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Interfacing the Honeywell EPLCG to Allen Bradley PLC-5 and SLC-500

 Introduction 
This article discusses some practical aspects of interfacing the Honeywell Enhanced PLC Gateway (EPLCG) with Allen Bradley PLC-5 and SLC-500 Programmable Logic Controllers.

This article discusses the Enhanced PLC Gateway (EPLCG), although the recommendations here can be also be applied to the standard PLC Gateway (PLCG).

The EPLCG supports communications to Allen-Bradley PLCs using the PLC-2 subset of the DF1 command set.

The DF1 protocol has a large, somewhat unwieldy command set, supporting almost 60 different commands. The EPLCG uses three of the oldest commands, namely those that were used for communicating to Allen Bradley's early PLC-2 programmable logic controllers:
 01Unprotected Read
 05Unprotected Bit Write
 08Unprotected Single Word Write

Newer Allen-Bradley PLCs, such as the PLC-5 and SLC500, still support the PLC-2 commands, although it may be necessary to make some modifications in the PLC and/or in the other network devices (such as the KF-2) in order to support this.


 Commands Supported by the EPLCG 

Unprotected Read
Reads one or more registers from the PLC.

Unprotected Bit Write
Writes one or more bits, using the Set/Reset mask fields.

Unprotected Write
Writes one or more registers.

Note that all three commands lack a file number.


 Commands Supported by various Allen Bradley PLCs 
The following table (an extract from the Allen Bradley DF1 protocol reference) lists the three commands used by the EPLCG, along with the support for those commands in various Allen Bradley PLCs.

Probably the most important thing to note is that the SLC 500 does not support command 05, Unprotected Bit Write. The implication of this is that it is not possible to build digital composite or digital output points in the EPLCG that write directly to a bit within a word in the SLC 500.


 General Tips 

 PLC-5 
The following recommendations may be useful:

  • Configure the PLC-5 for byte addressing
  • The PLC-2 compatibility file needs to be assigned the same number as the station address of the 1770-KF2 module.
  • The PCnPORTA parameter for the box (emulated DHP) should be the same as the PLC-5 station address. For example, if the PLC-5 address is 2, and it is connected to Hiway 7, Box 8, the EB file would look like this: 
    &T BOX                                                                          
&N $HY07B08                                                                     
HWYNUM   = 07
BOXNUM   = 08
BOXTYPE  = DHP         
BOXASSN  = THISHG      
LOADDEST = HG_HIWAY    
BOXSIZE  = EXTENDED    
BOXPROT  = ALLENBRD    
BOXSTART = COLD        
SCANTIME = 0 
PC1TYPE  = APLC        
PC1PORT  = 1 
PC1PORTA = 2   
...
  • The PCnTYPE parameter for the box (emulated DHP) should be set to APLC
  • The KF-2 needs to be set up to support Embedded Responses. This is set up using DIP switches in the KF-2.

The following extracts from the Honeywell documentation describe the recommended setup for the KF-2 or equivalent KE module:


 SLC-500 
Although the SLC-500 is not explicitly supported by the EPLCG, it is possible to use the PLC-2 Unprotected Read and Unprotected Write commands to communicate with the SLC-500.

As noted above, command 05, Unprotected Bit Write, is not supported by the SLC-500 and so requires a workaround. Providing that the digital points are only operated through graphics, one suggested workaround is to write directly to the word in the SLC-500 (via a PLCG counter point) from the GUS embedded display. The subpicture used for these SLC-500 digital points would have parameters for the tag name of the counter point, along with the value to write. For example, if writing to bit 0 then write 1, or if bit 4 then write 16 to the counter.

The following recommendations may be useful:

  • Configure the SLC-500 for byte addressing
  • All data to/from the SLC-500 must go via the Common Interface File, File 9. Note that this is an integer file.
  • The PCnTYPE parameter for the box (emulated DHP) should be set to APLC
  • If digital outputs are being used, this will require additional engineering to work around the lack of support for the Unprotected Bit Write command

Note that the KF-3 automatically handles Embedded Responses, so no action is required.


 References 
Refer to the following documents for more information:

  • Allen Bradley DF1 Protocol Reference Manual, document 1770-6.5.16
  • Honeywell Enhanced Programmable Logic Controller Implementation Guidelines, document EP12-500