FieldServer FS-8700-66 Release note

Driver Version:
1.05
Document Revision:
1
A Sierra Monitor Company
APPLICABILITY & EFFECTIVITY
Effective for all systems manufactured after July 2011
Driver Manual
(Supplement to the FieldServer Instruction Manual)
FS-8700-66 GE SNP

FS-8700-66 GE-SNP Driver Manual Table of Contents
FieldServer Technologies 1991 Tarob Court Milpitas, California 95035 USA Web: www.fieldserver.com
Tel: [phone] Fax: [phone] Toll Free: [phone] email: [email protected]m
TABLE OF CONTENTS
1GE_SNP Description ....................................................................................................................................... 3
2Driver Scope of Supply ................................................................................................................................... 3
2.1 Supplied by FieldServer Technologies for this driver.....................................................................................3
3Hardware Connections................................................................................................................................... 4
4Data Array Parameters................................................................................................................................... 7
5Configuring the FieldServer as a GE_SNP Client.............................................................................................. 8
5.1 Client Side Connection Parameters ...............................................................................................................8
5.2 Client Side Node Descriptor Parameters .......................................................................................................9
5.3 Client Side Map Descriptor Parameters.......................................................................................................11
5.3.1 FieldServer Related Map Descriptor Parameters .................................................................................11
5.3.2 Driver Related Map Descriptor Parameters .........................................................................................11
5.3.3 Timing Parameters...............................................................................................................................11
5.3.4 Map Descriptor Example 1 –Simple Read ...........................................................................................12
5.3.5 Map Descriptor Example 2 –Simple Write ..........................................................................................12
5.3.6 Map Descriptor Example 3 –Handling BITs .........................................................................................13
6Configuring the FieldServer as a GE_SNP Server........................................................................................... 14
6.1 Server Side Connection Descriptors ............................................................................................................14
6.2 Server Side Node Descriptors ......................................................................................................................15
6.3 Server Side Map Descriptors........................................................................................................................15
6.3.1 FieldServer Specific Map Descriptor Parameters .................................................................................15
6.3.2 Driver Specific Map Descriptor Parameters .........................................................................................16
6.3.3 Timing Parameters...............................................................................................................................16
6.3.4 Map Descriptor Example......................................................................................................................17
Appendix A. Useful Features................................................................................................................................ 18
Appendix A.1. SNP Node Names..............................................................................................................................18
Appendix A.2. Scaling ..............................................................................................................................................19
Appendix B. Troubleshooting............................................................................................................................... 20
Appendix C. Reference......................................................................................................................................... 21
Appendix C.1. Error Messages .................................................................................................................................21
Appendix C.2. Driver Stats .......................................................................................................................................23
Appendix C.3. Server Response NAK, Major and Minor Error Codes ......................................................................25
Appendix C.3.1. Major Error Status Codes .......................................................................................................25
Appendix C.3.2. Minor Error Status Codes .......................................................................................................25
Appendix C.3.3. Minor Error Status Codes: Program Load and Store Requests ...............................................27
Appendix C.4. Compatibility Matrix.........................................................................................................................28

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1GE_SNP DESCRIPTION
The GE-SNP Serial driver allows the FieldServer to transfer data to and from devices over either RS-232 or RS-485
using GE-SNP Serial protocol. The FieldServer can emulate either a Server or Client.
The FieldServer provides functions to read and write PLC memory and change the privilege level. Standard SNP
mailbox messages are used. The driver does not support Datagram messages and cannot parse them. These
messages are defined by the SNP protocol to allow multiple data types to be packed into one message. They are
not commonly used by the HMI and 3rd party applications and are inconsistent with the FieldServer’s Write
Through and Port Expander capabilities.
The driver can expose communications statistics in a Data Array so that downstream devices can monitor them.
2DRIVER SCOPE OF SUPPLY
2.1 Supplied by FieldServer Technologies for this driver
FieldServer Technologies PART #
Description
FS-8915-10
UTP cable (7 foot) for RS-232 use
FS-8917-03
RJ45 to DB9M connector adapter
FS-8700-66
Driver Manual.

