same update process and interval as actual IO).

DeltaV
Version 7.3
 
 
 
 
 
 
Free time
26
 
Serial Card Port
 
 
AB RIO Racks
15
 
Scan
20
ms (10-40)
 
Other DeltaV IO Modules
30
 
COS Gen rate
3
 
 
 
 
 
 
 
 
Tieback Timing
Average
STD
N
Min
Max
 
 
435
48
1503
300
561
 

 

Most tieback time’s fell slightly below 400 or 500 ms. Base time is based on the DeltaV module scan rate (and order), VIM update rate, and RIO bus scan rate. Variation was due to PC tieback process and communications interval variations.

 

DeltaV tieback module read IO from a serial card input “landing” module and wrote the outputs to another serial card output “landing” module. This requires three DeltaV modules; each is running at 100 ms. Input GESTERC17-6-1-DI/I-060-3 maps to rack 15 DI 3, GESTERC17-6-1-DO/O-062-4 maps to rack 15 DO 4.  The tieback is monitored in the simulation PC rather than using the timing functions in this module.

 

The response time variation is based on four asynchronous process scans:

 

  1. DeltaV module scan rate, 3 modules each running at 100 ms (300 ms total).
  2. VIM (rate running at about 10-30 ms for 15 racks.)
  3. VIM to PIO module (10 MB Ethernet connection)
  4. RIO bus communications (about 105 ms for 15 racks)

Grouping of tieback time is significantly better with ModbusTCP (435+-48 n= 1503 [range 300-561]) than RS485 (943+-156, n=5659 [range 481-1482]) with the Digester database. There are two visible bands of tieback timing, the second band is due to the instances where the asynchronous updates to the DeltaV miss an input scan and therefore are processed 100 ms latter.

RS485 Serial Card Interface Tieback timing (w/15 racks of IO)

This graph was produced using a customer database with the DeltaV programmable serial card (series 2) interface and PIO flash version 1.1.17. Mimic was used to simulate the DeltaV IO cards. AB 1771 RIO simulator running on a PC (this provides the same update process and interval as actual IO).

DeltaV
Version 7.3
 
 
 
 
 
 
Free time
26
 
Serial Card Port
 
 
AB RIO Racks
15
 
Scan Port 1
282
ms (270-300)
 
Other DeltaV IO Modules
30
 
Scan Port 2
253
ms (150-260)
 
 
 
 
 
 
 
Tieback Timing
Average
STD
N
Min
Max
 
 
943
156
5659
481
1482
 

 

DeltaV tieback module read IO from a serial card input “landing” module and wrote the outputs to another serial card output “landing” module. This requires three DeltaV modules; each is running at 100 ms. Input GESTERC17-6-1-DI/I-060-3 maps to rack 15 DI 3, GESTERC17-6-1-DO/O-062-4 maps to rack 15 DO 4. The tieback is monitored in the simulation PC rather than using the timing functions in this module.

 

 

 

  1. DeltaV module scan rate, 3 modules each running at 100 ms (300 ms total).
  2. DeltaV serial card (rate running at about 300 ms for 7 racks on each port.)
  3. Serial card PIO module scan (500K per port, 2 ports)
  4. RIO bus communications (about 105 ms for 15 racks)

The speed and grouping of tieback time is significantly worse than with ModbusTCP (943+-156 n= 5659 [range 481-1482]) versus (435+-48 n= 1503 [range 300-561]). Five distinct bands of group values are visible, these represent the various permutations of the 4 asynchronous process scans that make up the values.

VIM Interface Tieback Time vs IO racks

Compare Tieback time to number of IO racks communicating. IO was simulated on a PC using a RIO card. This provides the same update process and interval as actual IO. The test was run with the RIO bus set to either 115K or 230.4K. All racks were processed on one VIM, all serial card datasets configured on one port (2) under one device. The other port was disabled. With ModbusTCP tieback time is independent of number of racks from 1 to 6 (10 at 230.4K), while with the serial card there is a direct correlation between number of racks and length of average time. With ModbusTCP connection, the maximum ranges from about 200 to 400 ms, while with RS485 this starts at 200, but climbs to 1200 ms.

