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Port Status

The port status consists of a status word and the number of characters in the input buffer
that have not been retrieved by the application (characters which have been received and
are available).

word 1

Port status word (see below)

word 2

Characters available in the input buffer

The Port Status Word can be:

Bit

Name Definition

Meaning

Read In
progress

Set

Read Bytes or Read String invoked

Cleared

Previous Read bytes or String has timed out, been
canceled, or finished

Read Success Set

Read Bytes or Read String has successfully
completed

Cleared

New Read Bytes or Read String invoked

Read Time-
out

Set

Receive timeout occurred during Read Bytes or Read
String

Cleared

New Read Bytes or Read String invoked

Write In
progress

Set

New Write Bytes invoked

Cleared

Previously invoked Write Bytes has timed out, been
canceled, or finished

Write
Success

Set

Previously invoked Write Bytes has successfully
completed

Cleared

New Write Bytes invoked

Write Time-
out

Set

Transmit timeout during Write Bytes

Cleared

New Write Bytes invoked

Character
Available

Set

Unread characters are in the buffer

Cleared

No unread characters in the buffer

OverFlow
error

Set

Overflow error occurred on the serial port or internal
buffer

Cleared

Read Port Status invoked

Framing Error Set

Framing error occurred on the serial port

Cleared

Read Port Status invoked

Parity Error

Set

Parity error occurred on the serial port

Cleared

Read Port Status invoked

CTS is active

Set

CTS line on the serial port is active or the serial port
does not have a CTS line

Cleared

CTS line on the serial port is not active

4 - 0 U

not used, should be 0

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25.4.6

Write port control Function (4304)

This function forces RTS for the specified port:

Example Command Block for the Write Port Control Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0002

Data block length

address +1 0000

0000

NOWAIT mode

address +2 0008

0008

Status word memory type (%R)

address +3 0000

0000

Status word address minus 1 (%R0001)

address +4 0000

0000

Not used

address +5 0000

0000

Not used

address +6 4304

10D0

Write port control command

address +7 xxxx

xxxx

Port control word

Port Control Word

RTS

The Port Control Word can be:

RTS

Commanded state of the RTS output

1 = Activates RTS

0 = Deactivates RTS

0-14

Unused (should be zero)

Operating Note

For Micro PLC port 2 (RS-485), the RTS signal is also controlled by the transmit driver.
Therefore, control of RTS is dependent on the current state of the transmit driver. If the
transmit driver is not enabled, asserting RTS with the Write Port Control COMMREQ will not
cause RTS to be asserted on the serial line. The state of the transmit driver is controlled by
the protocol and is dependent on the current Duplex Mode of the port. For 2-wire and 4-
wire Duplex Mode, the transmit driver is only enabled during transmitting. Therefore, RTS
on the serial line will only be seen active on port 2 (configured for 2-wire or 4-wire Duplex
Mode) when data is being transmitted. For point-to-point Duplex Mode, the transmit driver
is always enabled. Therefore, in point-to-point Duplex Mode, RTS on the serial line will
always reflect what is chosen with the Write Port Control COMMREQ.

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25.4.7

Cancel COMMREQ Function (4399)

This function cancels the current operations in progress. It can be used to cancel both read
operations and write operations. If a read operation is in progress and there are unprocessed
characters in the input buffer, those characters are left in the input buffer and available for
future reads. The serial port is not reset.

Example Command Block for the Cancel COMMREQ Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0002

Data block length

address +1

0000

NOWAIT mode

address +2

0008

Status word memory type (%R)

address +3

0000

Status word address minus 1 (%R0001)

address +4

0000

Not used

address +5

0000

Not used

address +6

4399

112F

Cancel operation command

address +7

0001

Transaction type to cancel

All operations

Read operations

Write operations

Operating Notes

This function does not update the status of words of the cancelled COMMREQs.

CAUTION

If this COMMREQ is sent in either Cancel All or Cancel Write mode when a Write Bytes (4401)
COMMREQ is transmitting a string from a serial port, transmission is halted. The position
within the string where the transmission is halted is indeterminate. In addition, the final
character received by the device the Micro PLC is sending to is also indeterminate.

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25.4.8

Autodial Function (4400)

This feature allows the VersaMax Micro PLC to automatically dial a modem and send a
specified byte string.

To implement this feature, the port must be configured for Serial I/O.

For example, pager enunciation can be implemented by three commands, requiring three
COMMREQ command blocks:

•

Autodial:

04400
(1130h)

Dials the modem.

