CENTAC Communicating with the Centac Control Systems

Transcription

1 CENTAC Communicating with the Centac Control Systems INGERSOLL-RAND AIR COMPRESSORS

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3 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS Copyright Notice Copyright 1992 Ingersoll-Rand Company THE CONTENTS OF THIS MANUAL ARE PROVIDED AS IS AND WITHOUT ANY IMPLIED WARRANTIES WHATSOEVER. Ingersoll-Rand air compressors are not designed, intended, or approved for breathing air applications. Ingersoll-Rand does not approve specialized equipment for breathing air applications and assumes no responsibility or liability for compressors used for breathing air service.

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5 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS Table of Contents Introduction 1 RS-232C Interfacing Methodologies 1 Hardware Configurations 1 Custom Software 2 Programmer Skill for Custom Software 2 RS-232 to Serial Device 3 INCOM to MINT Box to Serial Device 4 RS-232C or MINT Box to Serial Device via Phone Modems 5 MODEM to Phone Modem to Serial Device 6 MP3 to Fisher-PRoVOX 20 SERIES Controller 6 Current RS-232C Projects 8 Connections 8 Direct Connection 9 Short Haul MODEM Connection 12 Centac Energy Master (CEM) 16 Software 16 CEM Features 16 Hardware 18 CEM 18 Which Method Should Be Used and When? 19 Communication Features 20 Glossary 21

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7 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 1 Introduction Customers want to communicate to our control systems for remote compressor control and monitoring. This communication capability provides for flexibility in the customer's compressed air operation through remote start and stop, data gathering for preventative maintenance, and incorporation into plant-wide control system. There are two major avenues for communicating with Centac control systems. Each requires hardware for the control panel, and a communications device with software to perform the desired panel functions. Centac Energy Master (CEM) is a system written by Ingersoll-Rand specifically for Centac air compressors with MP3 panels and an IBM Compatible Personal Computer. The RS-232C interface is a system that can communicate with any serial device that has an RS-232C port. This system's software must be written by the customer or his representative to suit his individual needs for remote control and monitoring. Since the customer writes this interface, the system can be as flexible as the customer desires. The following sections indicate the features of each communication method, the hardware and software requirements for the various RS-232C interfaces and finally the hardware requirements for CEM. The intent of this document is to assist in selecting the appropriate communication method for a specific Centac Control System. RS-232C Interfacing Methodologies Hardware Configurations For the descriptions that follow, a serial device can be a Personal Computer (PC), Programmable Logic Controller (PLC), Distributed Control System (DCS) or any other device that can transmit, receive and interpret an RS-232C formatted signal. In the descriptions that follow, the PC and PLC serial devices are not specific to manufacturers or operating systems; however, some specific interfaces have already been developed. These interfaces will enable faster development for custom software. There are many ways of interfacing to Centac control systems through an RS-232C port. Most of the following methodologies are currently available; but please be aware, other possible configurations can exist. The primary RS-232C interfaces are: RS-232 to a Serial Device INCOM to a MINT Box then to a Serial Device MODEM to a MODEM on the Serial Device Each of these major types have strengths and weaknesses depending upon the number of air compressors a facility has, the distance the air compressors are from the serial device and the customers networking philosophy.

