Halma ALICAT BC Series User manual

OPERATING MANUAL
FOR MASS FLOW INSTRUMENTS
BASIS 2 METERS AND CONTROLLERS

2 Introduction

instrument.
If you have any questions, or if something is not working as
expected, please contact us. We are eager to help you in any
way possible.
Contact Information
World Headquarters,
Tucson, Arizona, USA
[e-mail]
alicat.com
7641 N Business Park Dr.
Tucson, AZ 85743 USA
[phone]
Europe
[email protected]
Geograaf 24
6921 EW
Duiven, The Netherlands
[phone]
China & SE Asia
[e-mail]
alicat.com.cn
3rd Floor, Building 18,
FAMILY Science and
Technology Innovation Park,
No. 155, Yuanke Road
Minhang District
Shanghai, PRC 201109
[phone]
India
[e-mail]
Alicat Scientic India Pvt. Ltd.
101, Hamilton A Bldg,
Near Hiranandani Hospital,
Hiranandani Estate, Patli
Pada, Ghodbunder Road,
Thane West-400607
Maharashtra, India
[phone]
Calibration
BASIS 2 instruments are factory calibrated. When operated
per instructions, regular maintenance is not required. Periodic
inspection and recalibration can ensure ongoing performance and
accuracy. Fill out the Service Request Form at alicat.com/service
or contact us directly to send in your instrument for recalibration.
For information about our limited lifetime warranty, visit
alicat.com/warranty.
Serial #:
Next Calibration:
This instrument comes with a NIST-
traceable calibration certicate.
This instrument conforms to the European
Union’s Restriction of Use of Hazardous
Substances in Electrical and Electronic
Equipment (RoHS) Directive 2011/65/EU.
This instrument complies with the
requirements of the Low Voltage Directive
2014/35/EU and the EMC Directive 2014/30/
EU and carries the CE Marking accordingly.
This instrument complies with the
requirements of the European Union’s
Waste Electrical & Electronic Equipment
(WEEE) Directive 2002/96/EC
DOC-MANUAL-BASIS2 Rev 0, 2024.01.11
BASIS 2 GAS MASS
FLOW INSTRUMENTS

3 Introduction
Introduction
Your new BASIS 2 B- or BC-Series instrument is a highly accurate mass ow device that you
can control through several digital communication methods or through analog signals. Log
data to your PC with a serial data connection to control the instrument and to store ow data.
This manual covers the following instruments:
• BC-Series: gas mass ow controllers
• B-Series: gas mass ow meters
For support or questions regarding the use or operation of this instrument, please contact
us using the information on page 2.
Alicat offers many combinations of instrument sizes, accessories, connections, and
congurations. These custom solutions can meet a variety of application challenges brought
forth by users pushing the boundaries of our standard oerings. If you have an idea for
a new process or a challenging application, contact Alicat for specialized engineering and
application support.
This manual covers the general operation of factory OEM instruments. Some custom setups
may not be covered by this document. If you need further assistance, please contact Alicat
support.

4 Table of Contents
Contents
Contact Information 2
Calibration 2
Introduction 3
Getting Started 5
Mounting 5
Filters 6
Gas Connections 6
Maximum Pressure 6
 6
Control 8
 8
Establishing Communication 8
Alicat ASCII Serial Communication 10
Establishing Communication 10
Data Frame Format 10
Query Full Data Frame 11
 11
Setpoint 11
Setpoint Source 11
Maximum Setpoint Ramp Rate 12
Communication Watchdog 12
Hold Valve Position / Resume Flow 13
Loop Control Gains 13
Reference Temperature 13
Readings Averaging 13
 13
Start a Measurement 14
Measurement Averages and Ranges 14
Taring the Instrument 14
Autotare 15
Active Gas 15
Totalizer Reset 15
Totalizer Batch 15
 16
Full Scale Flow 16
Unit ID 16
Serial Number 16
Version Number 16
MODBUS Address 16
Serial Baud Rate 17
Command Protocol 17
 17
MODBUS-RTU 18
Communicating Via MODBUS-RTU 18
MODBUS Address 18
Changing the Baud Rate 18
Setpoint Type 19
Changing the Setpoint 19
Communication Watchdog 19
Taring the Instrument 19
Flow Averaging 20
Totalizer 20
Batch Dispensing 20
Changing the Gas 20
Switch Command Protocol 20
User Registers 21
Instantaneous Data Registers 21
Measurement Data Registers 22
 24
Analog Signals 28
Changing the Setpoint 28
Reading the Flow 28
Troubleshooting 29
Maintenance 30
Cleaning 30
Recalibration 30
Reference 31
Engineering Units 31