•

Write Bytes:

04401
(1131h)

Specifies an ASCII string, from 1 to 250 bytes in length, to
send from the serial port.

•

Autodial:

04400
(1130h)

It is the responsibility of the Micro PLC application
program to hang up the phone connection. This is
accomplished by reissuing the autodial command and
sending the hang up command string.

Autodial Command Block

The Autodial command automatically transmits an Escape sequence that follows the Hayes
convention. If you are using a modem that does not support the Hayes convention, you may
be able to use the Write Bytes command to dial the modem.

Examples of commonly used command strings for Hayes-compatible modems are listed
below:

Command String

Length

Function

ATDP15035559999<CR>

16 (10h)

Pulse dial the number 1-503-555-9999

ATDT15035559999<CR>

16 (10h)

Tone dial the number 1-503-555-9999

ATDT9,15035559999<CR>

18 (12h)

Tone dial using outside line with pause

ATH0<CR>

5 (05h)

Hang up the phone

ATZ <CR>

4 (04h)

Restore modem configuration to internally saved
values

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Example Autodial Command Block

This example COMMREQ command block dials the number 234-5678 using a Hayes-
compatible modem.

Word

Definition

Values

0009h

CUSTOM data block length (includes command string)

0000h

NOWAIT mode

0008h

Status word memory type (%R)

0000h

Status word address minus 1 (Register 1)

0000h

not used

0000h

not used

04400 (1130h) Autodial command number

00030 (001Eh) Modem response timeout (30 seconds)

0012 (000Ch)

Number of bytes in command string

5441h

A (41h), T (54h)

5444h

D (44h), T (54h)

3332h

Phone number: 2 (32h), 3 (33h)

3534h

4 (34h), 5 (35h)

3736h

6 (36h), 7 (37h)

0D38h

8 (38h) <CR> (0Dh)

25.4.9

Write bytes Function (4401)

This operation can be used to transmit one or more characters to the remote device through
the specified serial port. The character(s) to be transmitted must be in a word reference
memory. They should not be changed until the operation is complete.

Up to 250 characters can be transmitted with a single invocation of this operation. The
status of the operation is not complete until all of the characters have been transmitted or
until a timeout occurs (for example, if hardware flow control is being used and the remote
device never enables the transmission).

Example Command Block for the Write Bytes Function

Value
(decimal)

Value
(hexadecima
l)

Meaning

address

0006

Data block length (includes characters to send)

address +1

0000

NOWAIT mode

address +2

0008

Status word memory type (%R)

address +3

0000

Status word address minus 1 (%R0001)

address +4

0000

Not used

address +5

0000

Not used

address +6

4401

1131

Write bytes command

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Value
(decimal)

Value
(hexadecima
l)

Meaning

address +7

0030

001E

Transmit time-out (30 seconds). See note
below.

address +8

0005

Number of bytes to write

address +9

25960

6568

‘h’ (68h), ‘e’ (65h)

address +10

27756

6C6C

‘l’ (6Ch), ‘l’ (6Ch)

address +11

0111

006F

‘o’ (6Fh)

Although printable ASCII characters are used in this example, there is no restriction on the
values of the characters that can be transmitted.

Operating Notes

Note:

Specifying zero as the Transmit time-out sets the time-out value to the amount of time actually
needed to transmit the data, plus 4 seconds.

CAUTION

If an Initialize Port (4300) COMMEQ is sent or a Cancel Operation (4399) COMMREQ is sent
in either Cancel All or Cancel Write mode while this COMMREQ is transmitting a string from
a serial port, transmission is halted. The position within the string where the transmission
has halted is indeterminate. In addition, the final character received by the device the Micro
PLC is sending to is also indeterminate.

25.4.10

Read bytes Function (4402)

This function causes one or more characters to be read from the specified port. The
characters are read from the internal input buffer and placed in the specified input data area.

The  function  returns  both  the  number  of  characters  retrieved  and  the  number  of
unprocessed characters still in the input buffer.  If zero characters of input are requested,
only the number of unprocessed characters in the input buffer is returned.

If insufficient characters are available to satisfy the request and a non-zero value is specified
for the number of characters to read, the status of the operation is not complete until either
sufficient characters have been received or the time-out interval expires. In either of those
conditions, the port status indicates the reason for completion of the read operation. The
status word is not updated until the read operation is complete (either due to timeout or
when all the data has been received).