8 2 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Custom Software All RS-232C interfaces require custom interface software and custom application software. The interface software allows a specific serial device and operating system to transmit, receive, and interpret data from a Centac control system. The application software tells the Centac control system what to do; for example, start compressor when ready, stop compressor after midnight and retrieve the current data and save to a disk file. Currently there are hundreds of different serial devices using different operating systems and languages. Therefore, the practicality of having an interface for many systems is limited. Custom interfaces must be written as required by the hardware and operating system used. In order to assist the interface developer, the MP3 and MVP RS-232C INTERFACE MANUAL is provided after an order is placed for any RS-232C methodology. This manual provides protocol and data interpretation information for Centac control panels and a prewritten example of the interface for a DOS based IBM Compatible Personal Computer. Once the interface is written, application software must be written to interpret the transmitted and received signals into useful information. The application software is only limited by the capabilities of the hardware and the imagination of the developer. For example, one developer may have two compressors. In this application the developer wants a screen to display the compressor interstage pressure and temperatures for both machines with various other compressor data. A second developer has five compressors. He also wants to display the same data, but this time for all five machines. The only way this is done is through changing the application software (custom modification). The developer may write functions to read and display data, log that data to some magnetic media for storage, change compressor setpoints, sequence the compressors for efficient operation and network additional RS-232C devices, such as pumps, dryers, etc., into the system. All of these functions require specially written application software for the intended use. Programmer Skill for Custom Software Programmers familiar with the RS-232C environment are needed to write interface drivers and applications. The interface manual and software provided after purchase of an RS- 232C system covers the details of interfacing Ingersoll-Rand Company's Centrifugal Compressor control products to other systems that have an RS-232C port. This manual will not teach you every detail needed for RS-232C communications, or programming in general. There are many texts available that address these topics.

9 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 3 RS-232 to Serial Device This method requires an RS-232 card for each compressor, RS-232C cable and a port on the serial device (see diagram). The RS-232 card mounts in the compressor panel on the MP3 or MVP board and has a female 25 pin (DB25) RS- 232C connector. The communicates at 1200 Baud and requires the Receive Data (RD), Transmit Data (TD) and Common Ground (GND) signals. Therefore, a three-wire RS-232C cable is all that is required for operation. Normally, the maximum distance for RS-232C communications is 50 feet (15.24 meters). Extended distance cable can be obtained to reach a maximum distance of 500 feet (152.4 meters). Signal strength beyond this distance is too weak to maintain error free communication. There is no physical limit to the number of compressors that can be connected in this manner. The only stipulation is that a separate serial port exists for each RS-232. Short Haul Modem RS-232Microcontroller RS-232C Cable Dual Shielded Twisted Pair Wire (Total of 4 wires) RS-232C Cable Short Haul Modem Serial Ports Short Haul Modem Short Haul Modem pair wires, for a total of four wires. 1 2 RS-232Microcontroller SERIAL DEVICE RS-232 Microcontroler RS-232C Cable Serial Ports SERIAL DEVICE This stipulation imposes a practical limit for most PCs. A typical PC has one serial port as standard. An additional port can be added with a special card up to a total of four serial ports. The Programmable Logic Controller (PLC) presents different limitations. Some PLCs can add cards for additional ports, so the practical limit for PLCs can be much higher. The PLC manufacturer must provide this information for each case. Serial devices that use RS-422, RS-485 and other standards, require an in line converter for signal conversion. Each of these non RS-232C signals must be evaluated on a case by case basis. For greater distances, the addition of Short Haul Modems placed between the RS-232C and the Serial Device can be added. The Short Haul Modems can extend the RS-232C formatted signal up to 4 to 5 miles (6.5 to 8.0 km). The Short Haul Modems are connected by two sets of shielded twisted 1 2 RS-232 Microcontroler

10 4 COMMUNICATING WITH CENTAC CONTROL SYSTEMS INCOM to MINT Box to Serial Device This arrangement requires an INCOM, twisted pair wire, MINT Box, and a port on the Serial Device (see diagram). INCOMMicrocontroller INCOMMicrocontroller INCOMMicrocontroller INCOM Network Twisted Pair Wire MINT BOX RS-232C Cable Serial Port 1 SERIAL DEVICE The INCOM mounts in the compressor panel the same as all of the other configurations and has a two wire connector. The communicates at 1200 Baud to the MINT Box. Each INCOM has three dials for setting the address for the device. The reason for the address is that the MINT must know which machine it is talking too. The MINT is a networking device that control up to 4095 INCOM devices and converts the INCOM signal into an RS-232C signal. An INCOM device is an INCOM card or MINT Box. Therefore, it is conceivable to have multiple MP3 panels and multiple MVP panels all on the same system. The MINT has an additional feature not found in the other serial methodologies. The MINT is configurable for 300, 600, 1200 and 2400 Baud and contains an internal buffer to store data. NOTE: Regardless of the MINT Baud rate, the INCOM network is still operating at 1200 Baud. The maximum distance for the twisted pair INCOM network is 2500 feet (762 meters). Under special circumstances this distance can be extended, but requires approval. The MINT Box has a female 25 pin (DB25) connector and is a serial connection. The maximum distance for this serial connection is 50 feet (15.24 meters) unless extended distance cable, maximum distance 500 feet (152.4 meters), is used. This methodology should be used for multiple compressor applications. As a rule of thumb, when more than two compressors, you should use an INCOM to MINT Box arrangement; and less than three, an RS-232.

