Service Request Transport Protocol : Working & Its Applications

The Service Request Transport Protocol or SRTP was developed by “GE Intelligent Platforms” formerly known as GE Fanuc used for communication with programmable logic controllers. The common devices which are used in different industries like aerospace, water management,, manufacturing, and travel are programmable logic devices which are known as PLCs. This article discusses an overview of an SRTP or service request transport protocol – working with applications.

What is Service Request Transport Protocol?

The protocol which is used to transfer the data from PLCs (programmable logic controllers) is known as the service request transport protocol. This protocol is simply used over Ethernet and it supports almost all equipment of GE automation once equipped through an Ethernet port. This Ethernet port supports GE SRTP which is a successor to the serial protocols like SNP & SNPX for Ethernet media. Any client of SRTP is capable of system memory reading & writing for any remote capable devices of SRTP.

How Does SRTP Work?

The GE-SRTP Driver protocol works by transferring the data to & from the device on Ethernet by allowing the FieldServer through the GE-SRTP driver. Almost all automation equipment of GE simply supports this protocol once equipped with an Ethernet Port.

The FieldServer can follow either a Client or Server. The driver acts like a Client who is capable of reading & writing the memory of the system for any no. of remote SRTP devices. Once the FieldServer acts like a Client then it can scale word data.

Once the FieldServer is arranged to operate as an SRTP Server, then the driver allows system messages written to the memory of the system & responds to read requests. The driver cannot follow a Client & Server at the same time on a similar connection to a similar IP address.

The driver is capable of representing its communication information so that it can be observed through a downstream device. In the FieldServer data arrays, Driver statistics & diagnostics are represented so they may be read & monitored through remote devices or systems.

Signal Flow from HMI to PLC using SRTP Protocol

A PLC includes various registers where these registers are programmed & accessed through an HMI (human-machine interface). Generally, an HMI is a software device that runs on a PC. The signal flow diagram from the HMI to the PLC using SRTP is shown below.

HMI computer runs with Windows XP which is operated by the operators of the system to communicate through the PLC. Here, a simulated human machine was created using Wonderware Intouch v9.5. So, the communication between the GE Fanuc Series 90-30 & the workspace needsWonderware IO server which can include various input sources. In this, we used Wonderware Intouch software, Microsoft Excel & the Dynamic Data Exchange protocol. The GE-SRTP protocol is used between the Wonderware IO server & the GE Fanuc Series 90-30) for transporting the data.

Difference B/W SRTP Vs RTP

The difference between SRTP and RTP includes the following.

Supported Data Types & Functions

The data types and functions supported by SMBus include the following.

• %R – Register memory.
• %AI – Analog Input Memory.
• %AQ – Analog output memory.
• %I – Discrete input memory.
• %Q – Discrete output memory.
• %T – Discrete temporary memory.
• %M – Discrete momentary memory.
• %SA – Discrete system memory group A.
• %SB – Discrete system memory group B.
• %SC – Discrete system memory group C.
• %S – Discrete system memory.
• %G – Discrete global data table.

Similarly, unsupported functions & data types are programming messages. A data transfer device like FieldServer doesn’t require programming messages.

Advantages and Disadvantages

The advantages of service request transport protocol include the following.

• High Quality.
• It is very simple to implement as well as to use.
• It is reliable & has Low Latency.
• Open Source.
• Interoperable & Content Agnostic.
• Highly protected.

The disadvantages of service request transport protocol include the following.

• It needs more memory & programming effort.
• Its latency is high because of its connection-oriented nature.

Applications of Service Request Transport Protocol

The applications of service request transport protocol include the following.

• Service Request Transport Protocol is used to transfer data from PLCs (programmable logic controllers).
  This type of protocol is used over Ethernet and all the GE automation equipment supports these protocols once equipped through an Ethernet port.
• This protocol is simply used by several Ethernet-connected controllers.

Thus, this is an overview of a service request transport protocol or SRTP. The SRTP protocol was tested against the Fanuc Robot R-30iB for reading, and writing different values from types of memory and both reading & writing of text values worked. Here is a question for you, what is HTTP?