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3HARDWARE CONNECTIONS
The FieldServer is connected to the GE PLC in one of two ways.
Almost every GE PLC has a RS-232 and RS-485 port. Possible cable configurations are illustrated below.
A converter is required to connect between RS-232 and RS-485. An example of connection configurations for the
Model480TLED connector is depicted overleaf. Any equivalent connector can be used.
DATE: 2/2/04
BY: MF
BASE NAME:
FILE NAME:
GE SNP
CONNECTION DIAGRAM
FieldServer GE-PLC
RS-485 port
RS-232 port RS-232 port
RJ11
RS-485 port
15 pin female
Pin 1 at bottom
Cable 1
Cable 2
Cable 3
Cable 4
Tx
Rx
Gnd
+
-
-
+
Tx
Rx
Gnd
10
11
12
13
7
FieldServer GE
Cable 1
FieldServer
-
Tx
Rx
Gnd
Rx
Tx
Gnd
5
2
3
4
GE
Cable 2
CTS
1
6RTS
*
+
-
-
+

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BASE NAME:
FILE NAME:
GE SNP
CONNECTION DIAGRAM
RS-485 to RS-232 using
Model4850 TLED connector.
FS-8700-66-1
DATE: 2/16/08
BY: MC
RS-232 TO RS-422/485
OPTICALLY ISOLATED
CONVERTER
Recceive
Transmit
RS-232
SR-422/485
MODEL 485OTLED
electronics
B&B
TDA(-)
TDB(+)
RDA(-)
RDB(+)
422/485
GND
SD
RTS
+12V
R-232
GND
O
N
12345678
4800 9600 19.2K 38.4K ECHO 4W/2W 4W/2W TERM.
JP1
TB1
TB2
R1 or R2
+ (B)
- (A)
GND
FieldServer
2
3
4
7
DB25F
91035
7
Rx
Tx
RTS
GND
Note: Pins 4&5 and pins 6,8&20 are
bridged internally to allow for RS-232
devices that need hardware control.
RS-232 Device
2
3
5
J2
Rx
Tx
RTS
GND
RS- 232 TO RS-422/485
OPTICALLY ISOLATED
CONVERTER
Recceive
Transmit
RS-232
SR-422/485
MODEL485OTLED
electronics
B&B
TDA(-)
TDB(+)
RDA(-)
RDB(+)
422/ 485GND
SD
RTS
+12V
R-232
GND
O
N
12345678
4800 9600 19.2K 38.4K
ECHO
4W/2W 4W/2W
TERM.
JP1
TB1
TB2
+ (B)
- (A)
GND
Remote Serial
Device–2 wire
2
3
4
7
DB25F
91035
TDA(-)
TDB (+)
RDB(+)
RDA(-)
GND
Remote Serial
Device–4 wire
FS-8917-01
2
3
5
7
21787
J2
From FieldServer
Converter for Cable 3: From RS-232 on FieldServer to RS-485 on device.
Converter for Cable 4: From RS-485 on FieldServer to RS-323 on device.

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FieldServer Technologies 1991 Tarob Court Milpitas, California 95035 USA Web: www.fieldserver.com
Tel: [phone] Fax: [phone] Toll Free: [phone] email: [email protected]m
DATE: 2/2/04
BY: MF
(408)-262-2299
BASE NAME:
FILE NAME:
GE SNP
CONNECTION DIAGRAM
P1-P8
RJ45
PLC
Pri or 2ndary port
CPU or 2ndary port
Horner
232
to
422 9 pin
15 pin
Horner
232
to
422
9 pin
15 pin
RJ45
black
yellow
blue
white
green
3
2
5
9 pin
Ethernet cable segment
9 pin
25 pin
PC
Com1
6
7
2
3
5
RJ45
black
yellow
blue
white
green
9 pin
RJ45
black
yellow
blue
white
green
9 pin
9 pin
null modem
5 3 2 7 6
5 3 2
Typical
FieldServer
Cable
GE LOGICMASTER
PC Software to PLC

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4DATA ARRAY PARAMETERS
Data Arrays are “protocol neutral” data buffers for storage of data to be passed between protocols. It is necessary
to declare the data format of each of the Data Arrays to facilitate correct storage of the relevant data.
Section Title
Data_Arrays
Column Title
Function
Legal Values
Data_Array_Name
Provide name for Data Array
Up to 15 alphanumeric characters
Data_Array_Format
Provide data format. Each Data Array can only take
on one format.
FLOAT, BIT, UInt16, SInt16,
Packed_Bit, Byte, Packed_Byte,
Swapped_Byte
Data_Array_Length
Number of Data Objects. Must be larger than the
data storage area required by the Map Descriptors
for the data being placed in this array.
1-10, 000
Example
// Data Arrays
Data_Arrays
Data_Array_Name
, Data_Array_Format
, Data_Array_Length
DA_AI_01
, UInt16,
, 200
DA_AO_01
, UInt16
, 200
DA_DI_01
, BIT
, 200
DA_DO_01
, BIT
, 200