11 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 5 RS-232C or MINT Box to Serial Device via Phone Modems INCOMMicrocontroller INCOMMicrocontroller INCOMMicrocontroller INCOM Network Twisted Pair Wire Telephone Wire RS-232C Cable MINT BOX Phone Modem RS-232C Cable Typical RJ11 Telephone Jacks Phone Modem Serial Port SERIAL DEVICE The above configuration will allow long distance communications through a phone modem with either an RS-232C Card or a MINT Box. Extended operation and control is not recommended in this configuration due to the nature of phone line communications. Line noise could shut the communication down at any time without notice causing delays in data retrieval, which may cause problems depending upon the application. 1 NOTE Special consideration must be given in programming and device connection for proper operation in this configuration.

12 6 COMMUNICATING WITH CENTAC CONTROL SYSTEMS MODEM to Phone Modem to Serial Device When the compressor is many miles away from the serial device, the MP3 or MVP requires a MODEM and a telephone jack in the vicinity of the compressor. A phone modem and a port are required on the serial device. Typical RJ11 Telephone Jacks Phone Modem RS-232C Cable The MODEM mounts in the compressor panel in the same manner as the RS-232, but has a RJ-11 telephone jack instead of the female DB25 connector. The communication takes place at 1200 Baud. The phone signal quality is the limiting factor in this configuration. At 1200 Baud, most phone signals are of high enough quality to satisfactorily communicate with the compressor. The practical number of compressors for this application is one. It is theoretically possible to do multiple machines, but the expense of multiple phone lines and multiple phone modems prohibit such an arrangement. MP3 to Fisher-PRoVOX 20 SERIES Controller The previous RS-232C Interfaces are generic for unspecified target systems (serial devices). This last methodology is for a specific serial device and MP3 panels. This interface comes in two variations: Telephone Wire Serial Port 1 MODEMMicrocontroller SERIAL DEVICE RS-232 to PLC INCOM MINT Box to PLC The hardware requirements are similar to the previously discussed methods; i.e., RS-232 to Serial Device and INCOM to MINT.

13 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 7 RS-232Microcontroller RS-232Microcontroller RS-232Microcontroller RS-232Microcontroller RS-232C Cable FISHER PROVOX PLC External Interface Cards On the Fisher-PRoVOX 20-SERIES Controller end, an External Interface Card (CL6921) is required with an EPROM written specifically for this application. For the MINT Box configuration, a maximum of four MP3's can be networked. Other than this card and EPROM, the maximum distances and operating procedures are the same as any RS-232C device. The reason that only four MP3s can be networked with a MINT Box is due to the Fisher- PRoVOX 20-SERIES PLC configuration and limitations. The External Interface Card (CL6921) has a fixed number of input/output (I/O) points that can be addressed. This I/O limitation fixes the maximum number of compressors that can be addressed on a single card to four. If more machines are required, additional External Interface cards can be purchased.

14 8 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Current RS-232C Projects The following table lists interfaces currently being developed for the MP3 by Ingersoll-Rand Company and our customers. Not all of these interfaces are complete. However, the list indicates that developments are under way for a diverse number of serial devices. Hardware Operating Environment Written by: Completed 386 PC Compatible DOS - Genesis, graphics based control system Ingersoll-Rand 386 PC Compatible Windows - Genesis Ingersoll-Rand No 386 PC Compatible Windows - Visual Basic Customer Yes 386 PC Compatible QNX Operating System, similar to UNIX Allen-Bradley PLC PLC 5, with RB Flexible Interface Card Customer Customer Allen-Bradley PLC PLC 5, with DB Basic Module Customer Yes Fisher-ProVOX 20 Series PLC No Yes Yes Proflex Ingersoll-Rand Yes Honeywell PLC Honeywell PLC Language Customer No SIEMENS PLC Siemens PLC Language Customer No Square-D PLC Square-D PLC Language Customer No Connections The correct wiring between the RS-232C or MINT Box to the serial device is critical for proper communications. A standard nine or twenty-five wire RS-232C cable is not recommended. To make sure the interface wiring is correct, only use a three wire cable as outlined below.