5 Getting Started
Getting Started
The major instrument components are shown in the diagram below.
Most communication with the instrument is completed through digital communication
or analog signals, as described later in this manual. The LED on the top of the instrument
provides the following feedback:
• Solid Green: Power is applied, no errors
• Flashing Green: Power is applied, serial activity is occurring
• Fast Red Blink: Mass overrange, temperature overrange, totalizer overrange, etc.
• Slow Red Blink: Valve override in eect
Mounting
Mount the instrument so that ow passes through the ow body in the direction shown by
the ow arrow. The ow body must be mounted horizontally, with the ow arrow parallel
to the ground.
Use the (2) M3 threaded mounting holes on the bottom to mount the instrument to at panels
(see gure above). For dimensions see the BASIS 2 specication sheet at alicat.com/specs.

6 Getting Started
Filters
When pressure drop is not a concern, use in-line sintered lters to prevent large particulates
from entering the ow body of the controller. Suggested maximum particulate size is 
microns for all ow ranges.
Gas Connections
BASIS 2 series instruments do not require straight runs of pipe upstream or downstream.
Instruments are shipped with plastic plugs seated in the processing ports. To decrease the
chance of contaminating the ow stream, do not remove these plugs until you are ready to
install the instrument.
After removing the plugs, immediately connect the instrument to upstream and downstream
plumbing, leak-tight. Verify that the ow matches the direction indicated on the instrument
body. Standard gas ow instruments have female inlet and outlet ports.
Connection options include:
Model NPT SAE
SCCMSLPM 1/8" SAE4
5 SLPMSLPM 3/8" SAE6
If you are using a tting that does not have a face seal, use thread-sealing Teon tape to
prevent leakage around the port threads, but  the rst two threads entering
the instrument. This will minimize the possibility of tape entering the ow stream and
clogging the ow path.
Face seal ttings can be installed without Teon tape applied to the threads.

Caution: Pipe dopes or sealants are not recommended on the process connections.
These compounds can cause permanent damage to the instrument should they get into
the ow stream.
Maximum Pressure
The maximum common mode pressure for BASIS 2 instruments is 145 PSIG.
!
Warning: Using the ow instrument above the maximum specied pressure may
permanently damage the device. If the line pressure is above these limits, use a pressure
regulator upstream of the instrument to reduce the pressure.
Power and Signal Connections
Power is supplied to the instrument through the 6-pin JST-GH connector on the top of
the instrument. The table below indicates how to connect the instrument to power and
communication.
The instrument power requirement for BASIS 2 controllers is 12–24 VDC, 210 mA. The power
requirement for BASIS 2 meters is 12–24 VDC, 12 mA.
!
Warning: Only connect power to pin 4. Do not enable power to the instrument until all
necessary pins have been properly connected.
A mass flow instrument with
female NPT connections.