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5CONFIGURING THE FIELDSERVER AS A GE_SNP CLIENT
For a detailed discussion on FieldServer configuration, please refer to the FieldServer Configuration Manual. The
information that follows describes how to expand upon the factory defaults provided in the configuration files
included with the FieldServer (See “.csv” sample files provided with the FieldServer).
This section documents and describes the parameters necessary for configuring the FieldServer to communicate
with a GE_SNP Server.
The configuration file tells the FieldServer about its interfaces, and the routing of data required. In order to enable
the FieldServer for GE_SNP communications, the driver independent FieldServer buffers need to be declared in the
“Data Arrays” section, the destination device addresses need to be declared in the “Client Side Nodes” section, and
the data required from the Servers needs to be mapped in the “Client Side Map Descriptors” section. Details on
how to do this can be found below.
Note that in the tables, * indicates an optional parameter, with the bold legal value being the default.
5.1 Client Side Connection Parameters
Section Title
Connections
Column Title
Function
Legal Values
Port
Specify which port the device is
connected to the FieldServer
P1-P8, R1-R21
Protocol
Specify protocol used
SNP
Baud*
Specify baud rate
110 –19200, standard baud rates only (Vendor
limitation), 19200
Parity*
Specify parity
Even, Odd, None, Mark, Space
Data_Bits*
Specify data Bits
7, 8
Stop_Bits*
Specify stop Bits
1(Vendor limitation)
Poll _Delay*
Time between internal polls
0-32000 seconds, 1 second
Timeout
The timeout specified for the connection is used as the data link layer timeout. It is also
used as the timeout for each Map Descriptor if a timeout is not explicitly defined for a Map
Descriptor.
Example
// Client Side Connections
Connections
Port
, Protocol
, Baud
, Parity
, Poll_Delay
P1
, SNP
, 19200
, Odd
, 0.100s
1
Not all ports shown are necessarily supported by the hardware. Consult the appropriate Instruction manual for details of the ports available
on specific hardware.

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GE Port Defaults
Parameter
Series 90-30 PLC’s
Series 90-70 PLC’s
Baud
19,200
19,200
Data Bits
8
8
Parity
Odd
Odd
Stop Bits
1
1
Max Link Idle Time
10 Secs
5 Secs
GE Port Capabilities
Parameter
Legal Values
Baud
300, 600, 1200, 2400, 4800, 9600, 19200
Data Bits
8
Parity
Odd, Even, None
Stop Bits
1, 2
Max Link Idle Time
1 to 60 Secs
5.2 Client Side Node Descriptor Parameters
Section Title
Nodes
Column Title
Function
Legal Values
Node_Name
Provide name for Node. The driver uses the
Node name to establish a connection with the
Server if the Node_ID is non-zero.
If the Node_ID is zero then the driver ignores
the Node name when logging onto the PLC and
logs on using a Null nameThsu the driver is able
to log onto a Node whose name is unknown.
Up to 32 alphanumeric characters
Node_ID*
The parameter is used in conjunction with
Node_Name to control how the driver logs onto
the PLC
0,1
Protocol
Specify protocol used
SNP
Port
Specify which port the device is connected to
the FieldServer
P1-P8, R1-R22
SNP_Longbreak*
Number of characters used to calculate the
time for the specified baud rate at which the
FieldServer will send a break signal to the PLC.
1-65535, 4
SNP_Break_Offdelay*
Delay (in milliseconds) before a message is sent
to the PLC from the FieldServer after the break
is released.
1-65535, 70
2
Not all ports shown are necessarily supported by the hardware. Consult the appropriate Instruction manual for details of the ports available
on specific hardware.