15 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 9 Direct Connection The connections between the User's serial device and the RS-232C or MINT Box are accomplished with a three-wire RS-232C cable. Because this cable does not enable the hardware handshaking lines, the MINT Box may operate unpredictably. Refer to the interface diagram for wiring schematic with DB25 and DB9 connectors on the User's RS-232C device. User's RS-232C Device with DB25 Connector User's Interface Cable RS-232C or MINT Box with DB25 Connector 2 TxD (Transmit) > 2 3 RxD (Receive) < Signal Common > 7 (ground) User's RS-232C Device with DB9 Connector User's Interface Cable RS-232C or MINT Box with DB25 Connector 3 TxD (Transmit) > 2 2 RxD (Receive) < Signal Common > 7 (ground)

16 10 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Figure 17-1: DB9 to DB 25 interconnecting RS232C cable makeup

17 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 11 Figure 17-2: DB-25 to DB-25 interconnecting RS232C cable makeup

18 12 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Short Haul MODEM Connection To operate successfully with a Short Haul Modem, the RS-232C connections are identical to the direct connection listed previously. The Short Haul Modem on the Serial Device side should be set as Data Communication Equipment (DCE). The Short Haul Modem on the INCOM device end should be configured as Data Terminal Equipment (DCE). To disable handshaking from those Short Haul Modems that have that characteristic, place a jumper between line 8 (Carrier Detect) and line 20 (Data Terminal Ready) on the Modem configured as DTE. For additional information on wiring, consult the Short Haul Modem manual provided with the device. Serial Device DCE User's Interface Cable Card DTE T+ R+ T- R- R+ T+ R- T-

19 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 13 Figure 17-3: Short haul MODEM connections

20 14 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Figure 17-4: Setup of typical card with Baud rate selection

21 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 15 Figure 17-5: Jumper setup on CONI card with Baud rate selection RS-232 or MINT Box to Phone Modem Connection The connections are similar to the direct connection with the exception on lines 2 (TxD, Transmit Data) and 3 (RxD, Receive Data) must be switched because the phone modem reverses these lines.

22 16 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Centac Energy Master (CEM) Software CEM is prewritten, packaged software available for compressors with MP3 panels. This software features Controlled Surge Reset and Reload, Compressor Rotation, Automatic Timed Logging of Compressor Data, Event Logging of Trip, Alert, and Surge, Load Sharing, Adaptation to Compressor Loading Schedule, and Schedule Sequence Rotation of Compressors. Even with this feature rich set of capabilities, some users want more or different options. For example, some users might want to display multiple compressors on a single screen, but today each compressor is on a separate screen (see CEM Instruction Manual for details). A user might want to log specific data rather than all data. The options are endless. Adding each of these features requires reprogramming and testing (custom programming). Currently, these custom modifications are not available. CEM has many features and should meet most needs, but for those that need special modifications, the RS-232C Interface is the route for them. CEM changes require development of a new program. CEM Features Unmanned Operation Automatic control of compressors including starting / stopping / loading / unloading. The CEM automatically controls all phases of the compressors' operation. The compressors will start, stop, load, and unload automatically depending on the user's demand for air. This allows for operator-free operation. Controlled surge reset and reload. When a surge occurs, the compressor automatically resets and reloads. On surging of the lead compressor the user has the ability to control how often it will be allowed to surge before a sequence rotation occurs (lag compressors will be reset indefinitely). The sequence does not rotate due to a surge of a lag compressor. Compressor rotation in the event of a failure on the lead air compressor. When the lead compressor trips or alarms, the system automatically rotates compressors. This makes the next available compressor the lead compressor and the old lead compressor goes to the end of the rotation sequence. If the new lead compressor is not running loaded, the CEM program will start and/or load it. Automatic timed logging of compressor data. Compressor data can be logged: saved, printed, or displayed at specified intervals between 1 second and 48 hours. The user can log data in two different formats on all compressors or only on running compressors. This log supplies a record of compressor performance for future evaluation.

