How do I integrate Allen-Bradley SLC500 to Verbatim Gateway via DH485 over RS485?

How do I integrate Allen-Bradley SLC500 to Verbatim Gateway via DH485 over RS485?

This technical note explains how to interface the RACO® Catalyst® system with many of the Allen-Bradley® MicroLogix™ family of PLCs. This allows the Catalyst® to perform its alarm notifications and data logging functions using the existing wiring to Allen-Bradley® MicroLogix™ as inputs via data communications.

ImageRequired Components 

RS485 networks are preferred to over RS232 when required to communicate multiple slaves. (i.e. PLCs)
They also have a greater immunity to noise, thus allowing for significantly longer cable length and faster communication speed. The following components are required.


RACO® supplied components:

  • Catalyst® (C10-S-020-1450, C10-1-020-1450, C10-2-020-1450 models)
  • Alarmware Software version 1.87
  • 707CAT-PC15 Cable (Qty-2)

Allen-Bradley® supplied components:

  • MicroLogix™ PLC as listed in the table below
  • 1761-NET-AIC
  • RSLogix™ 500 software version 8.10.00
  • 1761-CBL Cable

The table below lists the MicroLogix™ PLCs containing a native Modbus RTU Slave driver.

 Modbus RTU Slave Drive is available within the following MicroLogixTM PLCs

Model # Series #
 1100  A & B
 1200  A & B & C 
 1400  A
 1500 LSP  B & C
1500 LRP B & C

ImageConnecting Catalyst to MicroLogix™ PLC through an AIC Module


                                            (Click on the image above to enlarge)

When setting up Modbus RTU Master / Slave over an RS485 network, provide a 3 wire cable (twisted pair)to connect back to the Allen-Bradley® Advanced Interface Converter. (1761-NET-AIC)

External +24 vdc power is recommended and either a Simple NULL Modem cable (as illustrated) or use the Allen-Bradley® 1761-CBL cable to plug to the round 8-pin mini DIN receptacle, to connect between the PLC and AIC unit. Configure as per the illustration above.

Ensure the P10 Jumpers are set for RS-485 on Catalyst main board.

ImageCommunication Setup for the Allen-Bradley® MicroLogix™

In addition to cabling and interface setup, parametric changes are required within RSLogix™ 500 and Alarmware to synchronize the Modbus Drivers.

When configuring communications for the Allen-Bradley®, a communication cable and either the RSLogix™ 500 or RSLogix™ Micro software are required.

STEP #1 — Connect your MicroLogix™ to your PC using the required 1761-CBL cable

  • This cable connects to Channel 0 in the MicroLogix™, where the DF1 Full Duplex driver by default is ready to communicate to your RSLogix™ 500 software.

STEP #2 — Launch RSLogix™ 500 Software

STEP #3 — Set Communications Driver to Modbus RTU Slave

  • Open the Channel Configuration form and select an available channel other than Channel 0, where  possible.

Caution: It is not recommended to configure communications to Modbus should your PLC have only a single communications port, as Channel 0 should remain available to RSLogix™ 500 software for program upload / download and monitoring.


                                         (Click on the image above to enlarge)

As shown above, the Channel Configuration form has four tabs: General, Channel 0, Channel 1 and Channel 2. Since we will reserve Channel 0 for RSLogix™ 500  software, we will select the Channel 2 tab. (Select Channel 1 if Channel 2 does not exist.)

Once the Driver field is clicked, a drop-down menu of driver choices will be visible. Select Modbus RTU Slave, as shown.

Next, select a unique Node Address. Catalyst default is 1, so anything other than 1, up to 256, is acceptable. Both the Baud rate and parity should match with the Catalyst.

RACO recommends the following…

  • Baud rate = 19,200
  • Parity = EVEN
  • Stop Bits =1
  • Data Bits = 8
  • Control Line = No Handshaking

STEP #4 — Select PLC Data Files


                                             (Click on the image above to enlarge)

Available file numbers are 3, 7 and 9 – 255.When selecting the data files, ensure that they exist. If the files do exist, ensure the elements within the file exist, and are not being utilized elsewhere in your program.

In the example, at left, new data files were created and named exclusively for Modbus communications (highlighted in yellow).

Once a file number has been assigned, select the data type. Type the name you wish to call the file in the name field. Our name follows the assignment as per Modbus. (See Table #1.)

MicroLogix™ only allows Binary or Integer type data files to be linked to the Modbus convention. Floating point files are not currently accepted. (i.e. 5xxxx)

In our example, 128 elements were selected. In the case of both “Coil number” and “Contact number,” where address is only a single bit, this translates to 2048 available addresses (i.e. 128 * 16 bits).

