GDS Corp GDS-68XP User manual

GDS Corp.
2513 Hwy 646 Santa Fe, Texas 77510
409-927-2980 409-927-4180 (Fax) ww.gdscorp.com
Operation and Maintenance
Manual
GDS-68XP Process Monitor for Low O
2
Applications

GDS-68XP Operation & Maintenance Manual, Revision 3.4
Page 2
CAUTION: FOR SAFETY REASONS THIS EQUIPMENT MUST BE OPERATED AND
SERVICED BY QUALIFIED PERSONNEL ONLY. READ AND UNDERSTAND
INSTRUCTION MANUAL COMPLETELY BEFORE OPERATING OR SERVICING.
ATTENTION: POUR DES RAISONS DE SÉCURITÉ, CET ÉQUIPEMENT DOIT
ÊTRE UTILISÉ, ENTRETENU ET RÉPARÉ UNIQUEMENT PAR UN PERSONNEL
QUALIFIÉ. ÉTUDIER LE MANUE D’INSTRUCTIONS EN ENTIER AVANT
D’UTILISER, D’ENTRETENIR OU DE RÉPARER L’ÉQUIPEMENT.
REVISION HISTORY
Revision 3.0 6/5/13 Initial rewrite for version 3.0
Revision 3.1 11/1/13 Update for GASMAX CX
Revision 3.2 12/15/13 Update for v2.3 sequencer firmware
Revision 3.4 1/30/14 Update for v2.4x sequencer firmware
Copyright © 2014 GDS Corp. All Rights Reserved
P/N 1200-0732-05

GDS-68XP Operation & Maintenance Manual, Revision 3.4
Page 3
CONTENTS
1 SAFETY INFORMATION _______________________________________________ 8
2 GENERAL INFORMATION ______________________________________________ 9
Introduction ________________________________________________________________ 9
Explosion Proof Installation ____________________________________________________ 9
Intrinsically Safe Installation ___________________________________________________ 9
3 SPECIFICATIONS ____________________________________________________ 10
4 OPERATION _______________________________________________________ 12
Overview _________________________________________________________________ 12
Sa ple Draw Configuration ___________________________________________________ 14
Sensor Considerations _______________________________________________________ 14
5 INSTALLATION _____________________________________________________ 15
Selecting a Location _________________________________________________________ 15
Power & Signal Connections __________________________________________________ 16
On-De and Sequence Input __________________________________________________ 16
Ethernet Connection ________________________________________________________ 16
Relay Connections (Optional) _________________________________________________ 17
MODBUS Connections (Optional) ______________________________________________ 17
AC Heater (Optional) ________________________________________________________ 19
Purge Air Inlet______________________________________________________________ 19
Sa ple Inlet _______________________________________________________________ 20
Filter Drain & Filter Bypass ___________________________________________________ 21
Sa ple Exhaust ____________________________________________________________ 22
Installation Su ary: Do’s and Don’ts __________________________________________ 23

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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6 INITIAL SETUP______________________________________________________ 24
Overview _________________________________________________________________ 24
Sequencer Controls and Indicators _____________________________________________ 24
SEQUENCE ______________________________________________________________________ 24
Bypass _________________________________________________________________________ 25
Run ____________________________________________________________________________ 25
L1 and L2 _______________________________________________________________________ 25
GASMAX Controls and DIsplay ________________________________________________ 26
RAW SENSOR ____________________________________________________________________ 26
PPM OUTPUT ____________________________________________________________________ 26
DUAL DISPLA ___________________________________________________________________ 26
GDS-68XP STARTUP PROCEDURE ______________________________________________ 27
7 CALIBRATION ______________________________________________________ 28
Calibration Overview ________________________________________________________ 28
Calibration Procedure (CAL PORT SAMPLE) ______________________________________ 29
Calibration Procedure (INLET PORT SAMPLE) _____________________________________ 30
Calibration Procedure (STREAM SAMPLE) _______________________________________ 31
8 OPERATION an MAINTENANCE _______________________________________ 32
Nor al Operation __________________________________________________________ 32
Cold Weather Operation _____________________________________________________ 32
Nor al Maintenance ________________________________________________________ 32
Fault and overrange Conditions _______________________________________________ 33
Power Up _______________________________________________________________________ 33
Input Overrange FAULT ____________________________________________________________ 33
Sensor Recovery FAULT ____________________________________________________________ 33
Peak Algorithm fault ______________________________________________________________ 33
Sample or Purge Air Flow fault ______________________________________________________ 33
Calibration fault __________________________________________________________________ 33
Sensor Fail fault __________________________________________________________________ 33