23 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 17 Efficient Operation Load sharing of two to eight centrifugal air compressors. The CEM system shares the air load between all running compressors. This is done by having the lag compressors duplicate the position of the lead compressor's inlet valve. This provides stable load sharing and reduced blow off air, which saves money. Weekly schedule for the selectable control timers. The amount of time a compressor takes to start, load, unload, or stop is determined by selectable control timers. The CEM allows three different sets of values for each timer. The weekly schedule lets the user choose which set of values the compressors uses each hour of the week. This enables the user to adjust the CEM for known load changes, such as morning start-up, lunch breaks, and weekends. Advanced Compressor Protection Compressor data review. Compressor data: vibration, pressure, temperature, running hours, system pressure, setpoints, number of starts, and motor current is displayed on the computer screen. The user is able to view data from one compressor at a time or view multiple compressor data by setting the scroll rate to cycle through the data. Event logging of trip, alarm, and surge. CEM logs data whenever a compressor alarms, trips, or surges. The data is logged to a customer specified location: file, printer, or display. Scheduled sequence rotation of compressors. At a user-defined interval the CEM can rotate the compressors' sequence order. The first compressor becomes the last compressor in the sequence, and the other compressors move up. This feature equalizes the compressor runtime. Ease of Operation Change setpoints from computer's keyboard. This feature allows the user to change compressor setpoints from the computer. The compressor setpoints are pressure, current limit low, current limit high, proportional band, integral time, ramp rate, reload percent, and P1. Full manual control of the air compressors from the computer. The user can load, unload, reset, start, and stop any compressor from the computer regardless of whether the sequencer is active or inactive. NOTE The compressor panel must be in the remote mode before CEM will be allowed to start or stop the compressor.

24 18 COMMUNICATING WITH CENTAC CONTROL SYSTEMS Software Features Resident software frees computer for other tasks. The CEM system can be set up to run in the background of the computer while other programs run in the foreground. This makes the computer available for other uses without shutting down the CEM system or interfering with compressor operation. NOTE Running the CEM system in resident mode is not recommended unless all other software to be run with CEM is thoroughly tested for compatibility problems. Running the CEM system in resident mode is not recommended unless all other software to be run with CEM is thoroughly tested for compatibility problems. Screen blank option to save monitor. The computer monitor can be set up to go blank after 5 minutes of no keyboard use. The monitor is recalled by pressing any key. English or metric units available. CEM can be set up to use English or Metric units. Remote terminal access to compressor data. A remote terminal or computer system may access the compressor data through the RS-232 port. Help screens for operations. A complete on-line help system is available for every screen. Password protection. System security is provided through a password system. Up to nine different passwords allow selected entrance to the different screens. Easy to learn and use. The software is designed for easy usage and for quick learning. A minimal number of keystrokes is required to operate the system. Hardware The hardware requirements for CEM are similar to the RS-232C Interface hardware with a two exceptions. A CONI Card is used with CEM instead of a MINT Box. The CONI card performs the same function as the MINT, but is inserted into a PC. The Serial Device must be a PC with an AT bus structure. CEM The Centac Energy Master requires an INCOM for each compressor, twisted pair wire, and a PC with the AT bus structure and one empty 8-bit slot for the CONI Card.

25 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 19 INCOMMicrocontroller INCOMMicrocontroller INCOMMicrocontroller INCOM Network Twisted Pair Wire CONI CARD Personal Computer (with AT Bus) The INCOM mounts in the compressor panel and has a two wire connector. The communicates at 1200 Baud to the CONI Card in the PC. Each INCOM has three dials for setting the address for the MP3 panel. The maximum number of MP3 panels that can be networked to CEM is eight. This limitation is software driven, not hardware. The maximum distance for the shielded twisted pair INCOM network is 2500 feet (762 meters). Under special circumstances this distance can be extended, but requires approval for each application. Which Method Should Be Used and When? There is no simple answer to this question, because it depends upon several parameters and the user's preference. The following questions should be asked in making the decision. What is the purpose for the control system? Data Logging? Remote Operation? Load Sharing? What is the target environment? PC? PLC? Distributed Control System (DCS)?