7 Getting Started
Pin Function
1Analog Setpoint Input
Analog Signal Output
3Ground
4Power In, 12–24V
5Serial RS-232 (Rx) or RS-485 B (+)
6Serial RS-232 (Tx) or RS-485 A (-)

8 Control
Control
BASIS 2 Series controllers and meters can be operated using digital or analog communication.
Digital communication can be performed using Alicat FlowVision™ 2.0 software, ASCII serial
commands, MODBUS-RTU, or analog signals.
• FlowVision 2.0: page 8
• ASCII Serial Communication: page 10
• MODBUS-RTU: page 18
• Analog Signals: page 28
FlowVision 2.0
FlowVision 2.0 is Windows-based software that enables easy collection and analysis of data
from Alicat controllers and meters. FlowVision controls the instrument, runs scripts, and
logs and charts data. It also provides easy access to the gas calibration selector and PID
loop tuning to increase response speed and stability for some applications.
Establishing Communication
To connect your instrument to FlowVision 2.0:
 Download FlowVision 2.0 from  and install.
 With the instrument turned on and connected to the computer, start FlowVision 2.0.
 Click Devices in the top left corner.
 Click the green plus sign to add your instrument.
 Select Wired for the connection type.
 Specify the connection settings. By default, the Device ID is A, and the Baud Rate
is 38400. The Port is dependent on which COM port the instrument is connected to.
 Specify a Name if desired. FlowVision 2.0 generates a name if the eld is blank.
 Click Add to complete the process.
Please see the FlowVision 2.0 manual at  for detailed instructions on
operating your instrument through FlowVision.

9 Control
Note that the following three controls dier from the FlowVision manual for BASIS 2:
 Valve drive. The “Valve” button has the following options:
a. Open/exhaust
b. Close
c. Direct drive (sets the "Setpoint" entry eld to use a percentage instead of a unit of
measure)
d. Resume (clears the direct drive and returns to closed-loop control).
 The totalizer limit mode, shown in Instrument Settings > Totalizer, is unique to
BASIS 2 page 15.
 Setpoint ramping. The button near the “Setpoint” box will read Ramp: (disabled) or (%/
msec). Clicking the button will open Instrument Settings > Control, where a new
ramp rate can be set or cleared page 12.

10 Control
Alicat ASCII Serial Communication
Alicat ASCII Serial Terminal is a pre-congured program for serial communications via the
instrument’s 6-pin connector. Download Serial Terminal for free at alicat.com/drivers.
Serial commands are not case-sensitive. In the descriptions below, however, the commands
are shown in uppercase while command variables are shown in lowercase to help dierentiate
them.
All commands must be prexed with the unit_id, the unique identier for the instrument.
Instruments will also accept commands beginning with *, which is used as a broadcast value.
The unit_id must be the rst character after a carriage return ().
Unless otherwise noted, there must be a space between the command and any arguments.
All commands must end with a carriage return (), decimal value 13, hex value 0x0D).
After a carriage return is received, the instrument does not start acting upon or responding
to a command until the serial idle timeout is reached (the time needed to transmit 3.5 bytes).
All responses are a single line terminated with a carriage return. If a new command is started
before the carriage return of the response of the previously issued command has been
received, the behavior of the instrument is undened.
Establishing Communication
Connect the instrument to a computer through a COM port or virtual COM port, noting the
COM port number. Up to 26 instruments can be connected to a single COM port.
The following are the default connection settings:
• Baud: 38400 (see page 17)
• Data bits: 8
• Parity: none
• Stop bits: 1
• Unit ID: A (see page 16)
Data Frame Format
Data can be read from the instrument as the complete data frame, or you can choose specic
data to report. The complete data frame is:
A +24.57 +100.0 +0021513.0 +100.0 +55.13 N2 [ T O V ][ M O V ][O V R][ H L D][ V T M ]
unit _ idtemp. ow total setpointvalve _ drivegasErrorConditions
where:
unit _ id: The text ID of the instrument.
temperature: The current temperature of the instrument, in °C.
ow: The current ow reading of the instrument, in the congured units.
Total: The total volume (read from the totalizer)
setpoint: The current setpoint value, in the same units as ow.
valve _ d rive: The valve drive will be reported in percent of full drive.
gas: The currently selected gas.
If present, the following error condition abbreviations indicate:
TOV Temperature overrange.
MOV Mass ow overrange.
OVR Totalizer overrange.
HLD Valve hold is in use (see page 13).
VTM Valve thermal management is active (see note below).

