EMERSON PACSystems VersaMax MICRO PLC & NANO PLC. USER MANUAL (2019)

User Manual

Chapter 24

GFK-1645M

Dec 2019

Service Request Function

530

24.26.1

Input Parameter Block for SVCREQ 53

The input parameter block for SVCREQ 53 contains the following 5 input words.

Address

Memory type (number) of the PLC memory location of the first byte or word of
data to be written. Numbers are:

Memory

Decimal

Hex

%I (byte)

%Q (byte)

%T (byte)

%M (byte)

%G (byte)

%R (word)

%AI (word)

%AQ (word)

address + 1

Offset of the requested data within the memory type in PLC memory

address + 2

Length of data to be read, 1

–

10 bytes or words.

address + 3

Memory type of the PLC reference memory type for the completion status. See
the numbers above. For example, %R1 = memory type 8.

address + 4

Offset in PLC Memory to write completion status (zero based).
For example, %R1 = offset 0.

24.26.2

Status Data for SVCREQ 53

SVCREQ 53 returns one of the following status indications:

Word 1

Hex

Dec Status

Definition

0x0001

Full Success

All requested data was successfully written

0x0101

257

Partial Success

Less than requested data length was written. Can
occur if some requested data is already in flash.

0x0102

258

Insufficient
Destination Memory

Flash memory area full, connect with the programmer
and write all data to flash in order to erase and store
latest values.

0x0202

514

Invalid Length

Length is beyond minimum 1 and maximum 10

0x0302

770

Source/Dest
Mem Wrong

Wrong source or destination reference memory
specified in the SVCREQ.

Word 2

Number of bytes written

Word 3

Number of bytes remaining in flash

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

531

Chapter 25:

Serial I/O, SNP & RTU Protocols

This chapter describes the VersaMax Micro PLC Serial I/O feature, which can be used to
control the read/write activities of one of the Micro PLC ports directly from the application
program.

This chapter also contains instructions for using COMMREQs to configure the CPU serial
ports for SNP, RTU, or Serial I/O protocol.

•

Format of the COMMREQ Function

•

Configuring Serial Ports Using the COMMREQ Function

—

RTU Slave/SNP Slave Operation with a Programmer Attached

—

COMMREQ Command Block for Configuring SNP Protocol

—

COMMREQ Command Block for Configuring RTU Protocol

—

COMMREQ Command Block for Configuring Serial I/O

•

Serial I/O COMMREQ Commands

—

Initialize Port

—

Set Up Input Buffer

—

Flush Input Buffer

—

Read Port Status

—

Write Port Control

—

Cancel Operation

—

Autodial

—

Write Bytes

—

Read Bytes

—

Read String

Details of RTU and SNP protocol are described in the Serial Communications User’s Manual

(GFK-0582). For additional information about Modbus Master communications, please refer
to the document Modbus RTU Master Communications (GFK-2220).

At higher sweep times, serial communication throughput is less and response times are
larger. As a result of this, a communication request takes longer to complete.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

532

25.1

Format of the Communication Request
Function

Serial  I/O  is  implemented  through  the  use  of  Communication  Request  (COMMREQ)
functions.    The  operations of  the  protocol,  such  as  transmitting  a  character  through  the
serial  port  or  waiting  for  an  input  character,  are  implemented  through  the  COMMREQ
function block.

The COMMREQ requires that all its command data be placed in the correct order (in a
command block) in PLC memory before it is executed. The COMMREQ should then be
executed by a contact of a one-shot coil to prevent sending the data multiple times. A series
of Block Move (BLKMV) commands should be used to move the words to create a command
block in the Register tables.

The COMMREQ function has three inputs and one output. When the function receives
power flow, a command block of data is sent to the specified module.

Figure 371

25.1.1

Parameters of the COMMREQ Function

Input/
Output

Choices

Description

enable

flow

When the function is energized, the communications request is
performed.

R, AI, AQ

IN contains the first word of the command block.

SYSID

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

SYSID contains the rack number (most significant byte) and slot
number (least significant byte) of the target device. For the CPU,
SYSID must specify rack/slot 0.

TASK

R, AI, AQ, constant TASK specifies the port for which the operation is intended:

task 19 for port 1

task 20 for port 2

flow, none

FT is energized if an error is detected processing the COMMREQ:

•

The specified target address is not present (SYSID).

