FC 21 - M/A Station (Basic)

The M/A station (basic) 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 basic station generates a set point and provides manual / automatic transfers, control output adjustment in the manual mode, and set point adjustment in the automatic mode. If set point tracking is selected, S14 identifies the input that the set point will track (<S1> or <S2>).

 

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

 

When the control output track switch <S5> equals one, the control output tracks the value of the control output track signal <S4>.

 

The manual interlock <S6> may be used to hold the station in manual mode.

 

 

 

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

Always 0 for basic station

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.

 

 

 

21.1 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

SPT

Block number whose output the set point tracks, depending

on S14 (track set point).

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

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

NOTE: Specifications S7, S8 and S9 are tunable but not adaptable (function code 24).

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 SP <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 (unused in function code 21)

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.

 

 

21.2  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 21-1. 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 21-1 - 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)

 

21.3  Station Operating States

 

The station function block interfaces the controller module to the digital control station (DCS), analog control station (SAC), management command system (MCS), operator interface station

(OIS) or a computer through a network interface unit (NIU). The operating state of the station block consists of 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 block output is determined by the block inputs, messages received over the module bus and messages received from the DCS or SAC station.

 

21.4  Control Output Tracking

 

When the control output track state <S5> equals one, the output of the block overrides the current value of <S4>. The actual operating level and mode are saved, and will be restored when the track flag input goes to zero.

 

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

 

The output of the station is a percentage from 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.

 

21.5 Manual Interlock

 

When the manual interlock state <S6> equals one, the station is forced to the manual mode. When the interlock is released, the station remains in the manual mode.

 

21.6  Set Point Tracking

 

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

 

21.7  Examples

 

S2

Number of the block output the set point tracks as determined

by the set point track option (S14).

 

 

S14

Set point track option:

0 = no tracking

1 = track PV <S1> when station is in manual

2 = track <S2> in manual mode only

3 = track <S2> always (auto and manual) – no direct control

of set point from station

 

 

If S14 = 0

The set point can be varied using local or computer manual

control.

 

 

If S14 = 1

The set point tracks <S1> when the station is in manual. This

provides bumpless transfer when the station returns to automatic mode if the station and PID are correctly configured. In this mode the set point cannot be varied manually.

 

 

If S2 = Blk no. and

            S14 = 2

The set point tracks <S2> when the station is in manual. The

set point may be changed when the station returns to automatic. A bump in the set point value is possible when the station is switched to automatic mode.

 

 

If S2 = Blk no. and

            S14 = 3

The set point tracks <S2> when the station is in either automatic or manual mode. No direct set point adjustments can

be made from the station.

 

 

 

21.8  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 only report via an exception report (not reported to a DCS or SAC station).

 

Figure 21-1 illustrates a manual/auto station used in a single input/single output configuration. The PID block (function code 19) provides proportional, integral, and derivative control based on the process variable <S1> and set point <S2> inputs. The signal developed by the PID block is a corrected process variable value, used as the auto signal <S3> by the basic station block.

 

NOTES:

  1. A station output is zero to 100 percent. Therefore, if the 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 scale on hardware stations can be driven by the station output or by a feedback signal. This applies to two stations connected to a given termination unit. Interconnected stations do not have this option.

  2. The station can only be transferred to computer operation when a network interface unit and computer are involved in the transaction. Pressing the computer pushbutton on the station will not provide a usable output.