If the time-out interval is set to zero, the COMMREQ remains pending until it has received
the requested amount of data, or until it is cancelled.

If this COMMREQ fails for any reason, no data is returned to the buffer. Any data that was
already in the buffer remains and can be retrieved with a subsequent read request.

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Example Command Block for the Read Bytes Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0005

Data block length

address +1

0000

NOWAIT mode

address +2

0008

Status word memory type (%R)

address +3

0000

Status word address minus 1 (%R0001)

address +4

0000

Not used

address +5

0000

Not used

address +6

4402

1132

Read bytes command

address +7

0030

001E

Read time-out (30 seconds)

address +8

0005

Number of bytes to read

address +9

0008

Input data memory type (%R).

address +10

0001

Input data memory address (%R0001)

Return Data Format for the Read Bytes Function

The return data consists of the number of characters actually read, the number of characters
still available in the input buffer after the read is complete (if any), and the actual input
characters.

Address

Number of characters actually read

Address + 1

Number of characters still available in the input buffer, if any

Address + 2

first two characters (first character is in the low byte)

Address + 3

third and fourth characters (third character is in the low byte)

Address + n

subsequent characters

Operating Note

If the input data memory type parameter is specified to be a word memory type and an
odd number of bytes are actually received, then the high byte of the last word to be
written with the received data is set to zero.

As data is received from the serial port it is placed in the internal input buffer. If the buffer
becomes full, then any additional data received from the serial port is discarded and the
Overflow Error bit in the Port Status word (See Read Port Status Function) is set.

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25.4.11

Read String Function (4403)

This function causes characters to be read from the specified port until a specified
terminating character is received. The characters are read from the internal input buffer and
placed in the specified input data area.

If the terminating character is not in the input buffer, the status of the operation is not
complete until either the terminating character has been received or the time-out interval
expires. In either of those conditions, the port status indicates the reason for completion of
the read operation.

If the time-out interval is set to zero, the COMMREQ remains pending until it has received
the requested string, terminated by the specified end character.

If this COMMREQ fails for any reason, no data is returned to the buffer. Any data that was
already in the buffer remains and can be retrieved with a subsequent read request.

Example Command Block for the Read String Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0005

Data block length

address +1

0000

NOWAIT mode

address +2

0008

Status word memory type (%R)

address +3

0000

Status word address minus 1 (%R0001)

address +4

0000

Not used

address +5

0000

Not used

address +6

4403

1133

Read string command

address +7

0030

001E

Read time-out (30 seconds)

address +8

0013

000D

Terminating character (carriage return):
must be between 0 and 255 (0xFF),
inclusive

address +9

0008

Input data memory type (%R)

address +10

0001

Input data memory address (%R0001)

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Return Data Format for the Read String Function

The return data consists of the number of characters actually read, the number of characters
still available in the input buffer after the read is complete (if any), and the actual input
characters:

Address

Number of characters actually read

Address + 1

Number of characters still available in the input buffer, if any

Address + 2

first two characters (first character is in the low byte)

Address + 3

third and fourth characters (third character is in the low byte)

Address + n

subsequent characters

Operating Note

If the input data memory type parameter is specified to be a word memory type and an odd
number of bytes are actually received, then the high byte of the last word to be written with
the received data is set to zero.

Example

The following Block Move will set up values to perform a serial I/O port configuration. In this
example, port 2 is being configured. The entries of the first Block Move set up the following:

IN1

Length of the data block, which is always16 for a configuration COMMREQ

IN2

Wait/No Wait flag: 0 for No Wait

IN3

Status Word pointer: 8 signifies %R

IN4

Status word pointer offset; this number is zero based, so 0 points to %R1

IN5

Idle timeout value; not used with No Wait mode

IN6

Maximum communication time; not used with No Wait mode

IN7

Command Word; FFF0 hex is the command for serial port setup

In the second Block Move:

IN1

Protocol selector; 5 selects Serial I/O

IN2

Port mode; 0 selects slave mode

IN3

Data rate; 6 selects 19200

IN4

Parity; 1 selects odd

IN5

Flow control; 1 selects none

IN6

Turnaround delay; 0 selects none

IN7

Timeout; 0 selects none

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In the third Block Move:

IN1

Bits per character; 1 chooses 8 bits per character

IN2

Stop bits; 0 selects 1 stop bit

IN3

Interface; not used so set to 0

IN4

Duplex mode; 1 selects 4-wire

IN5

–

IN7

Not used

Figure 373

%R1 is used as the status word for the COMMREQ. The following rung clears the word before
the COMMREQ is issued:

Figure 374

The following logic issues a COMMREQ for port 2. The SYSID is set to rack 0 slot 1. The TASK
ID of 20 defines port 2. The IN-parameter points to %AI1 which is where all the configuration
data was placed with the Block Move statements. If an error occurs, the bit COMMREQP1bad
(%T6) will be set.