•

The specified task is not valid for the device (TASK).

•

The data length is 0.

•

The device’s status pointer address (in the command block)

does not exist. (applies only to HSC COMMREQs)

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

533

25.1.2

Command Block for the COMMREQ Function

The Command Block starts at the reference specified in COMMREQ parameter IN. The
length of the Command Block depends on the amount of data sent to the device.

The  Command  Block  contains  the  data  to  be  communicated  to  the  other  device,  plus
information  related  to  the  execution  of  the  COMMREQ.  The  Command  Block  has  the
following structure:

address

Length (in words)

address + 1

Wait/No Wait Flag

address + 2

Status Pointer Memory

address + 3

Status Pointer Offset

address + 4

Idle Timeout Value

address + 5

Maximum Communication Time

address + 6 to
address + 133

Data Block

Example of the COMMREQ Function

In the example, when %M0021 is ON, a Command Block located starting at %R0032 is sent
to port 2 (communications task 20) of the CPU (rack 0, slot 0). If an error occurs processing
the COMMREQ, %Q0110 is set.

Figure 372

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

534

25.2

Configuring Serial Ports Using the COMMREQ
Function

The following tables list the command block values required for setting up a Serial Port for
SNP, RTU, and Serial I/O. All values are in hexadecimal unless otherwise indicated. The
BLKMV commands that are used to create the command block are described in the
example.

25.2.1

Timing

If a port configuration COMMREQ is sent to a serial port that currently has an SNP/SNPX
master (for example, the programmer) connected to it, the serial port configuration
specified by the COMMREQ does not take effect until the Micro PLC detects a loss of the

SNP/SNPX master. This occurs after the configured T3’ times out, once the master

disconnects. The COMMREQ status word for the port configuration COMMREQ is updated
as soon as the CPU verifies that the specified configuration is valid. That means a COMMREQ
Successful value may be returned by the Port Configuration COMMREQ before the specified
configuration is actually installed.

25.2.2

Sending Another COMMREQ to the Same Port

The application program must wait at least 2 seconds after a new serial port protocol is
installed before sending any COMMREQs specific to that protocol to the port. This applies
to a new protocol installed by Storing a new hardware configuration or by a port
configuration COMMREQ. If the port is configured for Serial I/O, this waiting period must
also follow any Stop to Run mode transition of the Micro PLC.

25.2.3

Invalid Port Configurations

COMMREQs to configure protocols for the port(s) of the Micro PLC are limited to the
supported protocols for the given port. Any COMMREQs that request invalid port
configurations will be rejected.

Note:

For all Nano and Micro PLC CPUs, care must be taken setting timeouts and message lengths when
operating at 4800 bps or lower. Messages at this rate are longer in duration and may cause
various timeouts within a system. This can become worse on the Micro-64 CPU because the scan
times on this unit are typically slightly longer.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

535

25.2.4

RTU Slave/SNP Slave Operation with Programmer
Attached

A programmer (an SNP/SNPX master device) can be attached to port 1 or port 2 while RTU
Slave mode is active on the port. For multi-drop connections, the Micro PLC must have been
configured to use an appropriate PLC ID. Note that for a multi-drop SNP connection with the
port currently configured for RTU Slave, the SNP ID associated with the Micro PLC settings
must match the multi-drop ID.

The programmer must use the same serial communications parameters (baud rate, parity,
stop bits, etc.) as the currently active RTU Slave protocol for it to be recognized.

When  the  Micro  PLC  recognizes  the  programmer,  the  Micro  PLC  removes  the  RTU  Slave
protocol from the port and installs SNP Slave as the currently active protocol. The SNP ID,
modem turnaround time, and default idle time for this new SNP Slave session are obtained
from the configured Micro PLC settings, not the port 1 or port 2 configurations. Connection
should  be  established  within  12  seconds.  When  the  programmer  connection  has  been
enabled, normal programmer communications can take place.  (Failure of the programmer
to  establish  communications  within  12-seconds  is  treated  as  a  Loss  of  Programmer
Communications).

The programmer may send a new protocol via configuration or a Serial Port Setup
COMMREQ. (COMMREQs not supported by SNP Slave protocol are rejected). If a new
protocol is received, it will not take effect until the programmer is disconnected.