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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Sensor Replace ent ________________________________________________________ 34
9 USER MENUS ______________________________________________________ 35
Alar OUTPUTS Menu _______________________________________________________ 36
Channel Settings Menu ______________________________________________________ 37
Co Settings Menu ________________________________________________________ 39
Syste Settings Menu _______________________________________________________ 40
Diagnostics Menu ___________________________________________________________ 41
10 MODBUS REGISTERS_______________________________________________ 42
List of GDS-68XP MODBUS variables ____________________________________________ 42
11 TROUBLESHOOTING GUIDELINES ____________________________________ 50
Fault Indication on OUTPUT Channel (CH2) ______________________________________ 50
Fault Indication on raw Sensor Channel (CH1) ____________________________________ 50
Sensor Fails Calibration ______________________________________________________ 50
GDS-68XP and Receiving Device Displayed Values Don’t Match ______________________ 50
Controller MODBUS Data Incorrect _____________________________________________ 51
Controller Showing MODBUS COMM ERROR _____________________________________ 51
GDS-68XP Display Blank ______________________________________________________ 51
12 SPARE PARTS ____________________________________________________ 52
13 DRAWINGS AND DIMENSIONS ______________________________________ 57
14 WIRING DIAGRAMS _______________________________________________ 58
15 GASMAX CX GDS-68XP FACTORY DEFAULT SETUP _______________________ 59
16 SEQUENCER SETTINGS (DIP SWITCH VERSIONS) _________________________ 65
17 KNOWN ERRATA __________________________________________________ 66

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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TABLE OF FIGURES
FIGURE 3-1: TOXIC SENSOR CHARACTERISTICS ........................................................................................ 11
FIGURE 3-2: SEQUENCE TIMING .............................................................................................................. 11
FIGURE 3-3: RECOMMENDED MINIMUM SEQUENCE TIMES .................................................................... 11
FIGURE 4-1: GDS-68XP FLOW DIAGRAM (PRESSURE) ............................................................................... 12
FIGURE 4-2: GDS-68XP SAMPLE TIMING .................................................................................................. 13
FIGURE 4-3: GDS-68XP MAXIMUM TIMING VALUES ................................................................................ 13
FIGURE 4-4: GDS-68XP FLOW DIAGRAM (SAMPLE DRAW) ....................................................................... 14
FIGURE 5-1: GDS-68XP (HIGH PRESSURE, STANDARD FILTER) .................................................................. 15
FIGURE 5-2: GDS-68XP POWER AND SIGNAL WIRING .............................................................................. 16
FIGURE 5-3: RELA / MODBUS CONNECTIONS ......................................................................................... 17
FIGURE 5-4: MODBUS WIRING JUNCTION BOX ........................................................................................ 18
FIGURE 5-5: 200W AC HEATER WIRING ................................................................................................... 19
FIGURE 5-6: INLET TUBING RECOMMENDATIONS.................................................................................... 20
FIGURE 5-7: FILTER DRAIN & EXHAUST TUBING RECOMMENDATIONS ..................................................... 21
FIGURE 5-8: GDS-68XP EXHAUST TUBING ................................................................................................ 22
FIGURE 6-1: GDS-68XP USER INTERFACE ................................................................................................. 24
FIGURE 6-2: SEQUENCE TIMING .............................................................................................................. 25
FIGURE 6-3: L1 / L2 INDICATOR OPERATION ............................................................................................ 25
FIGURE 6-4: GDS-68XP GASMAX DISPLA SEQUENCE .............................................................................. 26
FIGURE 7-1: CALIBRATION PROCEDURE (CAL PORT) ................................................................................ 29
FIGURE 7-2: CALIBRATION PROCEDURE (INLET PORT).............................................................................. 30
FIGURE 7-3: CALIBRATION PROCEDURE (SAMPLE STREAM) ..................................................................... 31
FIGURE 8-1: FAULT CONDITIONS ............................................................................................................. 33
FIGURE 9-1: MAIN MENU TREE ............................................................................................................... 35
FIGURE 9-2: ALARM OUTPUTS MENU TREE ............................................................................................. 36
FIGURE 9-3: CHANNEL SETTINGS MENU TREE (1) .................................................................................... 37
FIGURE 9-4: CHANNEL SETTINGS MENU TREE (2) .................................................................................... 38
FIGURE 9-5: COMM SETTINGS MENU ...................................................................................................... 39
FIGURE 9-6: S STEM SETTINGS MENU TREE ............................................................................................ 40
FIGURE 9-7: DIAGNOSTICS MENU TREE ................................................................................................... 41
FIGURE 12-1: GDS-68XP SAMPLE DRAW COALESCING FILTER (SPARE PARTS) ........................................... 52
FIGURE 12-2: GDS-68XP LOW PRESSURE COALESCING FILTER (SPARE PARTS) .......................................... 53
FIGURE 12-3: GDS-68XP HIGH PRESSURE COALESCING FILTER (SPARE PARTS).......................................... 54
FIGURE 12-4: GDS-68XP HIGH PRESSURE B PASS FILTER (SPARE PARTS) .................................................. 55