Figure 375

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Chapter 26:

PID Function

This chapter describes the PID (Proportional plus Integral plus Derivative) function, which is
used for closed-loop process control. The PID function compares feedback from a process
variable with a desired process set point and updates a Control Variable based on the error.

•

Format of the PID Function

•

Operation of the PID Function

•

Parameter Block for the PID Function

•

PID Algorithm Selection

•

Determining the Process Characteristics

•

Setting Parameters Including Tuning Loop Gains

•

Sample PID Call

26.1

Format of the PID Function

The PID function uses PID loop gains and other parameters stored in an array of 40 16 bit
words to solve the PID algorithm at the desired time interval. All parameters are 16-bit
integer words. This allows %AI memory to be used for input Process Variables and %AQ to
be used for output Control Variables.

Figure 377

The PID function does not pass power flow if there is an error in the configurable
parameters. It can be monitored using a temporary coil while modifying data.

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26.1.1

Parameters of the PID Function

Input/
Output

Choices

Description

enable

flow

When enabled through a contact, the PID algorithm is performed.

I, Q, M, T, G, R, AI,
AQ, constant

The control loop or process set point. Set using process variable
counts, the PID function adjusts the output control variable so that
the process variable matches the set point (zero error).

I, Q, M, T, G, R, AI,
AQ

Process Variable input from the process being controlled, often a
%AI input.

MAN

flow

When energized to 1 (through a contact), the PID block is in
manual mode. If the PID block is on manual off, the PID block is in
automatic mode.

flow

If energized along with MAN, it adjusts the control variable up by 1
CV per solution.

flow

If energized along with MAN, it adjusts the control variable down by
1 CV per solution.

Address R

Location of the PID control block information (user and internal
parameters). Uses 40 %R words that cannot be shared.

flow, none

OK is energized when the function is performed without error. It is
Off if errors exist.

I, Q, M, T, G, R, AI,
AQ

The control variable output to the process, often a %AQ output.

As scaled 16 integer numbers, many parameters must be defined in either Process Variable
(PV) counts or units or Control Variable (CV) counts or units. For example, the Set Point (SP)
input must be scaled over the same range as the Process Variable as the PID block calculates
the error by subtracting these two inputs. The Process Variable and Control Variable Counts
may be

–

32000 or 0 to 32000 matching analog scaling or from 0 to 10000 to display

variables as 0.00% to 100.00%. The Process Variable and Control Variable Counts do not
have to have the same scaling, in which case there will be scale factors included in the PID
gains.

Incremented (UP parameter) or decremented (DN parameter) by one (1) per access of the PID function.

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26.2

Operation of the PID Function

26.2.1

Automatic Operation

The PID function can be called every sweep by providing power flow to Enable and no power
flow to Manual input contacts. The block compares the current PLC elapsed time clock with
the last PID solution time stored in the internal RefArray. If the difference is greater than the
sample period defined in the third word (%Ref+2) of the RefArray, the PID algorithm is solved
using the time difference. Both the last solution time and Control Variable output are
updated. In Automatic mode, the output Control Variable is placed in the Manual Command
parameter %Ref+13.

26.2.2

Manual Operation

The PID block is placed in Manual mode by providing power flow to both the Enable and
Manual input contacts. The output Control Variable is set from the Manual Command
parameter %Ref+13. If either the UP or DN inputs have power flow, the Manual Command
word is incremented or decremented by one CV count every PID solution. For faster manual
changes of the output Control Variable, it is also possible to add or subtract any CV count
value directly to/from the Manual Command word

The PID block uses the CV Upper and CV Lower Clamp parameters to limit the CV output. If
a positive Minimum Slew Time is defined, it is used to limit the rate of change of the CV
output. If either the CV amplitude or rate limit is exceeded, the value stored in the integrator
is adjusted so that CV is at the limit. This anti-reset windup feature means that even if the
error tried to drive CV above (or below) the clamps for a long period of time, the CV output
will move off the clamp as soon as the error term changes sign.