11 Control

Note: Valve thermal management activates if a setpoint is given for a certain amount
of time with no ow detected. To prevent the valve from getting excessively hot when
driven fully, thermal management is activated, which pulses the valve open/closed until
a change in ow is detected. Once the change is detected, the valve returns to normal
control mode.
Single spaces separate each parameter, and each value is displayed in the chosen instrument
engineering units.
Query Full Data Frame
To read (poll) the entire data frame: unit_id
The returned data will be in the format described just above.
Query Specic Data
To read (poll) particular data: unit_id DV specific_data
specific_data is the data to be returned, per the table below.
 
+1 
 Current setpoint
+4 Current temperature
+8 Current valve drive
+16 Gas
 Total volume
+64 Current batch remaining
 Instrument Status
Add the table values to request multiple data, which will be returned in the order they appear
in this table. The total value must be an integer from 1 to 255:
Example: unit_id DV 12(returns the current temperature followed by valve drive)
Setpoint
The setpoint (controllers only) is the ow rate that the controller attempts to achieve and
maintain by proportionally changing the position of the valve.
To read the setpoint: unit_id S
To set the setpoint: unit_id Ssetpoint
setpoint is the desired ow rate. It is a oating-point value from 0 to the full scale of the
instrument, in the selected ow units for the device. The instrument will accept up to 2.5%
overrange.
Example: aS 15.44 (set a setpoint of +15.44 [ow units] for instrument A)
Setpoint Source
The setpoint source is the input which will control the setpoint (controllers only).
To read the setpoint source: unit_id LSS
To set the setpoint source: unit_id LSS setpoint_source
setpoint_source determines which source will control the setpoint, per the table below:

12 Control
 Notes
aThe analog input is used as the setpoint source. Digital setpoint commands are not
accepted.
sSaved digital. Digital setpoint commands are accepted; the most recent command
will be used when the instrument is powered up. The analog input is disconnected
from the setpoint.
CAUTION: If the setpoint is consistently changed more often than every few
minutes, the nonvolatile memory may be damaged and may no longer correctly
store the setpoint.
uUnsaved digital. Setpoint changes from the digital setpoint commands are accepted

analog input is disconnected from the setpoint.
This setting is recommended when the setpoint will often be changed more often
than every few minutes.
Example: aLSS a (the analog input will be the source for the setpoint for
instrument A)
Maximum Setpoint Ramp Rate
This value determines the maximum rate at which the setpoint can change (controllers
only). It applies to both increasing and decreasing setpoints, whether the setpoint was given
digitally or via the analog input.
To read the current setpoint ramp rate: unit_id SR
: unit_id SR max_ramp_rate time_units
max_ramp_rate is the new maximum setpoint ramp rate. The value is a non-negative oating-
point value in the default units of the loop control variable (see page 13). The instrument
will use the closest available ramp rate.
time_units are the units for the ramp rate, per the table below:
Unit Label  Notes
ms 3millisecond
s4second
m5minute
Example: aSR 600 3 (set the maximum ramp rate to 600 [ow units] /ms for
instrument A)
To disable any limiting of the setpoint ramp rate: unit_id SR 0
Communication Watchdog
A communication watchdog (controllers only) will force a zero setpoint and close the controller
valve if no serial communication is received within a user-dened time period.

Note: The Communication Watchdog is only available when the setpoint is being set via
MODBUS communication and the Setpoint Source is set to "unsaved digital" (see page 11).
All other setpoint sources (ASCII, saved digital, analog) are not impacted by the
Communication Watchdog.
: unit_id WD
Example: aWD(read watchdog time on instrument A)
: unit_id WD watchdog_time
watchdog_time is the desired watchdog duration, from 0–5000 milliseconds.