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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FIGURE 12-5: GDS-68XP SENSOR HEAD EXPLODED VIEW ......................................................................... 56
FIGURE 13-1: GDS-68XP DIMENSIONS (NEMA 4X ENCLOSURE) ................................................................ 57
FIGURE 14-1: GDS-68XP WIRING DIAGRAM ............................................................................................. 58

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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1SAFETY INFORMATION
Important – Read Before Installation
Users should have a detailed understanding of GDS-68XP operating and maintenance instructions. Use
the GDS-68XP only as specified in this manual or detection of gases and the resulting protection provided
may be impaired. Read the following WARNINGS prior to use.
WARNINGS
•The GDS-68XP process monitor described in this manual must be installed, operated and
maintained in accordance with information contained herein. Installation in any hazardous area
must comply with all applicable restrictions, requirements and guidelines for said hazardous
areas. It is the end user customer’s final decision to ensure that the GDS-68XP is suitable for the
intended use.
•The GDS-68XP is designed and constructed to measure the level of certain gases in backgrounds
that contain low amounts of free oxygen. Accuracy in atmospheres containing steam or inert
gases cannot be guaranteed.
•Do not paint enclosure, transmitter or sensor assembly.
•Do not operate the GDS-68XP if its enclosure is damaged or cracked or has missing components.
Make sure the cover, internal PCB’s and field wiring are securely in place before applying power.
•Do not expose the GDS-68XP to electrical shock or continuous severe mechanical shock. Protect
the GDS-68XP from dripping liquids and high power sprays.
•Calibrate with known target gas at start-up and check on a regular schedule, at least every 90
days. More frequent inspections are encouraged to spot problems such as dirt, oil, paint, grease
or other foreign materials in the sample tubing or in the sensor head.
•Periodically test for correct operation of the system’s alarm events by exposing the sample
extraction point to a calibration gas concentration above the High Alarm set point.
•Use only for applications described within this manual.