After the programmer is removed, there is a slight delay (equal to the configured SNP T3’

timeout) before the Micro PLC recognizes its absence. During this time, no messages are
processed on the port. The Micro PLC detects removal of the programmer as an SNP Slave
protocol timeout. Therefore, it is important to be careful when disabling timeouts used by
the SNP Slave protocol.

When the Micro PLC recognizes the disconnect, it reinstalls the RTU Slave protocol, unless a
new protocol has been received. In that case, the Micro PLC installs the new protocol
instead.

Note:

If an RTU slave receives a message with length greater than 265 bytes it responds with an error
code 0x03 indicating "Invalid Data Field"

Example

Port 1 is running RTU Slave protocol at 9600 baud.

A programmer is attached to port 1. The programmer is using 9600 baud.

The Micro PLC installs SNP Slave on port 1 and the programmer communicates
normally.

The programmer stores a new configuration to port 1. The new configuration sets
the port for SNP Slave at 4800 baud (it will not take effect until the port loses
communications with the programmer).

When the Micro PLC loses communications with the programmer, the new
configuration takes effect.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

536

25.2.5

COMMREQ Command Block for Configuring SNP
Protocol

Values

Meaning

Address

10H

Data Block Length

Address + 1

0 = No Wait

WAIT/NOWAIT Flag

Address + 2

0008 = %R, register memory

Status Word Pointer Memory Type

Address + 3

Zero-based number that gives the address
of the COMMREQ status word (for
example, a value of 99 gives an address of
100 for the status word)

Status Word Pointer Offset

Address + 4

0 (Only used in Wait/No Wait mode)

Idle Timeout Value

Address + 5

0 (Only used in Wait/No Wait mode)

Maximum Communication Time

Address + 6

FFF0H

Command Word (serial port setup)

Address + 7

0001

Protocol: 1=SNP

Address + 8

0000=Slave, 0001=Master

Port Mode

Address + 9

10=175000, 7=38400, 6=19200, 5=9600,
4=4800, 3=2400, 2=1200, 1=600, 0=300

Data Rate (bps)

Address + 10

0 = None, 1 = Odd, 2 = Even

Parity

Address + 11

1 = None

Flow Control

Address + 12

0 = None, 1 = 10ms, 2 = 100ms, 3 = 500ms Turnaround Delay

Address + 13

0 = Long, 1 = Medium, 2 = Short, 3 = None Timeout

Address + 14

1 = 8 bits

Bits Per Character

Address + 15

0 = 1 Stop Bit, 1 = 2 Stop bits

Stop Bits

Address + 16

not used

Interface

Address + 17

not used

Duplex Mode

Address + 18

user-provided

Device identifier bytes 1 and 2

Address + 19

user-provided

Device identifier bytes 3 and 4

Address + 20

user-provided

Device identifier bytes 5 and 6

Address + 21

user-provided

Device identifier bytes 7 and 8

The device identifier for SNP Slave ports is packed into words with the least significant character in the least significant byte of the word.

For example, if the first two characters are “A” and “B,” the Address

+ 18 will contain the hex value 4241.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

537

25.2.6

COMMREQ Command Block for Configuring RTU
Protocol

Values

Meaning

First 6 words

Reserved for COMMREQ use.

Address + 6

FFF0H

Command

Address + 7

Protocol: 0003=RTU

Address + 8

Port Mode: 0000=Slave

Address + 9

10=175000, 7=38400,
6=19200, 5=9600, 4=4800,
3=2400, 2=1200, 1=600,
0=300

Data Rate

Address + 10

0 = None, 1 = Odd, 2 = Even

Parity

Address + 11

0 = Hardware, 1 = None

Flow Control

Address + 12

0-255 (units of 10ms.
For example, 10=100ms)

Turnaround delay. For release 2.02 and later
CPUs only.
0 for all other CPUs.