13 Control
Example: aWD 10(create a watchdog with 10 ms timeout for instrument A)
: unit_id WD 0
Hold Valve Position / Resume Flow
The controller valve can be held at any percentage of full drive (controllers only).
To hold the valve at a particular drive percentage: unit_id HPUR valve_drive
valve_drive is the percentage of full drive, a oating-point value from 0–100 percent.
Example: aHPUR 10(holds the valve for instrument A at 10% of full drive)
To resume closed loop control: unit_id C
The valve will be returned to normal closed loop ow control.
Loop Control Gains
The loop control variables determine how the instrument will maintain the flow rate
(controllers only).
To read the closed loop control gains: unit_id LCG
To set the closed loop control gains: unit_id LCG pgain igain
pgain is the value to use for the proportional closed loop gain. It is an integer from 0–65535.
The larger the pgain, the faster the controller will predict necessary future corrections
based on the current rate of change in the system. A larger p gain may result in overshoot
and oscillations.
igain is the value to use for the integral closed loop gain. It is an integer from 0–65535.
The larger the igain, the more aggressively the controller will correct errors between the
commanded setpoint and the measured process value.
Example: aLCG 500 5000(sets pgain to 500 and igain to 5000 for instrument A)
Reference Temperature
The reference temperature is used to calculate standard or normal mass ow.
To read the temperature reference: unit_id RT
To set the temperature reference: unit_id RT ref_temperature
ref_temperature is the reference temperature. It is a oating-point number in °C.
Example: aRT 22(sets the temperature reference to 22°C for Instrument A)
Readings Averaging
Averaging smooths the reading response for a rapidly changing ow
: unit_id DCA
: unit_id DCA average_tc
average_tc is the averaging time constant, the time required for the reported value to reach
63.2% of the nal value when there is a step change in the reading. The value is an integer
from 0–9999 milliseconds.
Example: aDCA 400(sets the time constant to 400 ms for instrument A)
Congure Measurement Triggering
Measurements can be triggered based on one or more events (controllers only).
To read the current triggering mode: unit_id MT
To set the current triggering mode: unit_id MT mode
mode determines how a measurement will be triggered, per the table below:

14 Control
Mode Description
+1 Begin a new measurement when the digital setpoint is changed.
 Begin a new measurement when the valve is being held and the
hold percentage is changed.
+4 
measurement is read.
The table values may be added together to trigger measurements based on multiple methods
(the value must be an integer between 0 and 7).
Example: aMT 3 (triggers a measurement for instrument A when either the digital
setpoint is changed or the hold percentage is changed)
Start a Measurement
: unit_id DVAS time
time is the duration to collect data in the measurement, in milliseconds. The measurement
will end when the time has elapsed.
Example: aDVAS 4000(starts a measurement with 4000ms collection time)
Measurement Averages and Ranges
The average values or the range of values can be reported over the measurement time
period, for the current or most recent measurement
: unit_id DVAA
The data returned will be:
unit _ id time avg _ tempavg _ ow
where:
unit _ id: The text ID of the instrument.
time: The elapsed time that data has been acquired, in milliseconds..
avg _ temp: The average temperature during the measurement, in °C.
avg _ ow: The average ow during the measurement.
unit_id DVAR
The data returned will be:
unit _ id time min _ temp max _ temp min _ ow max _ ow
where:
unit _ id: The text id of the instrument.
time: The elapse time data has been taken for this measurement, in milliseconds.
min _ temp: The minimum temperature during the measurement, in °C.
max _ temp: The maximum temperature during the measurement, in °C.
min _ ow: The minimum ow during the measurement, in instrument ow units.
max _ ow: The maximum ow during the measurement, in instrument ow units.
Taring the Instrument
Taring adjusts the ow sensor oset so that the current ow value reads zero. This action
gives the instrument a zero reference, ensuring that the instrument provides its most
accurate ow measurements. It is recommended to tare the instrument:
• After installation
• If the instrument is hit with any signicant impact
• After any signicant changes to the temperature or pressure.