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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2GENERAL INFORMATION
INTRODUCTION
The GDS-68XP provides a safe and reliable way to measure the level of hydrogen sulfide, mercaptan, THT
and other gases in a process stream that contains low levels of oxygen. The GDS-68XP uses a programmed
sequencer to alternatively apply sample and purge air to a highly sensitive and reliable electrochemical
sensor. Applying purge air periodically refreshes the sensor’s chemistry, significantly extends the life of
the sensor and reduces the total amount of sample gas
released into the atmosphere.
The GDS-68XP can be configured to draw samples from
non-pressurized (ambient) sources or to accept positive
pressure samples, from +5 psig to +1500 psig. Multiple
filter options are available and the unit features a low-flow
warning switch and long life brushless DC pump for sample
and purge air.
The GDS-68XP features the new dual-channel GASMAX CX
networked gas monitor. In addition to a new high visibility
color display, the GASMAX CX includes an Ethernet port with built-in MODBUS/TCP interface and web
server for complete integration into user networks. GDS Corp software can be used to remotely monitor
the status and performance of GDS-68XP systems if a network connection is available.
EXPLOSION PROOF INSTALLATION
The GDS-68XP is designed for use in Class 1 Division 1 hazardous areas. Installation in these areas should
follow best industry standard practices and all appropriate electrical codes. Generally, these codes require
rigid metal conduit, poured seals and other installation elements necessary to ensure safety. For
maximum protection against RF interference or electrical surge, the GDS-68XP enclosure and
interconnecting conduit must be properly grounded.
INTRINSICALLY SAFE INSTALLATION
The GDS-68XP is not designed or certified for use as an Intrinsically Safe device.

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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3SPECIFICATIONS
Model GDS-68XP Process Monitor for Low Oxygen Applications
Power Input 24VDC ± 5% at < 12 watts
Display High resolution color LCD with engineering units, bargraph and 30-minute trend
Sensor Types Electrochemical sensors for toxic gases
Inlet Pressure
Requirements
Sample draw: Ambient to +5 inches of water column
Low pressure / no regulator: +5 inches water column to +25 psig (± 10% max)
High pressure / standard filter: +10 psig to +1500 psig
High pressure / combination filter: +10 psig to +1500 psig
Draw Distance Demonstrated up to 100 feet of ¼” OD tubing
Accuracy +/- 5% of full scale (typical)
Standard
Output
Three-wire 4-20mA current source outputs with fault and overrange indication.
Maximum loop resistance is 750 ohms with standard 24VDC supply.
Ethernet RJ-45 with built-in MODBUS/TCP interface and web server
Optional Relay / MODBUS interface with 3x 5A SPDT programmable alarm relays.
Temperature
(operating)
0°C to +50°C standard (no enclosure)
Note: Ambient temperature below 0°C may keep purge air pump from starting
-20°C to +50°C with NEMA 4X enclosure and optional 200W AC heater. Heater
thermostat is set to 50°F and is not user-adjustable.
Temperature
(inert)
-20°C to +55°C
In cold weather, GDS Corp recommends turning on the AC heater (if installed) for
several hours before applying DC power
Memory On-board non-volatile memory retains all user settings
Materials Instrument housings: Aluminum
Tubing & fittings 316 stainless steel
Dimensions Painted steel plate: 21” x 21” x 8”,
NEMA 4x non-metallic enclosure, 24” x 24” x 8”
Approvals GASMAX CX Gas Monitor CSA Certified Class I, Div 1 & 2 Groups B, C, D.
Sequencer enclosure CSA certified for use in Class I Div 1 areas.
Flame arrestors UL certified for use in Class 1 Div 1 areas.
Warranty Two years on electronics