Address + 13

not used

Timeout

Address + 14

not used

Bits per Character

Address + 15

0 = 1, 1 = 2

Stop Bits

Address + 16

not used

Interface

Address + 17

0 = 2-wire, 1 = 4-wire

Duplex Mode

Address + 18

Station Address (1-247)

Device Identifier

Address + 19 - 21

not used

Device Identifier

Address + 22

–

255 (units of 10ms)

Receive to Transmit Delay

Address + 23

0-255 (units of 10ms)

RTS Drop Delay

Release 2.02 and later Micro PLC CPUs support Receive to Transmit Delay and RTS Drop Delay functionality for RTU and Serial IO through

the Serial Port Setup COMMREQ. Both of these optional parameters are described in Receive to Transmit Delay and RTS Drop Delay. When
configuring the protocol, these two optional parameters can be added to the port setup COMMREQ. The data block length (Address + 0)
for a COMMREQ that includes the Receive to Transmit Delay and RTS Drop Delay parameters should be 12H, not 10H. Either length (10H
or 12H) is valid for this COMMREQ.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

538

Receive to Transmit Delay and RTS Drop Delay

Release 2.02 and later Micro PLC CPUs support Receive to Transmit Delay and RTS Drop
Delay functionality for RTU and Serial IO through the Serial Port Setup COMMREQ.

When configuring the protocol, these two optional parameters can be added to the port
setup COMMREQ. The data block length (Address + 0) for a COMMREQ that includes the
Receive to Transmit Delay and RTS Drop Delay parameters should be 12H, not 10H. Either
length (10H or 12H) is valid for this COMMREQ.

Receive to transmit delay - This is the time interval that defines the end of each received
response message. It is measured from the end of the last received character. RTS is off and
the transmitter is silent during this interval. If a new query is ready for transmission, RTS is
asserted no earlier than the end of this interval.

Zero specifies the default, defined as 3.5 character times at the specified data rate,
assuming 11 bits per character.

Data Rate

Default (10ms Units)

Data Rate

Default (10ms Units)

1200
2400
4800
9600

322
161
80
40

19200
38400
57600
115200

21
10
7
3

If the required delay is greater than the default value at the current data rate, increase the
specified value to required delay in 10ms units. If the required delay is less than the default
at the current data rate, no additional delay is necessary.

RTS Drop Delay -This is the time from the end of the last transmitted character to the time
when RTS is turned off (dropped). The RTS Drop Delay is also specified in 10ms units. The
receiver is disabled during transmission and remains disabled during the RTS drop delay
time.

Data Rate

Default (10ms Units)

Data Rate

Default (10ms Units)

1200
2400
4800
9600

92
46
23
12

19200
38400
57600
115200

6
3
2
1

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

539

25.2.7

COMMREQ Command Block for Configuring Serial I/O
Protocol

Values

Meaning

First 6 words

Reserved for COMMREQ use.

Address + 6

FFF0H

Command

Address + 7

0005

Protocol: 0005=Serial IO

Address + 8

0 = Slave

Port Mode

Address + 9

10=175000, 7=38400,
6=19200, 5=9600, 4=4800,
3=2400, 2=1200, 1=600, 0=300

Data Rate (bps)

Address + 10

0 = None, 1 = Odd, 2 = Even

Parity

Address + 11

0 = Hardware, 1 = None

Flow Control

Address + 12

0-255 (units of 10ms). For
example, 10 = 100ms).

Turnaround Delay. For release 2.02 and
later CPUs only. 0 for all other CPUs.

Address + 13

0 = Long

Timeout

Address + 14

0=7 bits, 1=8 bits

Bits per Character

Address + 15

0 = 1 stop bit, 1 = 2 stop bits

Stop Bits

Address + 16

must be set to 0

Interface

Address + 17

0 = 2-wire, 1 = 4-wire

Duplex Mode

Address + 18

—

not used

Device Identifier

Address + 22

0-255 (units of 10ms)

Receive to transmit delay / RTS Drop Delay

Release 2.02 and later Micro PLC CPUs support Receive to Transmit Delay and RTS Drop Delay functionality for RTU and Serial IO through

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

540

25.3

Calling Serial, I/O COMMREQs from the PLC
Sweep

Implementing a serial protocol using Serial I/O COMMREQs may be restricted by the Micro
PLC sweep time. For example, if the protocol requires that a reply to a certain message from
the remote device be initiated within 5ms of receiving the message, this method may not
be successful if the sweep time is 5mS or longer, since timely response is not guaranteed.