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Example
// Client Side Nodes
Nodes
Node_Name
, Node_ID
, Protocol
, Port
, SNP_Longbreak
, SNP_Break_Offdelay
PLC 1
, 1
, SNP
, P1
, 50
, 200

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5.3 Client Side Map Descriptor Parameters
5.3.1 FieldServer Related Map Descriptor Parameters
Column Title
Function
Legal Values
Map_Descriptor_Name
Name of this Map Descriptor
Up to 32 alphanumeric characters
Data_Array_Name
Name of Data Array where data is to be stored in
the FieldServer
One of the Data Array names from
Section 4
Data_Array_Offset
Starting location in Data Array
0 to maximum specified in Section
4
Function
Function of Client Map Descriptor
Rdbc, Wrbc, Wrbx
5.3.2 Driver Related Map Descriptor Parameters
Column
Title
Function
Legal Values
Node_Name
Name of Node to fetch data from
One of the Node names
specified in Section 5.2
Data_Type
Data type
Discrete Inputs (%I)
Discrete Outputs (%Q)
Discrete Temporaries (%T)
Discrete Internals (%M)
Genius Global Data (%G)
Analog Inputs (%AI)
Analog Outputs (%AQ)
Registers (%R)
%SA Discrete
%SB Discrete
%SC Discrete
%S Discrete (%S)
Length
Length of Map Descriptor. Length may not exceed the table in the
PLC
1-1000
Address
Starting address of read block / write block. The 1st element of
each Data type Table is referred to as address one.
1 , 2 , 3
Positive whole numbers
Format*
BIT tables are normally read by reading whole Bytes at a time (If
the Length is 10 then 10 Bytes of data are read and placed in 10
Data Array locations). This parameter can be used to read BITs
instead. (If the Length is 10 then 10 BITs are read and each BIT is
stored in its own location.)
BIT, Byte
The format for %AI, %AQ
and %R cannot be changed.
GE_Func*
This parameter is only specified when a write Map Descriptor is
created to change the privilege level.
CPL, RW
5.3.3 Timing Parameters
Column Title
Function
Legal Values
Scan_Interval
Rate at which data is polled
≥0.1s

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5.3.4 Map Descriptor Example 1 –Simple Read
This example provides a Map Descriptor to read 10 Bytes of Discrete Input states, starting at the very first Discrete Input. The data is stored in a Data Array called DA_DI
and the first input is stored at location 100 in the array (101st element). The PLC is polled every 2 seconds
Map_Descriptor_Name
, Data_Array_Name
, Data_Array_Offset
, Function
, Node_Name
, Address
, Length
, Scan_Interval
, Data_Type
Read D1
, DA_D1
, 100
, Rdbc
, PLC-1
, 1
, 10
, 2.0
, %1
5.3.5 Map Descriptor Example 2 –Simple Write
This example writes data from the FieldServer Data Array called DA_AO to the PLC identified as NODE1. The write is repeated every 5 seconds. Ten word values are
written to the PLC’s %AQ Data Table starting at location 20.
Map_Descriptor_Name
, Data_Array_Name
, Data_Array_Offset
, Function
, Node_Name
, Address
, Length
, Scan_Interval
, Data_Type
Write_AO
, DA_AO
, 0
, Wrbc
, Node1
, 20
, 10
, 5.0
, %AQ
Use the % symbol as
you would if you were
programming a GE
PLC.
The name of the Data Array in
which the driver will store the
data. The name must
correspond to a Data Array
defined in Section 4
Map Descriptor names
may be used in driver error
messages. It is useful to
use unique names.
RDBC = read block
continuous. The driver will
read data from the PLC
continuously.
The Node as defined
in Section 5.2. The
Node Name
connects the Map
Descriptor to a Node
which in turn
connects the Map
Descriptor to a port.
The address and length
specify the first element
and the number of
elements that must be
read from the PLC’s data
tables. GE PLC’s reference
data tables starting at
element 1. Unless
otherwise specified, the
driver reads bytes and
words. This Map
Descriptor reads bytes of
data covering %I1 to %I79
Read will be
performed every
2.0s.
Write to PLC Table element number 20 to 29
inclusive (10 elements)
Write continuously
Write to the %AQ
(Analog Output) table
in the PLC.
Location in the Data Array
where the first element of
data will be stored. An
offset of 100 indicates the
101st element of the array.