15 Control
To tare the instrument: unit_id V duration
duration is the time in milliseconds, to collect values for calculating the tare oset. The value
is a positive integer from 1–32767.
Example: aV 1000 (tares instrument A, with the oset calculated over 1 second)
Autotare
If autotare is enabled (controllers only), the instrument will automatically zero its measurement
after being at a 0 setpoint for 2 seconds.
To read the current autotare configuration: unit_id ZCA
To enable autotare: unit_id ZCA 1
To disable autotare: unit_id ZCA 0
Active Gas
BASIS 2 instruments are calibrated to measure any of 9 gases:
#Short Name Long Name
Air Air (Clean Dry)
1Ar Argon
COCarbon Dioxide
3NNitrogen
4OOxygen
5NO Nitrous Oxide
6HHydrogen
7He Helium
8CHMethane
To read the active gas: unit_id GS
To set the active gas: unit_id GS gas_number
gas_number is an integer from the table above.
Example: aGS 8 (ow methane on instrument A)
When reading or setting the active gas, the data reported will be:
unit _ idgas _ numbergas _ name
Totalizer Reset
The instrument totalizer can measure the total ow that has passed through the instrument
since it was powered on or since it was last reset. The totalizer value is returned as part of
the data frame (see page 10). Totalizer counts are reset if power to the instrument is lost.
To reset the total volume: unit_id T
Totalizer Batch
Batch dispensing ows a set volume of gas, after which the valve will close and the ow will
stop (controllers only).
To read the remaining batch volume: unit_id DV 64 (see page 11) .
To set the totalizer batch: unit_id TB batch_volume
batch_volume is a oating-point value, in instrument volume units.
To disable the batch function: unit_id TB 0

16 Control
Example: aTB 200 (set the batch volume to 200 [volume units] for instrument A)
Totalizer Conguration
To read the current totalizer configuration: unit_id TC
To set the current totalizer configuration: unit_id TC limit_mode
limit_mode is the action when the totalizer reaches the maximum allowed value:
 Description
Keep totalizer value at the maximum value and do not indicate an OVR error.
1Reset totalizer value to zero and do not indicate an OVR error.
Keep totalizer value at the maximum value and indicate an OVR error.
3Reset totalizer value to zero and indicate an OVR error.
Example: aTC 1 (when the totalizer for instrument A reaches max, reset the
total volume to zero and do not indicate an OVR error)
Full Scale Flow
To read a full scale value: unit_id FPF value
value is the instrument measurement to read full scale:
 

1Largest total volume/totalizer batch
Maximum temperature
Example: aFPF 2(returns the max temperature for Instrument A)
The data returned will be:
unit _ idvalueunits
where units is a text string indicating the units for the full scale measurement.
Unit ID
Each instrument has a unique, single-letter ID using the standard 26-letter English alphabet
(e.g., A, B, C, etc.). The default ID is “A”. If more than one instrument on a COM port has the
same ID, data and commands become corrupted and do not report or execute properly.
When connecting multiple instruments on the same COM port, connect one instrument at a
time. Provide a unit ID command for each instrument before connecting the next instrument.
To give the most recently connected instrument an unused ID:
To change the unit id: unit_id @ new_id
Serial Number
Each instrument has a unique serial number.
To read the serial number: unit_id SN
Version Number
Each rmware release has a unique number.
: unit_id VE
MODBUS Address
The MODBUS address is the ID to which the instrument will respond when using MODBUS
communications (see page 18).

