Emerson VSG & VSSG Specifications download pdf (Page 135)

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Section 12 • Communications with a Central Controller/DCS

Single Screw Compressor PLC Operation Manual • Vilter/Emerson • 35391CM

Communication with a Central

Controller/DCS

IIt is possible for a central controller/DCS to read live

data and other information from the compressor PLC as

well as send commands to the compressor PLC. The fol-

lowing section outlines the PLC addresses that are used

for interface between the Compressor PLC and central

controller/DCS.

VPN ACCESS

VPN access allows Vilter engineers and technicians con-

nect remotely to the compressor PLC for greater ease of

support and troubleshooting. If possible, it is recom-

mended that VPN access be provided.

To allow VPN access to a compressor PLC, the following

are required:

• A physical Ethernet Connection from the compressor

PLC to the site control network

• IP address, Subnet Mask, and Gateway address must

be deﬁ ned for each compressor PLC and HMI on the

site control network

• A VPN server linked to the site control network

• VPN server login information

SETTING UP COMMUNICATIONS

The compressor PLC communicates using the Ethernet

IP protocol. Communication to other devices compat-

ible with Ehternet IP is as simple as setting up CIP mes-

sage instructions and does not require any additional

hardware other than that required for the physical

connection.

Interface Data in the compressor PLC is organized into

arrays for simple access of data. For example, all live

instrument data is arranged into a single block 200 el-

ements long. To get all of the data, only one message

would need to be conﬁ gured to read an array of regis-

ters, Data type REAL, Length of 200. See the communi-

cation tables below for speciﬁ cs.

NOTE

Setting up communications using a protocol other

than Ethernet IP is possible (Example Modbus RTU

or Modbus TCP), but additional hardware, software,

programming, and setup are required.

WATCHDOG TIMER

A watchdog timer is used to monitor the status of the

communication link between the compressor PLC and

Central Controller/DCS. It is only necessary to write

watchdog logic when the Central Controller/DCS is

sending commands to the compressor PLC. If reading

data only, the Watchdog timer is not used.

The watchdog timer works by “passing a bit” back and

forth between the Compressor PLC and central control-

ler/DCS. If the compressor PLC sees the bit in the same

state for a settable watchdog time, the communications

are faulted. The Communication fault action is select-

able between “Alarm and Revert to Local Mode” or “Trip

Machine.” If “Alarm and Revert to Local Mode” is se-

lected, the compressor will continue to run if running. If

“Trip Machine” is selected, the compressor will trip and

shut down. The settable watchdog time and commu-

nications fault action are selected on the Conﬁ guration

Screen.

The watchdog bits that are used are:

• Compressor to Central/DCS: BOOL_OUT[0].0

• Central/DCS to Compressor: BOOL_IN[0].0

When the Compressor PLC sees BOOL_IN[0].0 OFF it will

turn BOOL_OUT[0].0 ON. To maintain the handshake,

the Central Controller/DCS shall Turn BOOL_IN[0].0 ON

when it sees BOOL_OUT[0].0 ON.

Data that can be Read from the

Compressor PLC

LIVE INSTRUMENT DATA

Tags “REAL_OUT[0]” through “REAL_OUT[99]” present

the live data values as they appear on the HMI screen

in the selected engineering units. The Tags “REAL_

OUT[100]” through “REAL_OUT[199]” present all live

data values in base units, which are PSIA for pressures

and Degrees Fahrenheit for Temperatures. For data other

than Pressures and Temperatures, the engineering units

are ﬁ xed and not selectable, and will appear the same in

“REAL_OUT[0]” through “REAL_OUT[99]” as they will in

“REAL_OUT[100]” through “REAL_OUT[199].” The en-

gineering units selected are presented in the tags “INT_

OUT[0]” through “INT_OUT[99].”

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Emerson VSG & VSSG Specifications Page 135

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