When using Serial, I/O protocol with a VersaMax Nano or 14 point Micro PLC, the user switch
must be configured.  A Nano PLC requires an external switch connection.  When a Nano PLC
or  14-Point  Micro  PLC  is  in  Run  mode,  Serial  I/O  protocol  is  active;  after  a  Run-to-Stop
transition, the Nano PLC or 14-Point Micro PLC automatically reverts to SNP.  An SNP master
device  (VersaPro,  for  instance)  is  not  able  to  communicate  with  a  Nano  PLC  or  14-Point
Micro PLC that is configured for Serial I/O when it is in Run mode.

Since the Serial I/O is completely driven by the application program, in STOP mode a port
configured as Serial I/O automatically reverts to SNP slave, to facilitate programmer
communication. Therefore, while in Stop mode, Serial I/O protocol is not active; it is only
active when the PLC is in Run mode.

Calling the Serial, I/O from a Timed Interrupt Block

The protocol can also be implemented from within a timed interrupt block. Whenever the
interrupt block executes, the status of the previous COMMREQ or the status of the port is
queried to determine if the last requested action is complete. It if is, a new port command
can be initiated with a COMMREQ function block.

25.3.1

Compatibility

The COMMREQ function blocks supported by Serial I/O are not supported by other currently
existing protocols (such as SNP slave, SNP master, and RTU slave). Errors are returned if they
are attempted for a port configured for one of those protocols

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

541

25.3.2

Status Word for Serial I/O COMMREQs

A value of 1 is returned in the COMMREQ status word upon successful completion of the
COMMREQ. Any other value returned is an error code where the low byte is a major error
code and the high byte is a minor error code.

Major Error
Code

Description

1 (01h)

Successful Completion (the expected completion value in the COMMREQ status word).

12 (0Ch)

Local error

—

Error processing local command. Minor error code identifies the specific error.

1 (01h)

Wait-type command is not permitted. Use No-Wait command.

2 (02h)

COMMREQ command is not supported.

5 (05h)

Error writing COMMREQ status word to PLC memory.

6 (06h)

Invalid PLC memory type specified.

7 (07h)

Invalid PLC memory offset specified.

8 (08h)

Unable to access PLC memory.

12 (0Ch)

COMMREQ data block length too small.

14 (0Eh)

COMMREQ data is invalid.

13 (0Dh)

Remote error

—

Error processing a remote command. Minor error code identifies the error.

2 (02h)

Number of bytes to read exceeds input buffer size, or more than 250 bytes requested to be
written.

3 (03h)

COMMREQ data block length is too small. String data is missing or incomplete.

4 (04h)

Receive timeout awaiting serial reception of data

8 (08h)

Unable to access PLC memory.

12 (0Ch)

COMMREQ data block length too small.

48 (30h)

Serial output timeout. The port was unable to transmit the string. (Could be missing CTS signal
when the serial port is configured to use hardware flow control.)

50 (32h)

COMMREQ timeout. The COMMREQ did not complete within 20-second time limit.

14 (0Eh)

Autodial Error

—

An error occurred while attempting to send a command string to an attached external

modem. The minor error code identifies the specific error.

2 (02h)

The modem command string length exceeds end of reference memory type.

3 (03h)

COMMREQ Data Block Length too small. Output command string data missing or incomplete.

4 (04h)

Serial output timeout. The port was unable to transmit the modem autodial output.

5 (05h)

Response was not received from modem. Check modem and cable.

6 (06h)

Modem responded with BUSY. Modem is unable to complete the requested connection. The
remote modem is already in use; retry the connection request later.

7 (07h)

Modem responded with NO CARRIER. Modem is unable to complete the requested connection.
Check the local and remote modems and the telephone line.

8 (08h)

Modem responded with NO DIALTONE. Modem is unable to complete the requested connection.
Check the modem connections and the telephone line.

9 (09h)

Modem responded with ERROR. Modem is unable to complete the requested command. Check
the modem command string and modem.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

542

Major Error
Code

Description

10 (0Ah)

Modem responded with RING; the modem is being called by another modem. Modem is unable
to complete the requested command. Retry the command later.

11 (0Bh)

Unknown response from modem. Modem unable to complete the command. Check modem
command string and modem. Response should be CONNECT or OK.

50 (32h)

COMMREQ timeout. The COMMREQ did not complete within 20 seconds.