This operation, with the Manual Command tracking CV in Automatic mode and setting CV
in Manual mode, provides a bumpless transfer between Automatic and Manual modes. The
CV Upper and Lower Clamps and the Minimum Slew Time still apply to the CV output in
Manual mode and the internal value stored in the integrator is updated. This means that if
you were to step the Manual Command in Manual mode, the CV output will not change any
faster than the Minimum Slew Time (Inverse) rate limit and will not go above or below the
CV Upper or CV Lower Clamp limits.

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26.2.3

Time Interval for the PID Function

The PID will not execute more often than once every 10 milliseconds. If it is set up to execute
every sweep and the sweep is under 10 ms, the PID function will not run until enough sweeps
have occurred to accumulate an elapsed time of 10 ms. For example, if the sweep time is 9
ms, the PID function executes every other sweep, so the overall elapsed time between
executions is 18 ms. A specific PID function should not be called more than once per sweep.

The longest possible interval between executions is 10.9 minutes. The PID function
compensates for the actual time elapsed since the last execution within 100µS.

The PID algorithm is solved only if the current Micro PLC elapsed time clock is at or later than
the last PID solution time plus the sample period. If the sample period is set to 0, the
function executes each time it is enabled; however, it is restricted to a minimum of 10 ms
as noted above.

26.2.4

Scaling Input and Outputs

All  parameters  of  the  PID  function  are  16  bit  integer  words  for  compatibility  with  16  bit
analog  process  variables.  Some  parameters  must  be  defined  in  either  process  variable
counts or units or control variable counts or units.

The set point input must be scaled over the same range as process variable, because the PID
function calculates error by subtracting these two inputs. The process variable and control
variable counts do not have to use the same scaling. Either may be -32000 or 0 to 32000 to
match analog scaling, or from 0 to 10000 to display variables as 0.00% to 100.00%. If the
process and control variables do not use the same scaling, scale factors are included in the
PID gains.

Example of the PID Function

The example shown below includes typical inputs.

Figure 378

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26.3

Parameter Block for the PID Function

The parameter block for the PID function occupies 40 words of %R memory. Many of the 40
words are used by the Micro PLC and not configurable. Every PID function call must use a
different 40-word memory area even if all 13 configurable parameters are the same.

The first 13 words of the parameter block must be specified before executing the PID
function. Zeros can be used for most default values. Once suitable PID values have been
chosen, they can be defined as constants in a BLKMOV so they can be changed by the
program as needed.

26.3.1

Internal Parameters in RefArray

The PID function reads 13 parameters and uses the rest of the 40-word RefArray for internal
PID storage. Normally you would not change these values. If you call the PID block in Auto
mode after a long delay, you may want to use SVCREQ 16 to load the current PLC elapsed
time clock into %Ref+23 to update the last PID solution time to avoid a step change on the
integrator. If you have set the Override low bit of the Control Word (%Ref+14) to 1, the next
four bits of the Control Word must be set to control the PID block input contacts, and the
Internal SP and PV must be set as you have taken control of the PID block away from the
ladder logic.

Parameter Low Bit Units Range

Description

Address

Loop
Number

Integer

0 to 255.

Optional  number  of  the  PID  block.  It  provides  a
common  identification  in  the  PLC  with  the  loop
number defined by an operator interface device.

Address +1

Algorithm

Set by the PLC

1 = ISA algorithm
2 = independent algorithm

Address+2

Sample
Period

10ms

0 (every sweep)
to 65535 (10.9
Min) At least
10ms.

The shortest time, in 10mS increments, between
solutions of the PID algorithm. For example, use a 10
for a 100mS sample period.

Address+3
Address+4

Deadband +
and
Deadband -

PV Counts

0 to 32000
(+ never
negative)
(- never positive)

INT  values  defining  the  upper  (+)  and  lower  (-)
Deadband  limits  in  PV  Counts.  If  no  Deadband  is
required, these values must be 0.  If the PID Error (SP -
PV) or (PV - SP) is above the (-) value and below the (+)
value, the PID calculations are solved with an Error of
0. If non-zero, the (+) value must greater than 0 and
the  (-)  value  less  than  0  or  the  PID  block  will  not
function.
Leave these at 0 until the PID loop gains are set up or
tuned.   A Deadband might be added to avoid small CV
output changes due to variations in error.

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