17 Control
To read the MODBUS address: unit_id MA
To set the MODBUS address: unit_id MA address
address is the new MODBUS address, an integer from 1–247.
Serial Baud Rate
To read the current baud rate: unit_id NCB
To set the baud rate: unit_id NCB baud_rate
baud_rate is the communication rate, per the table below:
 Corresponding Baud Rate
 4800
1 9600
 19200
3 38400 (default)
4 57600
5 115200
Example: aNCB 2(changes baud rate on Instrument A to 19200)

Note: When changing the instrument’s baud rate, the baud rate for the COM port must
change in time to successfully receive the response conrming the new baud rate. If it
does not, then the baud rate will change on the instrument, but the response from the
serial terminal will not be understood. Since the serial terminal is set to communicate
at a specic baud rate during the initial conguration, you will have to redene the
terminal baud rate to continue communications.
Command Protocol
The instrument can be switched from BASIS Protocol 2 (described in this manual) to Protocol
1 (described in the BASIS 1 manual).
To read the current command protocol: unit_id P
: unit_id P 1
: unit_id P2

Note: There is a space between the command and argument when switching from
protocol 2 to 1 but NOT when switching from protocol 1 to 2.
Example: aP2(switch Instrument A to be controlled through BASIS 2 protocol)
BASIS 1 ASCII commands can be found in the BASIS 1 user manual, available at
alicat.com/manuals.

Caution: The BASIS 1 protocol only works with ow units of SCCM, SLPM, NCCM, and
NLPM. An error will be returned if you attempt to switch to BASIS 1 protocol for an
instrument with dierent ow units.
Load Factory Default as Current Conguration
When you reset the instrument, data from the factory area of memory is copied into the
user settings area. The instrument will be completely congured to the factory-set values.
Power cycling the instrument is recommended to ensure that all values are properly used.
To reset the instrument to factory defaults: unit_id FACTORY RESTORE
This command must be entered in ALL CAPITAL LETTERS and with exactly one space between
the words of the command.

18 Control
MODBUS-RTU
MODBUS is an application-layer messaging protocol that formats data for communications
over serial RS-232 or RS-485. The instrument supports MODBUS-RTU protocol, with data
transmitted through Pin 6 of the 6-pin connector.
MODBUS-RTU can be used to control:
• taring
• data collection
• setpoint control
• totalizer
• batch dispensing
• changing the gas
• control loop adjustment.

Note: For more information on MODBUS-RTU communication commands, please see
the MODBUS FAQ at alicat.com/using-your-alicat/faq-modbus/ and the MODBUS-RTU
manual at alicat.com/manuals.
Communicating Via MODBUS-RTU
The following MODBUS commands are supported:
3Read holding register
6Write single register
16 Write multiple registers
Use the instrument registers to interact with the instrument.
Note that the response to function code 6 (Write Single Register) will echo the value provided
by the master in the write and not the value of the register after the write. For example, if
the setpoint register were written to a value that is not valid for the instrument, it will be
adjusted inside the register; however, the value returned will be the original setpoint value.
If the real value is desired, a read of the register must be issued. This is consistent with the
MODBUS specication and demanded by some MODBUS implementations.
Reading undened registers may return 0 or a Bad Address error.
MODBUS Address
Each instrument must have a unique address, which will be an integer from 1 to 247.
To find the MODBUS address: read register 45.
To assign a MODBUS address: write an integer from 1 to 247 to register 45.
Changing the Baud Rate
BASIS 2 units can communicate at the baud rates shown in the table below.
 Corresponding Baud Rate
 4800
1 9600
 19200
3 38400 (default)
4 57600
5 115200

19 Control
To read the current baud rate: read register 21.
To change the baud rate: write a value from the table above to register 21.