Card
Port
DS
Description
 
57
2
1
Global status
 
 
2
BT1-2
 
 
 
3
DI Racks 0-9
 
 
4
DO Racks 0-9
 
 
5
DI Racks 10-19
 
 
6
DO Racks 10-19
DS 5&6 were added when racks 10-19 were added

 

 

 
RIO = 115K
RIO = 230.4K
Racks
Mean
Std
Mean
Std
1
200
12
200
12
2
199
16
199
16
3
200
16
200
16
4
201
20
201
20
5
202
25
202
25
6
202
26
202
26
7
212
30
212
30
8
242
42
242
42
9
242
35
242
35
10
262
47
262
47
11
299
25
299
25
12
296
40
296
40
13
309
45
309
45
14
321
47
321
47
15
317
43
317
43
16
334
43
334
43
17
345
38
345
38
18
352
42
352
42
19
369
37
369
37
20
389
29
389
29

 

VIM Interface Tieback Time vs IO racks (two ports)

Compare Tieback time to number of IO racks communicating. IO was simulated on a PC using a RIO card.This provides the same update process and interval as actual IO. All racks were processed on one VIM, all serial card datasets configured on two ports under one device each. Started with all datasets configured, removed datasets as no longer required for rack access. Disabled port when it was no longer necessary. Did not include Global or BT datasets.

Card
Port
DS
Description
 
57
1
1
DI Racks 0-9
 
 
2
DO Racks 0-9
 
 
3
DI Racks 20-29
 
 
4
DI Racks 30-31
 
2
1
DI Racks 10-19
 
 
2
DO Racks 10-19
 
 
3
DO Racks 20-29
 
 
4
DO Racks 30-31

 

Tieback Time (n=200)

 

Racks
Mean
Std
1
187
33
2
195
23
3
196
20
4
198
19
5
199
21
6
200
24
7
200
21
8
204
26
9
215
34
10
217
40
12
279
32
14
270
45
16
276
46
18
301
51
20
331
58
24
356
73
28
419
86
32
494
99
 

 

Serial Card interface

 

Compare Tieback time to number of IO racks communicating. IO was simulated on a PC using a RIO card. This provides the same update process and interval as actual IO. All racks were processed on one DeltaV programmable serial card (series 2), all serial card datasets were configured on two ports (running at 500K) under one device each. Started with all datasets, removed datasets as no longer required for rack access. Disabled port when it was no longer necessary. Did not include Global or BT datasets.

DeltaV
 
 
 
PIO
 
 
Processor Free Time
 
100
 
 
Process Time
4
Port Scan Time
Avg
23
 
 
Ports
2
 
Max
30
 
 
 
 
 
Min
10
 
RIO
 
 
Poll Rate
 
41 / sec
 
 
Baud
115
 
Card
Port
DS
Description
 
57
1
1
DI Racks 0-9
 
 
2
DO Racks 0-9
 
 
3
DI Racks 20-29
 
 
4
DI Racks 30-31
 
2
1
DI Racks 10-19
 
 
2
DO Racks 10-19
 
 
3
DO Racks 20-29
 
 
4
DO Racks 30-31

 

Tieback time (ms, n=200)

 

Racks
Mean
Std
 
Racks
Mean
Std
 
1
188
28
 
12
705
156
 
2
203
31
 
14
803
212
 
3
257
40
 
16
899
226
 
4
289
44
 
18
897
198
 
5
347
51
 
20
909
214
 
6
387
48
 
24
1027
248
 
7
382
91
 
28
1050
199
 
8
485
71
 
32
1244
313
 
9
529
105
 
 
 
 
 
10
586
117