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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Type Target Gas Min Span Max Span Te p Range War -Up
15 Hydrogen Sulfide 0-10 ppm 0-5000 ppm 0°C to + 50°C 2 to 4 hours
30 Mercaptan 0-15 ppm 0-30 ppm 0°C to + 40°C 4 to 8 hours
31 THT 0-15 ppm 0-30 ppm 0°C to + 40°C 12 to 24 hours
40 Odorants 0-15 ppm 0-50 ppm 0°C to + 50°C 2 to 4 hours
41 Spotleak 0-15 ppm 0-50 ppm 0°C to + 50°C 2 to 4 hours
Figure 3-1: Toxic Sensor Characteristics
SEQUENCE
Switch Sequence
Time
Purge / Hold
Time
Total Cycle Time
(Approximate)
0On-Demand Sequence 1 hour
1 12-15 min 40 min 1 hour
2 12-15 min 105 min 2 hours
3 12-15 min 165 min 3 hours
4 12-15 min 225 min 4 hours
5 12-15 min 285 min 5 hours
6 12-15 min 345 min 6 hours
7 12-15 min 465 min 8 hours
Figure 3-2: Sequence Timing
RECOMMENDED MINIMUM SAMPLE SEQUENCE TIMES
GAS T PE MINIMUM COMMENTS
15 H DROGEN SULFIDE, LOW RANGE (< 50 PPM) 1 HOUR FOR MAXIMUM SENSOR LIFE, USE LONGEST
POSSIBLE SEQUENCE
15 H DROGEN SULFIDE (> 500 PPM) 2 HOURS FOR MAXIMUM SENSOR LIFE, USE LONGEST
POSSIBLE SEQUENCE
31 TETRAH DROTHIOPHENE, 0-50 MG/M3 2 HOURS TWO HOURS IS REQUIRED FOR COMPLETE
SENSOR RECOVER FROM METHANE
30 MERCAPTAN, 0-50 MG/M3 OR 0-3 LB/MCF 4 HOURS FOUR HOURS IS REQUIRED FOR COMPLETE
SENSOR RECOVER FROM METHANE
40,
41
GENERAL ODORANTS, INCLUDING SPOTLEAK 2 HOURS TWO HOURS IS REQUIRED FOR COMPLETE
SENSOR RECOVER FROM METHANE
Figure 3-3: Recommended Minimum Sequence Times

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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4OPERATION
OVERVIEW
The GDS-68XP uses low cost, reliable electrochemical sensors to detect hydrogen sulfide, mercaptan, THT
and other trace gases in process streams that contain little or no oxygen. As electrochemical sensors
require oxygen to operate, the GDS-68XP alternatively exposes the sensor to the process flow and then
purges the sensor with ambient air to maintain the sensor in a sensitive, oxygenated state.
Figure 4-1: GDS-68XP Flow Diagram (Pressure)
At the beginning of each sequence, the GDS-68XP performs Conditions the Sensor by allowing a small
sample of gas to enter the sensor flow cell, followed by a fixed 5 minute delay. The 68XP then enters
Sa ple Charge during which sample gas flows into the sensor flow cell for two minutes, or until sensor
output exceeds 25% of scale. Once sample pre-charge is complete, the sequencer enters Read mode
where the sequencer processor attempts to identify a stable peak value. Once the sample value is
Purge Air
Pump
Flow
Switch
Cal Gas Inlet
Sample
Inlet
Flow Meter
Sample Outlet
Sensor
Flow Cell
Run / Cal
Valve
3
-
Way
Valve
Pressur
e
Regulator
Purge Air
Inlet
Filter
RAW SENSOR
(Ch 1)
PPM OUTPUT
(Ch 2)

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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identified and stored, the GDS-68XP enters Recovery mode where purge air flows into the sample flow
cell. Once the sensor output drops below 10% of scale, the GDS-68XP transfers the stored reading to the
4-20mA and MODBUS output. In the event that an overrange, loss of flow, peak-find algorithm fault, non-
return-to-zero fault, sensor fault or calibration fault occurred during the sequence, a specific fault
indication value will be output in place of the measured value. Finally, the GDS-68XP enters Purge/Hold
mode, during which purge air is periodically passed across the sensor to prepare it for the next sequence.
The length of the Purge/Hold time is determined by the SEQUENCE switch setting.
Figure 4-2: GDS-68XP Sample Timing
GDS-68XP Sequence Ti ing
Condition
Sensor
Fixed
Delay
Sa ple
Charge
Sa ple
Read
Sensor
Recovery
Purge / Hold
Valve opens,
sample flows
to sensor
Purge air
applied to
sensor
Valve opens,
sample flows
to sensor
Sequencer finds
and stores max
gas reading;
valve closes
Purge air applied
to sensor until
output < 10% of
scale
Purge air applied
to sensor
(intermittent)
Fixed 5-10
seconds
Fixed 5
minutes
Two minutes
or value > 25%
Min: 2 seconds
Max: 5 minutes
Min: 1 second
Max: 10 minutes
Determined by
sequence switch
Figure 4-3: GDS-68XP Maximum Timing Values
Value or Fault indication transferred to Output
Condition
Sensor
Recovery
Purge / Hold
“RAW SENSOR”
“ OUTPUT”
Delay
Charge
New Cycle
Read