Note: When changing the instrument’s baud rate, the baud rate for the COM port must
change in time to successfully receive the response conrming the new baud rate. If it
does not, then the baud rate will change on the instrument, but the response from the
serial terminal will not be understood. Since the serial terminal is set to communicate
at a specic baud rate during the initial conguration, you will have to redene the
terminal baud rate to continue communications.
Setpoint Type
The setpoint type is the signal which will be used to dene the setpoint (controllers only).
To read the setpoint type: read register 516. The values are detailed below.
To change the setpoint type: write one of the following values to register 516 to set the
setpoint source:
0: Analog input.
1: Digital. The setpoint is saved in ash memory as a power up setpoint.
2: Digital. The setpoint is not saved in ash memory.
Changing the Setpoint
The setpoint is the flow rate that a controller attempts to achieve and maintain by
proportionally changing the position of the valve (controllers only).
To read the current setpoint: read register 2106. This is the current setpoint. It may dier
from the commanded setpoint due to limited resolution on congured maximum ramping,
indicated by register 524.
To set the desired setpoint: write the setpoint, as a signed 2-byte integer, to register
33524. The setpoint is equal to the value after formatting with the factory-set number of
decimal places.
Example: if ow units are SCCM, to obtain a setpoint of 10 SCCM, write 10 to
register 33524.
Communication Watchdog
A communication watchdog (controllers only) will force a zero setpoint and close the controller
valve if no serial communication is received within a user-dened time period. The time is
set in milliseconds, and valid values are 0–5000. If zero, the watchdog is disabled.

Note: The Communication Watchdog is only available when the setpoint is being set via
MODBUS communication and the Setpoint Source is set to "unsaved digital" (see page 11).
All other setpoint sources (ASCII, saved digital, analog) are not impacted by the
Communication Watchdog.
: read register 514.
: write a value from 0–5000 to register 514.
Taring the Instrument
Taring adjusts the ow sensor oset so that the current ow value reads zero. This action
gives the instrument a zero reference, ensuring that the instrument provides its most
accurate ow measurements. It is recommended to tare the instrument:
• After installation
• If the instrument is hit with any signicant impact
• After any signicant changes to the temperature or pressure.
To tare the instrument: write 0xAA55 (43605) to register 39. Note that performing a tare
will interrupt any currently-executing measurement.
To enable autotare (controllers only): write 1 to register 515. The instrument will tare ow

20 Control
automatically when the setpoint had been set to 0 for at least two seconds.
To disable autotare (controllers only): write 0 to register 515.
Flow Averaging
Flow averaging governs how the ow sensor reports data, an integer 0–2500 ms. A higher
ow averaging setting will smooth the data, which can be useful for rapidly uctuating ow
readings.
The time constant is approximately [value] milliseconds.
To read the current averaging: read register 55.
To change the averaging value: write an integer from 0–2500 to register 55.
Totalizer
The totalizer measures the total ow that has passed through the instrument since it was
powered on or since it was last reset. Totalizer counts are reset if power is lost.
To read the totalizer: read registers 2104–2105. The value will be in the volume units of
the instrument, scaled by the factory-set number of decimal places.
To reset the totalizer: write 0xAA55 (43605) to register 53.
Batch Dispensing
Batch dispensing (controllers only) ows a set volume of gas, after which the valve closes
and ow stops. You can repeat batches.
To read the volume remaining for the current batch: read registers 2108–2109. The
value will be an integer in the current totalizer volume units, with the same scaling as the
total volume (registers 2104–2105).
To set the desired volume for the totalizer batch: write the value to registers 521–522.
The value must be an integer in the instrument’s current total volume units and scale.
To reset the batch: write 0xAA55 (43605) to register 53.
Changing the Gas
BASIS 2 instruments are calibrated to measure any of 9 gases:
Gas Number Short Name Long Name
Air Air (Clean Dry)
1Ar Argon
COCarbon Dioxide
3NNitrogen
4OOxygen
5NO Nitrous Oxide
6HHydrogen
7He Helium
8CHMethane
To select a gas: write one of the gas numbers above, as an integer, to register 2100.
Switch Command Protocol
The unit can be switched from BASIS Protocol 2 (described in this manual) to Protocol 1
(described in the BASIS 1 manual).
To read the command protocol: read register 56.