Using Allen-Bradley Controllers via Message Block

Most Allen-Bradley PLCs and PC-based controllers (ControlLogix, CompactLogix, SLC5/05, PLC-5, SoftLogix, etc.) support serial RS-232 and Ethernet communications, either built-in or through an add-on module.

This topic describes how to communicate with the RMC from the Allen-Bradley PLCs via the Message (MSG) block. For information on using EtherNet/IP I/O with the Logix PLCs, see the Using Allen-Bradley Controllers via EtherNet/IP topic. For details on using Allen-Bradley's RSView operator interface with the RMC, see the RSView with the RMC topic.

The Allen-Bradley PLCs can read or write from registers in compatible remote devices such as other Allen-Bradley PLCs or the RMC. The RMC contains floating point (F) files, all of which are accessible over serial or Ethernet from the Allen-Bradley PLCs. See DF1 Addressing for details on finding those registers and their addresses.

If you need help setting up your Ethernet network, either consult your network administrator, or for simple stand-alone networks, see Setting Up a Standalone TCP/IP Network.

Delta provides example PLC programs to help you quickly set up the communications between your PLC and the RMC. See the downloads section of Delta's website at https://deltamotion.com.

The Allen Bradley PLCs and controllers listed above all use the same ladder logic block to communicate over Ethernet: the Message (MSG) block. This block takes a number of parameters, which are briefly described below. For a complete description of the parameters, refer to Allen-Bradley's Instruction Set Reference Manual for the appropriate PLC.

The MSG block parameters differ slightly depending on the controller and programming software. The parameters used by RSLogix 5 version 3.2.0.0, RSLogix 500 version 6.30.00, and RSLogix 5000 version 17.00.00 for the PLC-5, SLC 5/05, ControlLogix and CompactLogix controllers respectively are described below. The SoftLogix 5 parameters are similar.

The ControlLogix or CompactLogix MSG block is displayed as follows. More detailed information on the MSG block is available in Allen Bradley publication 1756-RM003L-EN-P, Logix5000 Controllers General Instructions.

The Message Control portion of the MSG requires a tag.
To create a tag:

Right-click the "?" and choose NewTag.
Type a name in the Name field.
Make sure the Data Type is MESSAGE.
Click Configure. The Message Configuration dialog will open.

Enter the following on the Configuration tab:
- Message Type: From this drop-down list, select PLC-5 Typed Read to read values from the RMC, or PLC-5 Typed Write to write values to the RMC. The SLC Typed Read or Write will also work, but is limited to 58 registers.
- Source Element (reads only): Enter the address of the first RMC register you want to read. See the Registers Maps for help on addresses.
- Source Element (writes only): Enter the tag in the ControlLogix or CompactLogix that you want to send to the RMC. In most cases, the element should be an array or structure composed entirely of REAL data types, since this is the type of the RMC data.
- Number of Elements: Enter the number of RMC registers to read or write in this field.
- Destination Element (reads only): Enter the tag in the ControlLogix or CompactLogix into which you want to read the RMC data. See the Source Element bullet above for suggestions on the data type of the tag.
- Destination Element (writes only): Enter the address of the first RMC register you want to write. See the Registers Maps for help on addresses.
Enter the following on the Communication tab:
- For Ethernet:
  Path:

- For Serial:
  Path: Enter the path from the ControlLogix or CompactLogix CPU to the RMC. The format of the path usually is as follows:

2, station address

where station address is the node address of the RMC. For example, if the node address is 3, the path is:

2, 3

Note:
The "2" is the port number for the serial port on the ControlLogix CPU.

- Communication Method: Select the CIP option.
Click OK.
To make changes to the Message Configuration dialog later, click the ellipsis button in the MSG block.