GDS-68XP Operation & Maintenance Manual, Revision 3.4
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SAMPLE DRAW CONFIGURATION
In addition to low and high positive pressure versions, the GDS-68XP can be configured to draw a sample
from ambient pressure sources. In this configuration the pump operates continuously to pull from both
purge air and sample sources (See Fig. 4-4).
Figure 4-4: GDS-68XP Flow Diagram (Sample Draw)
SENSOR CONSIDERATIONS
The GDS-68XP supports electrochemical sensors for hydrogen sulfide, THT and mercaptan. Each sensor
contains a fixed amount of chemical electrolyte that reacts with the target gas to create free electrons
that are amplified and measured. Once the electrolyte is depleted, sensor output will diminish and the
sensor must be replaced.
IMPORTANT: TOXIC SENSORS ARE SUBJECT TO ACCELERATED DETERIORATION IF POWER IS NOT APPLIED
WITHIN 3 MONTHS OF SHIPMENT FROM GDS CORP.
Purge Air
Pump
Flow
Switch
Cal Gas Inlet
Sample
Inlet
Flow Meter
Sample Outlet
Sensor
Flow Cell
Run / Cal
Valve
3
-
Way
Valve
Purge Air
Inlet
Fi
lter

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5INSTALLATION
SELECTING A LOCATION
Locate the GDS-68XP as close as practical to the source of the sample gas to minimize latency and ensure
that fresh sample is available for each sequence. The GDS-68XP plate and/or enclosure feature mounting
holes that can be used to securely attach the monitor to a bulkhead or wall. Make sure there is sufficient
clearance below and to the left side of the GDS-68XP to allow room for power & signal cables, purge air,
sample, filter bypass (if installed), filter drain and sample exhaust tubing.
If the unit is installed outside or where insects or dust may be present, be sure that the supplied purge air
filter is in place and that a cover is placed over the Cal Gas Inlet port.
Always mount the GDS-68XP in a vertical position to ensure proper operation of flow switch and filter
drains. The non-metallic enclosure option is strongly recommended if unit is to be installed outside or in
areas where high or low temperature extremes may occur.
NOTE: TEMPERATURES BELOW 0°C MA KEEP THE PURGE AIR PUMP FROM STARTING.
Always use recommended conduit and poured seals for signal and power wiring installation in hazardous
areas. Consult local codes and regulations where appropriate.
Figure 5-1: GDS-68XP (High Pressure, Standard Filter)
Purge Air
Inlet
Sa ple
Inlet
Power & Signal
Input
Filter Drain
Sa ple Exhaust
Regulator
Flow Meter
AC Heater
GASMAX
Sequencer
Sensor
Sensor
Flow
Cell

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POWER & SIGNAL CONNECTIONS
To access the power and signal wiring, remove the cover from the
explosion proof enclosure. Connect a source of +24VDC, ± 10% power
to wiring terminals J1-1 (+24V) and J1-3 (GND). The 4-20mA (source)
output is available at J1-2 (See Fig. 5-2).
ON-DEMAND SEQUENCE INPUT
If the “on-demand’ sequencer mode is selected the GDS-68XP will enter a hold mode until an external
contact closure between J6-1 and J6-2 occurs. This causes the GDS-68XP to perform a single sample
sequence. Upon completion, the DONE output will be pulled low to signal the remote controller that the
sequence is complete.
Figure 5-2: GDS-68XP Power and Signal Wiring
ETHERNET CONNECTION
The GASMAX CX gas monitor includes a standard RJ-45 Ethernet connection on the main I/O board. Both
fixed IP and DHCP-based dynamic IP addressing is supported. The GASMAX CX supports a full range of
MODBUS / TCP accessible register (see Chapter 10) as well as a built-in web server with data display and
gas detector configuration page.
J1
–
1: +24
VDC
Power
Input
J1 – 2: 4-20mA Analog Out
J1 – 3: Power / Signal Common
Reco ended
DC
Wire Gauge
< 100 ft #18 GA
100 to 500 ft #16 GA
500 to 1000 ft #14 GA
J6
–
1: START: Ground to Common to initiate sequence
J6 – 2: Common
J6 – 3: DONE: Open Collector Output, pulled low when sequence complete
Access to optional Alarm Relays,
MODBUS serial ports and Ethernet is
via ¾” NPT fitting on right side of
GASMAX