26 20 COMMUNICATING WITH CENTAC CONTROL SYSTEMS If the target environment is a PLC or DCS system, does it have support for a serial interface? Is other equipment (pumps, dryers, other compressors) involved in a centralized system? How many compressors are to be operated in the control system? What is the physical distance between the compressor(s) and the centralized system? feet (meters)? miles (kilometers)? Does the customer have resources or willing to write an interface and/or application? Communication Features Features RS-232C Interface CEM Software Written By Customer IR Maximum Devices to Control Device Types Supported MP3 Yes Yes MVP Yes No Unmanned Operation Yes Yes MP3 Faceplate Control and Monitoring Functions Compressor Alert and Shutdown Status Yes* Yes Run Time Yes* Yes Number of Starts Yes* Yes Stop Yes* Yes Reset Yes* Yes Acknowledge Yes* Yes Start Yes* Yes Actual Values Yes* Yes Alert Values Yes* Yes Shutdown Values Yes* Yes Unload Yes* Yes Modulate Yes* Yes Autodual Yes* Yes Bypass Valve Status Yes* Yes Inlet Valve Status Yes* Yes Change Set Points Yes* Yes

27 COMMUNICATING WITH THE CENTAC CONTROL SYSTEMS 21 Features RS-232C Interface CEM Other MP3 Functions Controlled Surge Reset and Reload No Yes Compressor Rotation No Yes Automatic Timed Logging of Compressor Data No Yes Event Logging of Trip, Alert, and Surge No Yes Load Sharing No Yes Adaptation to Compressor Loading Schedule No Yes Schedule Sequence Rotation of Compressors No Yes MVP Faceplate Control and Monitoring Functions Compressor Alert and Shutdown Status Yes* No Start Yes* No Stop Yes* No Load Yes* No Unload Yes* No Reset Yes* No Compressor Status Flags Yes* No Analog Buffer Yes* No Setpoint Buffer Yes* No Change Setpoints Yes* No Feature directly supported through interface software. NOTE "No" is not a physical limitation, but one imposed by the application software. Programming is required to achieve a "Yes". Glossary ASCII Byte DCS EPROM ERAM American Standard Code for Information Interchange. Eight bits of data. Distributed Control System. A centralized system for controlling various types of equipment; i.e., compressors, pumps, dryers, etc. Erasable Programmable Read Only Memory. Erasable Random Access Memory.

28 22 COMMUNICATING WITH CENTAC CONTROL SYSTEMS INCOM INCOM Device INCOM MINT Modem MP3 MVP Nibble PROM RAM RS-232 Serial Device System Pressure Word INdustrial COMmunications Network. A Westinghouse data protocol. Can be one of the following communication devices that have the capability of translating the Westinghouse INCOM protocol into the RS-232C protocol. The current devices available are MINT, RS-232, and a Modem. Product Operated Network Interface. Master INCOM Network Translator. A networking device allows input of 4095 devices that transmit the INCOM signal and translates the signal into an RS-232C formatted signal. Same as a except with a telephone jack interface instead of twisted pair wire interface. Ingersoll-Rand microprocessor that provides the control and monitoring for standard Centac air compressors. Specialized Ingersoll-Rand microprocessor that provides the control and monitoring for Centac models CV0 and CH3 air compressors. Half of a byte, or four bits of data. Programmable Read Only Memory. Random Access Memory. Product Operated Network Interface. A device that translates the INCOM signal into an RS-232C formatted signal. A Personal Computer (PC), Programmable Logic Controller (PLC), Distributed Control System (DCS) or any other device that can transmit, receive and interpret an RS-232C formatted signal. The actual operating discharge pressure of the compressor. Two bytes, or sixteen bits of data.