In the case of “Input Register number,” 128-16 bit integer addresses were available.

In the case of “Output Register number,” 128-16 bit integer addresses are available.

Note: Although we have assigned data tables 128 words in length, we are bound to the number of channels on the Catalyst end. Please refer to model numbers listed above to select a unit with the desired number of channels to suit your application.

Upon completion of your data file assignment you are ready at the Allen-Bradley® end to communicate.

ImageCommunication Setup for the RACO Catalyst 

STEP #1 — Launch Alarmware for Catalyst 1.87

  • Alarmware for Catalyst version 1.87 software.

STEP #2 — Connect Catalyst to PC

  • The Catalyst to Alarmware PC cable is required. (See below for pin-out.)
  • The Alarmware and cable are normally supplied with a new Catalyst. RACO cable part number 707CATPC15 (15-foot length).
  • Once connected to the Catalyst via Alarmware, we are ready to configure the necessary parameters for  communication.

Image  5_Catalyst_Alarmware_PC_cable.jpg

              (Click on the image above to enlarge)


STEP #3 — Modify Device Settings in Alarmware


                                    (Click on the image above to enlarge)

  • Open Alarmware and go to “Devices” via “Configuration.”
  • Once open, click the “Modbus Port” tab.
  • Configure the Modbus parameters beginning with the Baud Rate. Select the speed best suited for your application.
  • In this case, we have chosen the fastest available at 38.4K baud. The default Stop bits is 1 and Parity should be “Even.”
  • Next, select a Node address. Default is 1.
  • Do not check the “CTS always ON” box, as we do not require handshaking in this setup.

STEP #4 — Modify Channel Settings in Alarmware


                                       (Click on the image above to enlarge)

Finally, we will configure the Channel settings.As illustrated above, first select “Configuration” then select “Channels”.

  • Up to 256 remote channels are available (i.e. 1-256), depending on the Catalyst model you choose.
  • For the purpose of illustration, we have selected Channel Number 5. Next, we need to choose a mode (i.e. either Discrete or Analog).
  • This is done via SNA.
  • Enter the SNA as described below and then follow procedure to complete the Channel settings.


 Register Address  Description  Size/Limitation  Catalyst SNA
 0xxxx  Coil number xxxx  1-bit  1*node*0xxxx
 1xxxx  Input number xxxx  1-bit  1*node*1xxxx
 3xxxx  Input register number xxxx  16-bit  1*node*3xxxx
 4xxxx  Output register number xxxx  16-bit  1*node*4xxxx
 5xxxx Floating point register 32-bit 1*node*5xxxx
Note: The 5x and 4x references share the same data space. That is, an address of the form 5nnnn takes the 32 bits beginning at location 4nnnn and extending through 4nnnn + 1. These 32 bits are then interpreted as IEEE floating point number.


Catalyst Modbus SNA addresses begin with the number 1
Table #1 illustrates the Modbus data file convention within the Catalyst and the link to PLC via SNA (Source Net Address).

Although Holding Registers are not currently applicable to MicroLogix™, it should be noted that Output Register (4xxxx) and Holding Register (5xxxx) data types use the same data area at the Catalyst end.
The Holding Register (5xxxxx) uses two consecutive 16-bit addresses that are interpreted as a 32-bit floating point number. (i.e. 5xxxx & 5xxxx+1) Not unlike the Holding Register, the Output Register (4xxxx) also reserves two consecutive 16-bit words; however, the extended word is left blank

Similarly, when addressing the input register (3xxxx), allow for two consecutive 16-bit words, keeping in mind the word is reserved but not used. Hence, consecutive addressing of both Input and Output Registers will appear as follows. For example, first Input Register address point 30001, 2nd address point 30003, 3rd address point 30005, etc.

ImageConstructing the SNA (Source Network Address)

The SNA is comprised of … Net / Node / Address.

Net: This is either a 0 or 1.

0 when using local physical I/O
1 when using remote PLC I/O

Node: This is the node address of the PLC we wish to communicate.

(In this tech note we have configured our MicroLogix™ to have a Node address of 5.)

Address: Points to the Modbus address mapped to the PLC. (i.e. remote device)

The following two examples construct the SNA for both a discrete and analog PLC point as per file  assignment illustrated in this Tech Note.

Example #1
To configure a channel to communicate to B10:0/0, set the SNA as follows… 1*5*10001

Example #2
To configure a channel to communicate to N21:0, set the SNA as follows… 1*5*40001

Once channels are configured, they are ready to be assigned to your project, typically linked to Alarms and Messages.

Supporting Integration Notes: MicroLogix™ via Modbus over RS485