FC 22 - M/A Station (Cascade)

The M/A station (cascade) is one of three station functions (basic, cascade, and ratio) that provides an interface between a controller module and the digital control station (DCS), analog control station (SAC), operator interface station (OIS) and management command system (MCS).

 

The cascade station provides all the basic functions plus an additional mode that allows the set point to be controlled by another function block. When the station is in cascade mode, the value found in the block specified by <S2> will be the set point. In the basic (non-cascade) mode, the set point signal is equal to the value set by the operator.

 

Other options include a control output override input <S5> and a manual interlock input <S6>.

 

The control output override forces the control output value to track <S4> (e.g., when immediate action is required to maintain process safety).

 

The manual interlock may be used to hold the station in manual mode, which allows conditional logic to be used to prevent automatic operations.

 

 

 

Outputs:

Blk

Type

Description

N

R

Control output (CO) in percent (0% to 100%)

N+1

R

Set point in engineering units

N+2

B

Mode:

0 = manual

1 = automatic

N+3

B

Level:

0 = local

1 = computer

N+4

B

Station mode:

0 = basic

1 = cascade

N+5

B

Computer backup indicates computer time-out:

0 = no

1 = yes

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

0 - 255

Block address of process variable

S2

N

5

I

0 - 255

Block address of set point track value

S3

N

5

I

0 - 255

Block address of auto signal or input

S4

N

5

I

0 - 255

Block address of control output track signal

S5

N

0

I

0 - 255

Block address of control output track switch:

0 = normal

1 = track

S6

N

0

I

0 - 255

Block address of manual interlock:

0 = release

1 = manual lock

S7

Y1

100.000

R

Full

Process variable high alarm point in engineering units

S8

Y1

0.000

R

Full

Process variable low alarm point in engineering units

S9

Y1

4.0 E6

R

Full

Process variable set point deviation alarm point in

engineering units

S10

N

100.000

R

Full

Signal span of set point and process variable

S11

N

0.000

R

Full

Zero value of process variable

S12

N

0.000

R

Full

Zero value of set point

S13

N

0

I

0 - 255

Engineering unit identifier (*for console use only)

S14

Y

0

I

0 - 255

Set point track option:

0 = no

1 = process variable <S1>

2 = <S2>/manual only

3 = <S2>/manual and auto

S15

N

0

I

0 - 255

Computer time-out option:

0 = computer auto/manual mode unchanged

1 = computer manual

2 = computer auto

3 = computer cascade/ratio

4 = local auto/manual mode unchanged

5 = local manual

6 = local auto

7 = local cascade/ratio

S16

N

0

I

0 - 255

Control station address:

>15 = no station

 

NOTE:

1.   S7, S8 and S9 are tunable but not adaptable.

 

 

 

22.1 Explanation

 

Specifications

Spec #

Name

Description

S1

PV

Block number whose output is the process variable. This

input drives the digital station process variable indicator (in

engineering units).

S2

CASCADE

Specification S2 is the block address of the cascade input.

The output of this block is the set point when the station is in

cascade mode or set point track is enabled (see S14).

S3

COAUTO

Block number whose output value is the control output when

the station is in auto mode (usually the output of a PID block).

S4

COTRCK

Specification S4 is the block number whose output is the

control output when the station is tracking (control output

override mode), and is the initial value for the control output

on start-up.

S5

COFLG

Block  number  whose  output  value  determines  whether  the control output is to be tracked. If the output of the block is:

0 = no tracking (normal)

1 = track <S4>

S6

REMLOCK

Block number whose output value determines whether the

station locks in manual mode.

0 = no

1 = yes

S7

PVH

Engineering unit value of process variable at which a high

alarm will be generated and displayed.

S8

PVL

Engineering unit value of process variable at which a low

alarm will be generated and displayed.

S9

PVDEV

Engineering unit value of allowed deviation between process

variable and set point. A high deviation alarm occurs when

the process variable is greater than the set point and the

value of the difference between the two is greater than or

equal to this specification. A low deviation alarm occurs when

the process variable is less than the set point and the value of

the difference between the two is greater than or equal to this

specification. These two alarm conditions only report via an

exception report (not reported to a DCS or SAC station).

S10

EUSPAN

Value corresponding to engineering unit span for process

variable and set point.

S11

EUPVZ

Value corresponding to engineering unit zero for process variable.

S12

EUSPZ

Value corresponding to engineering unit zero for set point.

S13

EUID

Engineering units type.