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5.3.6 Map Descriptor Example 3 –Handling BITs
This example writes data from the FieldServer Data Array called DA_AO to the PLC identified as NODE1. The write is repeated every 5 seconds. Ten word values are
written to the PLC’s %AQ Data Table starting at location 20.
Map_Descriptor_Name
, Data_Array_Name
, Data_Array_Offset
, Function
, Node_Name
, Address
, Length
, Scan_Interval
, Data_Type
, Format
Read_DI
,DA_T
, 100
, Rdbc
, PLC-1
, 8
, 2
, 2.0
, %T
, Bit
The format parameter tells the driver to override the default
data type and read Bits specifically. In this case the Bits are
stored in the Data Array in two separate elements at DA_T[100]
and DA_T[101]

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6CONFIGURING THE FIELDSERVER AS A GE_SNP SERVER
For a detailed discussion on FieldServer configuration, please refer to the FieldServer Configuration Manual. The
information that follows describes how to expand upon the factory defaults provided in the configuration files
included with the FieldServer (See “.csv” files provided with the FieldServer).
This section documents and describes the parameters necessary for configuring the FieldServer to communicate
with a GE_SNP Client.
The configuration file tells the FieldServer about its interfaces, and the routing of data required. In order to enable
the FieldServer for GE_SNP communications, the driver independent FieldServer buffers need to be declared in the
“Data Arrays” section, the FieldServer virtual Node(s) needs to be declared in the “Server Side Nodes” section, and
the data to be provided to the Client needs to be mapped in the “Server Side Map Descriptors” section. Details on
how to do this can be found below.
Note that in the tables, * indicates an optional parameter, with the bold legal value being the default.
6.1 Server Side Connection Descriptors
Section Title
Connections
Column Title
Function
Legal Values
Port
Specify which port the device is connected to the
FieldServer
P1-P8, R1-R23
Protocol
Specify protocol used
SNP
Baud*
Specify baud rate
110 –115200, standard baud rates
only
Parity*
Specify parity
Even, Odd, None, Mark, Space
Data_Bits*
Specify data Bits
7, 8
Stop_Bits*
Specify stop Bits
1
Poll Delay
Time between internal polls.
0-3200s, 1s
Example
// Server Side Connections
Connections
Port
, Protocol
, Baud
, Parity
P1
, SNP
, 19200
, Odd
3
Not all ports shown are necessarily supported by the hardware. Consult the appropriate Instruction manual for details of the ports available
on specific hardware.

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6.2 Server Side Node Descriptors
Section Title
Nodes
Column Title
Function
Legal Values
Node_Name
Provide name for Node. The driver
uses the Node name to establish a
connection with the Server if the
Node_ID is non-zero. Refer to
Appendix A.1
Up to 32 alphanumeric characters
Node_ID
The parameter is used to control how
the driver logs onto the PLC.
0,1
Protocol
Specify protocol used
SNP.
Port
Port must be defined if the
FieldServer is emulating more than
one node and polling more than one
SNP device.
P1-P8, R1-R24
Example
// Server Side Nodes
Nodes
Node_Name
, Node_ID
, Protocol
, Port
PLC 1
, 1
, SNP
, P1
6.3 Server Side Map Descriptors
6.3.1 FieldServer Specific Map Descriptor Parameters
Column Title
Function
Legal Values
Map_Descriptor_Name
Name of this Map Descriptor
Up to 32 alphanumeric characters
Data_Array_Name
Name of Data Array where data is to be
stored in the FieldServer
One of the Data Array names from “Data
Array” section above
Data_Array_Offset
Starting location in Data Array
0 to maximum specified in “Data Array”
section above
Function
Function of Server Map Descriptor
Passive
4
Not all ports shown are necessarily supported by the hardware. Consult the appropriate Instruction manual for details of the ports available
on specific hardware.

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6.3.2 Driver Specific Map Descriptor Parameters
Column Title
Function
Legal Values
Node_Name
Name of Node to fetch data from
One of the Node names
specified in “Client Node
Descriptor” above
Data_Type
Data type
Use one of the Data Types specified in brackets.
Discrete Inputs (%I)
Discrete Outputs (%Q)
Discrete Temporaries (%T)
Discrete Internals (%M)
Genius Global Data (%G)
Analog Inputs (%AI)
Analog Outputs (%AQ)
Registers (%R)
%SA Discrete
%SB Discrete
%SC Discrete
%S Discrete (%S)
Length
Length of Map Descriptor –may not exceed the table length
in the PLC.
1 - 1000
Address
Starting address of read block / write block
The 1st element of each Data type Table is referred to as
address one.
1 , 2 , 3
Positive whole numbers
6.3.3 Timing Parameters
Column Title
Function
Legal Values
Scada_Hold_Timeout
Specifies time Server side waits before responding to Client that Node is
offline on FieldServer Client side.
>1.0s