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RELAY CONNECTIONS (OPTIONAL)
The optional GASMAX CX Relay / dual MODBUS RTU slave interface is connected “piggyback” to the back
of the GASMAX CX Display Assembly and supplies three level alarm relays, a FAULT relay and dual RS-485
Modbus RTU serial ports.
Figure 5-3: RELAY / MODBUS Connections
Relays K1, K2 and K3 provide a contact closure if the Alarm 1 (“K1”) or Alarm 2 (“K2”) or Alarm 3 (“K3”)
limits are exceeded. Alarms can be programmed to trigger above or below a certain value, work as normal
or ‘failsafe’ and can be made to latch if desired. Relay K3 indicates a FAULT condition in the sensor,
microprocessor or flow system.
Remote Alarm Reset can be used to acknowledge an Alarm 2 relay contact closure. Wiring from any
remote pushbutton to TB3 should be shielded and protected from noise spikes to prevent false Alarm
Reset commands.
WARNING: RELA CONTACTS ARE RATED FOR RESISTIVE LOADS ONL ! INDUCTIVE LOADS MA CAUSE
ARCING WHICH SHORTENS LIFE AND MA INTERFERE WITH SENSOR DATA.
MODBUS CONNECTIONS (OPTIONAL)
The dual optional GDS-68XP MODBUS RTU interface allows remote controllers or PLCs to monitor most
aspects of operation, including real-time data, range and alarm setpoints and alarm and fault status bits.
The GDS-68XP interface supports 9600 Baud RS-485 differential signaling only.
Access to each MODBUS RS-485 interface is via TB2 on the optional Relay / MODBUS board mounted on
the back of the GASMAX CX display module (See Fig. 5-3). Separate input and output terminals for
MODBUS “A” and “B” signals are available. A center terminal to tie incoming and outgoing shield
connections is also provided.
2
3
4
1
TB1
5
B
S
A
A
B
TB2
Relay Wiring:
1Fault 1 NC
2Fault 1 Common
3Fault 1 NO
4Relay 3 NC
5Relay 3 Common
6Relay 3 NO
7Relay 2 NC
8Relay 2 Common
9Relay 2 NO
10 Relay 1 NC
11 Relay 1 Common
12
Relay 1 NO
MODBUS interface
Re ote Alar Acknowledge
Modbus Wiring:
1 Modbus A
2 Modbus B
3 Shield Tie Point
4 Modbus A
5 Modbus B

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Page 18
MODBUS system architecture requires that the devices in any MODBUS loop be connected in a daisy-
chain layout. This minimizes signal reflections and improves signal noise margin. A MODBUS Termination
Jumper installs a load resistor across the MODBUS signal lines and should only be set to “A” (ON) at the
last device in the string (See Fig. 5-3).
Cable selection for MODBUS systems is important for both signal integrity and power distribution.
MODBUS / RS-485 transmissions use low-voltage differential signaling to achieve reasonable data rates
over very long distances, up to 4000 feet without a repeater.
For MODBUS data signals, GDS Corp recommends 20GA to
24GA twisted shielded cable. Daisy-chain power distribution
may require larger gauge wire since it is critical that the
supply voltage for the GDS-68XP at the far end of the string
not fall below 22VDC during power-up.
Note that while the GDS-68XP has two sets of wiring
terminals for MODBUS “A” and “B” signals, daisy-chain power
wiring requires that two wires be installed in the “+24” and
“GND” terminals on the GDS-68XP I/O Power Supply board.
This can be difficult if wire sizes are larger than #18GA. For
these reasons, if MODBUS is required GDS Corp recommends
the addition of the MODBUS Wiring Junction Box (see Fig. 5-
7). This option minimizes the need to access wiring inside the
GDS-68XP, provides individual wire landing points for
incoming and outgoing MODBUS and power wiring and
shields, and makes it easy to temporarily disconnect the GDS-68XP
power or MODBUS connections without affecting any other MODBUS
device.
+24 GND A
+24 GND A
PWR to
MODBUS to
+24
GND
“A”
“B”
+24
GND
“A”
“B”
Figure
5
-
4
: MODBUS Wiring
Junction Box