25.4

Serial I/O COMMREQ Commands

The following COMMREQs are used to implement Serial I/O:

•

Local COMMREQs - do not receive or transmit data through the serial port.

—

Initialize Port (4300)

—

Set Up Input Buffer (4301)

—

Flush Input buffer (4302)

—

Read port status (4303)

—

Write port control (4304)

—

Cancel Operation (4399)

•

Remote COMMREQs - receive and/or transmit data through the serial port.

—

Autodial (4400)

—

Write bytes (4401)

—

Read bytes (4402)

—

Read String (4403)

25.4.1

Overlapping COMMREQs

Some of the Serial I/O COMMREQs must complete execution before another COMMREQ can
be processed. Others can be left pending while others are executed.

COMMREQS that Must Complete Execution

•

Autodial (4400)

•

Initialize Port (4300)

•

Set Up Input Buffer (4301)

•

Flush Input buffer (4302)

•

Read port status (4303)

•

Write port control (4304)

•

Cancel Operation (4399)

•

Serial Port Setup (FFF0)

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

543

COMMREQs that Can be Pending While Others Execute

The table below shows whether Write Bytes, Read Bytes and Read String COMMREQs can
be pending when other COMMREQs are executed.

New COMMREQ

Currently-
pending
COMMREQs

Autodial
(4400)

Write
bytes
(4401)

Initialize
Port
(4300)

Set Up
Input
Buffer
(4301)

Flush
Input
buffer
(4302)

Read
port
status
(4303)

Write
port
control
(4304

Read
bytes
(4402)

Read
String
(4403)

Cancel
Operation
(4399)

Serial
Port
Setup
(FFF0)

Write bytes
(4401)

Yes

Read bytes
(4402)

Yes

Read String
(4403)

Yes

25.4.2

Initialize Port Function (4300)

This function causes a reset command to be sent to the specified port. It also cancels any
COMMREQ currently in progress and flushes the internal input buffer. RTS is set to
inactive.

Example Command Block for the Initialize Port Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0001

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 4300

10CC

Initialize port command

Operating Notes

Note:

COMMREQs that are cancelled due to this command executing do not have their respective
COMMREQ status words updated.

CAUTION

If this COMMREQ is sent 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.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

544

25.4.3

Set Up Input Buffer Function (4301)

This function can be used to change the size of the internal memory buffer where input data
will be placed as it is received. By default, the buffer is set to a maximum of 2K bytes.  As data
is received from the serial port it is placed in the input buffer.  If the buffer becomes full, 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.

Retrieving Data from the Buffer

Data can be retrieved from the buffer using the Read String or Read Bytes function. It is not
directly accessible from the application program.

If data is not retrieved from the buffer in a timely fashion, some characters may be lost.

Example Command Block for the Set Up Input Buffer
Function

Value
(decimal)

Value(hexadeci
mal)

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

4301

10CD

Setup input buffer command

address +7

0064

0040

Buffer length (in words)

Operating Notes

It is not possible to set the buffer length to zero. If zero is entered as the buffer length, the
buffer size will be set to the 2K bytes default.

If a length greater than 2K bytes is specified, an error is generated.

User Manual

Chapter 25

GFK-1645M

Dec 2019

Serial I/O, SNP & RTU Protocols

545

25.4.4

Flush Input buffer Function (4302)

This operation empties the input buffer of any characters received through the serial port
but not yet retrieved using a read command. All such characters are lost.

Example Command Block for the Flush Input Buffer Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0001

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

4302

10CE

Flush input buffer command

25.4.5

Read port status Function (4303)

This function returns the current status of the port. The following events can be detected:

A read request was initiated previously, and the required number of characters has
now been received or the specified time-out has elapsed.

A write request was initiated previously and transmission of the specified number of
characters is complete or a time-out has elapsed.

The status returned by the function indicates the event or events that have completed.
More than one condition can occur simultaneously, if both a read and a write were initiated
previously.

Example Command Block for the Read Port Status Function

Value
(decimal)

Value
(hexadecimal)

Meaning

address

0003

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

4303

10CF

Read port status command

address +7

0070

0046

Port status memory type (%I)

address +8

0001

Port status memory offset (%I0001)

EMERSON PACSystems VersaMax MICRO PLC & NANO PLC. USER MANUAL (2019) - page 36