The SLC 5/05 MSG block is displayed as the following:

Parameter	Description
Type	This parameter is always set to Peer-To-Peer for serial or Ethernet communication channels.
Read/Write	This parameter should be set to Read to read registers from the RMC and to Write to write registers to the RMC.
Target Device	This should be set to 500CPU for communicating with the RMC.
Local/Remote	This parameter has possible values of Local and Remote. It should be set to Local for communicating with the RMC.
Control Block	This parameter points to a block of integer-file registers (51 registers for Ethernet, 12 for serial). Set this to a block of registers, and then use the Setup Screen option in the MSG ladder logic block to modify those register values: This Controller: This section holds parameters for the SLC 5/05. Communication Command: This parameter will be set to 500CPU Read, or 500CPU Write, depending on what was selected in the MSG block itself (as described above). Data Table Address: Enter the address of the first Allen-Bradley PLC register to read RMC registers into, or to write to RMC registers from. Size in Elements: Enter the number of RMC registers to read or write in this field. The range enforced by the SLC is 1 to 256 values. Channel: Set this to the channel number of the serial Ethernet channel. For the SLC 5/05, this should be channel #1 for both serial and Ethernet. Target Device: This section holds parameters for the target device. Message Timeout: Indicate the number of seconds to wait for the RMC to respond before determining that the attempt failed. This can be set as low as a few seconds. Data Table Address: Enter the address of the first RMC register to read or write in this field. See the RMC Register Map (Allen-Bradley) for help on addresses. For Ethernet: Ethernet (IP) address: Set this to the IP address of the RMC you wish to communicate with. MultiHop: This parameter should be set to No. For Serial: Local Node Addr (dec): Enter the node address of this RMC. The node address of the RMC is set up in the Serial Port Settings Page. The node address entered on that screen is in decimal, so it is recommended that you enter the same number in the dec field. Local/Remote and Bridge Parameters: In most applications these will be set to Local and there will be no bridge parameters. If you are using a bridge then these parameters will need to be used. However, this is beyond the scope of RMC documentation. Refer to your Allen-Bradley documentation for instructions on using these fields.

Note:
For the SLC505, if the start of communications with the RMC exhibits several minutes of delay after power-up, it is because your SLC processor and/or firmware is old. Newer SLC processors do not have this problem.

The PLC-5 MSG block is displayed as follows:

The Control parameter points to a block of 51 N-file (integer) registers or two (2) MG-file (message) registers. Set this to an unused block of registers, and then use the Setup Screen option in the MSG ladder logic block to modify those register values:

Description

This PLC-5

This section holds parameters for the PLC-5:

Communication Command: From this drop-down list, select PLC-5 Typed Read to read values from the RMC, or PLC-5 Typed Write to write values to the RMC.
Data Table Address: Enter the address of the first Allen-Bradley PLC register to read RMC registers into, or to write to RMC registers from.
Size in Elements: Enter the number of RMC registers to read or write in this field. Transfers are limited to 1000 bytes for PLC-5 Typed Reads and Writes. Therefore, this limit is 500 integers, 250 floats, etc.
Port Number: Set this to the Ethernet channel number. For the PLC-5, this should be channel #2.

Target Device

This section holds parameters for the target device:

Data Table Address: Enter the address of the first RMC register to read or write in this field. See the Registers Maps for help on addresses.
For Ethernet:

MultiHop: This parameter should be set to No.

Ethernet (IP) address: Set this to the IP address of the RMC you wish to communicate with.

For Serial:

Local Node Addr (dec): Enter the node address of this RMC. The node address of the RMC is set up in the Serial Port Settings Page. The node address entered on that screen is in decimal, so it is recommended that you enter the same number in the dec field.

The MicroLogix MSG block is displayed as follows:

Serial Communications:

To edit the parameters of the message block, select the MSG block, enter an unused MSG file in the MSG File parameter, and double-click Setup Screen. This brings up a dialog with the following options:

This Controller: This section holds parameters for the MicroLogix.

Communication Command: From this drop-down list, select PLC-5 Typed Read to read values from the RMC, or PLC-5 Typed Write to write values to the RMC. The SLC Typed Read or Write will also work, but is limited to 58 registers.