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Page 19
AC HEATER (OPTIONAL)
The 200 watt AC-powered heater is recommended for outdoor applications where ambient temperatures
may fall below freezing for extended periods of time. The heater is available in either 110VAC or 230VAC
models. Access to the heater wiring is via a separate ¾” NPT fitting on the bottom of the heater junction
box. Note that all high voltage AC wiring must be kept separate from lower voltage DC and signal lines.
Figure 5-5: 200W AC Heater Wiring
Local codes and good wiring practice require an AC shutoff switch within sight of the heater assembly for
maintenance and testing. IMPORTANT: Keep all electrical fittings tight while circuits are alive.
PURGE AIR INLET
Purge air should be drawn from a source of ambient air that is clean and free of significant levels of
hydrogen sulfide or similar toxic gas. GDS Corp recommends placing the Purge Air inlet some distance
from the GDS-68XP, in a location that is free from background gas and protected from heavy rains, water
spray and snow or ice. Included with the unit is a ¼” screen fitting that ust be placed over the open end
of the purge air inlet to keep dust, dirt, water droplets and insects from entering the tubing and being
drawn into the purge air pump.
AC HOT
Access to heater wiring via ¾” NPT fitting on botto
of junction box (enclosure cutout not provided)
Heater Ele ent
Fixed Ther ostat (O
N below 50°F / 10°C)
AC NEUTRAL
AC GND
Ther ostat
Heater Ele ent
Black
Green
Red/White

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Page 20
SAMPLE INLET
The length of time it takes gas to flow from the sample source to the GDS-68XP inlet should not exceed 30
seconds to ensure that “fresh” sample is available at the beginning of each measurement cycle. For ¼” OD
stainless steel tubing and sample flow rates of approximately 0.5 liters per minute, the maximum length
should be no more than 60 feet (~20 m). Smaller diameter tubing will allow longer runs but may be
subject to clogging if the sample contains particulate or moisture. Larger diameter tubing should be
avoided due to the internal volume of entrained gas.
Figure 5-6: Inlet Tubing Recommendations
In the event that the GDS-68XP must be mounted at a greater distance from the sample pickup point, the
combination coalescing / bypass filter is recommended. This filter includes a bypass port and bypass valve
that can be opened slightly to enable a continuous flow of sample from the pickup point to the filter
input, ensuring that fresh sample is always available.
IMPORTANT: WHEN CONFIGURED FOR SAMPLE DRAW, THE GDS-68XP SAMPLE PUMP IS CAPABLE OF
PULLING UP TO 7.0 PSI VACUUM, ENOUGH TO LIFT WATER OVER 15 FEET. CARE SHOULD BE TAKEN NOT
TO SUBMERGE THE SAMPLE PICKUP POINT IN LIQUID. INJESTED LIQUID IS NOT COVERED UNDER
WARRANT .
If a unit that has been configured for sample draw is connected to a sample source with pressure higher
than ambient, an additional low-pressure regulator or flow restriction device should be installed in the
sample inlet line before the GDS-68XP. This will allow the sample flow to be adjusted to match the purge
air flow.
Inlet Delay Calculation
For ¼” OD stainless steel tubing,
allow 5 seconds delay for every
10 feet of sample line.
Max 60 Feet
With ¼” OD Tubing
Mount GDS
-
68XP
above sample
point if possible