S14

TRACKSP

Value that determines which input the set point will track in

manual mode. Selecting three causes set point tracking in

both manual and auto modes.

0 = no tracking

1 = track PV <S1> (manual mode)

2 = track CASCADE <S2> (manual mode)

3 = track <S2> always (auto and manual)

S15

COMPDEF

Value that determines the system default mode if the computer

times out while the loop is under computer control.

0 = computer auto/manual mode unchanged

1 = computer manual

2 = computer auto

3 = computer cascade/ratio

4 = local auto/manual mode unchanged

5 = local manual

6 = local auto

7 = local cascade/ratio (unused in function code 21)

S16

DIGSTAAD

Value of the hardware digital station address that the station

function block is representing. A number greater than 15

indicates no hardware station. When using a loop command

module, address 15 indicates a second displayed station.

When using a DCS or SAC station, valid addresses are zero

through seven.

 

 

22.2  Station output example:

 

A basic standard station is assigned to block number 30. The system then assigns the next five block numbers as listed in Table 22-1. If there is a need to assign more block numbers after the station block, six must be added to the stations block number to obtain the next available block number.

 

In this manner, each output of a station block has a unique reference number.

 

Table 22-1 - Station Output Description

 

Block Number

Station Output

30

CO (Control Output)

31

SP (Set Point)

32

A (Mode)

33

C (Level)

34

B (Station Mode)

35

CF (Computer Backup)

 

22.3  Station Operating States

 

This station function block acts as an interface between a controller module and its associated control station, operator interface station, management command system, or a computer through a network interface unit. The operating state of the station block is its level (local/computer), mode (manual/auto) and the station mode (basic/cascade/ratio). This operating state determines which outputs may be adjusted from the operator interface devices. The modification of the operating state and the block output is determined by the block input and messages received from associated interface devices.

 

22.4  Control Output Tracking

 

When the control output track state <S5> equals one, it does not affect the flag output of the block, but will override the current control output with the value in the block specified by <S4>. The actual operating state is saved, and will be restored when the track flag input goes to zero.

 

In the local manual and computer manual states, the control output is adjusted from the interface device unless a connected control station indicates that the analog output for the control is bypassed. In this case, the control signal comes from the control station bypass and the station tracks the control station control output signal.

 

While the station is under local manual control, the output is zero to 100 percent. If the input to the station is a calculation function, the input value must be scaled to percent, and the output of the station scaled to the required value, external to the station.

 

When the manual interlock state <S6> equals one, the station trips to the manual mode.

 

 

22.5  Set Point Tracking

 

The set point may be adjusted from the operator interface device only if set point tracking is not selected (zero for S14).  If one of the tracking options is selected, set point control will be determined by the option chosen.

 

22.6  Examples

 

S2

The number of the block containing the output that the set

point tracks in manual mode.

 

 

S14

Set point track option:

 

0 = no tracking. Set point can be varied using local manual control.

1 = track PV <S1> when station is in manual. Provides bumpless transfer when the station returns to automatic   mode; set point cannot be varied manually.

2 = track value in block selected in <S2> in manual mode only. Set point may be changed when the station returns to automatic mode. A bump is possible when transfer occurs.

3 = track value in block selected in <S2> always (auto and manual); no direct control of set point from station

 

 

 

 

22.7  Deviation Alarms

 

A high deviation alarm occurs when the process variable is greater than the set point, and the absolute value of the difference between the two is greater than or equal to S9. A low deviation alarm occurs when the process variable is less than the set point, and the absolute value of the difference between the two is greater than or equal to S9. These two alarm conditions report to the control station or the loop for display on a console.

 

Figure 22-1 illustrates cascade control of an input using the control output of a basic manual/auto station as the cascade input <S2> to a cascade manual/auto station.

 

NOTES:

  1. While a DCS or SAC station (in cascade operation) is under local manual control, the output is zero to 100 percent. Therefore, if the process variable input to the station is a calculation function, the value must be scaled to percent and the output of the station scaled to the required value, external to the station. The control output bar graph on a DCS or SAC station can be driven by the station output or by a feedback signal. This applies to the first two stations connected to a given termination unit. Stations connected by daisy chain do not have this option.

  2. The station can only be transferred to computer operation when there is a computer available through a network interface unit. Depressing the computer pushbutton on the station will not provide a usable output.

  3. When a station tag number is referenced in a trend block in an operator console, the process variable input is trended, not the output of the station block N. When the station is in cascade mode, the set point signal is equal to the cascade signal. In the automatic mode, the set point signal is equal to the set point output signal (N+1).