Data Table Address: Enter the address of the first Allen-Bradley PLC register to read RMC registers into, or to write to RMC registers from.

Size in Elements: Enter the number of RMC registers to read or write in this field. The MicroLogix can transfer 1 to 41 integers.

Channel: Set this to the channel number of the serial port you want to use. The MicroLogix 1500 LRP Series B can use either channel 0 or 1, but all other MicroLogix PLCs will always use channel 0.

Target Device: This section holds parameters for the target device.

Message Timeout: Indicate the number of seconds to wait for the RMC to respond before determining that the attempt failed. This can be set as low as a few seconds.

Data Table Address: Enter the address of the first RMC register to read or write in this field. See the Registers Maps topic for help on addresses.

Local/Remote and Bridge Parameters: In most applications these will be set to Local and there will be no bridge parameters. If you are using a bridge then these parameters will need to be used. However, this is beyond the scope of RMC documentation. Refer to your Allen-Bradley documentation for instructions on using these fields.

Ethernet Communications (MicroLogix 1100):

In the RSLogix project tree, in the Data Files, create a message File (MG) data file and a Routing Information (RI) data file.
Add the MSG block to the ladder logic. Select the MSG block, enter the an address in the Message data file you just created (e.g MG9:0), and double-click Setup Screen.

This brings up a dialog with a General tab with the following options:

This Controller: This section holds parameters for the MicroLogix.

Channel: To use Ethernet on the MicroLogix 1100, choose 1 (integral).

Communication Command: This parameter can be set to PLC5 Read, PLC5 Write, 500CPU Read, or 500CPU Write. The type of PLC selected is not important, but the Read or Write determines whether you will read registers from the RMC or write registers into the RMC.

Data Table Address: Enter the address of the first Allen-Bradley PLC register to read RMC registers into, or to write to RMC registers from.

Size in Elements: Enter the number of RMC registers to read or write in this field.

Target Device: This section holds parameters for the target device.

Message Timeout: Indicate the number of seconds to wait for the RMC to respond before determining that the attempt failed. This can be set as low as a few seconds.

Data Table Address: Enter the address of the first RMC register to read or write in this field. See the Registers Maps topic for help on addresses.

Local / Remote: Choose Local.

Routing Information File (RI): Enter the address in the Routing Information File that you intend to use (e.g. RI10:0).

On the Multi-hop tab, in the To Address box, enter the IP address of the target RMC. For example, 192.168.0.219.

The Allen-Bradley MSG block may take multiple ladder scans to complete. Therefore, it is important to enable the MSG block for the correct amount of time. Specifically, the MSG block must be energized until the message control's enable (EN) bit turns on. Following the ladder samples to ensure proper functionality:

Using the Examine If Open instruction as shown below fulfills two requirements of continuous MSG transactions. First, it will keep the block energized until the EN turns on, and second, it de-energizes the MSG block once the transactions is started so that when the transaction is completed (EN goes low again), the MSG block sees a rising edge on its input, thus repeating the transaction:

This sample takes care to keep the MSG block energized until the MSG block starts, as indicated by the enable (EN) bit turning on. Once this happens, the application-controlled TriggerOnce coil is turned off. The message control's Done (DN) or Error (ER) bits can be used to process the results of the transaction.

All items in the RMC have F-file addresses. Allen-Bradley defines F file data as 32-floating point values. All the RMC registers have F-file addresses, even if they are DWORDs or DINTs. For example, the Status Bits and Error Bits in the RMC are DWORDS. To read these values, read them using their F addresses as given in the RMC. Then, in the PLC, use the COP instruction (RSLogix5000), or CPW instruction (RSlogix500) to copy the data to a register or tag of the correct data type, for example, an N register, L register, or DINT tag. The COP instruction preserves all the bits correctly, and the resulting values will be correct.