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6.3.4 Map Descriptor Example.
Map_Descriptor_Name
, Data_Array_Name
, Data_Array_Offset
, Function
, Node_Name
, Address
, Length
, Data_Type
Server-R-Data
, TABLE_R
, 0
, Passive
, PLC-1
, 1
, 1000
, %R
If the Client is reading then
response data will be
obtained from this table.
If the Client is writing,
incoming data will be
stored in this array
Address1 corresponds to
offset 0 (1st location) in the
array. If the Client reads
%R15 then the driver
responds with data from
element 14 of the array.
This is a Server. It
responds to polls but
does no active work
itself.
This Map Descriptor can be
used to respond to a poll that
reads and writes to addresses 1
to 1000. If a poll attempts to
read data at address 1001,
then if no other Map
Descriptor covers that address
space, a “no datas” response
will be sent.
This Map Descriptor will be
used to process Client
read/writes of Register
(%R) data.

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Appendix A. Useful Features
Appendix A.1. SNP Node Names
The following notes describe how the Node_Name is used when the FieldServer is acting as a Server.
When a Client attempts to establish a connection it may send the name of the Node it wishes to connect to or a
Null Node name. If it sends a Null Node name then the FieldServer will connect if any SNP Nodes have been
defined for the same port as the Client using the Null name. The Node must be connected to a port. Only one SNP
node can be connected per port.
Example:
Ports
Port
, Baud
P1
, -
Nodes
Node_Name
, Port
Node1
, P1
If the Client uses the Node name during a connection attempt then the FieldServer uses the name to find a
matching Node. The name must be an exact match. If a match is found then the connection can be established.
In this case the Node does not have to be tied to a particular port and thus one Node could respond to different
Clients polling it on different ports. If this method is used and the Node is tied to a port then the Client must poll
on the same port.
Example: Client uses a specific name to connect. In this case provided there is a Node with a matching name the
connection will be made
Ports
Port
, Baud ….
P1
, ….
Nodes
Node_Name
, Protocol
Node1
, SNP
The Node must be connected to
the Client’s port P1. The Node
name is ignored and connection
is made to the Node through P1,
If there is a Node Called Node1
the connection will be made.

FS-8700-66 GE-SNP Driver Manual Page 19 of 28
FieldServer Technologies 1991 Tarob Court Milpitas, California 95035 USA Web: www.fieldserver.com
Tel: [phone] Fax: [phone] Toll Free: [phone] email: [email protected]m
Appendix A.2. Scaling
5
Scaling is applied by the driver when Bytes and registers are stored.
When BIT data is transferred in BIT format than scaling is not applied. When BIT data is transferred using bye
format’s then scaling may be applied.
Refer to the FieldServer Configuration Manual for more information on how scaling is handled by the FieldServer.
5
Valid for driver Version 1.02 and later.

FS-8700-66 GE-SNP Driver Manual Page 20 of 28
FieldServer Technologies 1991 Tarob Court Milpitas, California 95035 USA Web: www.fieldserver.com
Tel: [phone] Fax: [phone] Toll Free: [phone] email: [email protected]m
Appendix B. Troubleshooting
Every 2nd message Times Out A possible cause is that the poll frequency is so low that the time
between polls is greater than the Max Link Idle Time (of the PLC). If the time is exceeded the connection is
closed. Thus when the next poll is received by the PLC it ignores it and hence it times out in the
FieldServer. After timing out, the driver re-establishes the connection for the next poll.
Very slow Comms Rate A timer known as the T1 timer is the minimum time that must pass before a
response to a message can be sent. Both ends of a SNP connection have their own T1 timer (hard coded
in the driver). When a connection is made the timers are exchanged and the larger of the two is used.
The drivers T1 timer is set to a very small number. SNP message #6 indicates the size of the T1 timer. You
may need to configure the PLC to improve communications.
Multidrop RS-485 networks. SNP protocol specifies that there may only be one Master on a
network at a time. When using nameless Nodes (Node_ID=0) there may be only one PLC on the
connection.