Problem Solution Overview

We performed testing between the subject device and our 3100-MCM (Rev 2) product.

Solution Brief

Our testing generated the following results:

• We were able to successfully communicate with the device. (see below for configuration parameters used during the testing)
• The product supported Modbus functions 3,4,5 &6
• Only the RTU mode of the protocol is supported by the M-3420, with baud rates up to 9600 baud
• Read command lengths are limited to 15 words
• An RTS On Delay parameter value of 150 - 200 ms was used to slow down communications with the Beckwith M-3420. This is required as the Beckwith unit processes internal functional logic before communications logic
• An Intercharacter Timeout Delay parameter value of 20 ms was used in order to account for periodic delays between characters beyond the allowed 3.5 character widths

The purpose of our testing is to assure a level of compatibility in the communications between the two products. Through this testing, we attempt to identify problem areas that may arise when implementing the interface in the field.

Configuration:

The product testing was perfomed with the following settings used:

Baud : 9,600
Parity : None
Stop : 2
Mode : RTU Mode (ASCII Mode not available)
Physical Connection : RS-485 (2 wire connection w/ jumpers on M-3420 COM3 port)

The MCM module configuration used in our testing was as follows:

The test was performed using MCM Rev 2.0 with Port 2 configured to talk to the Beckwith unit in RS-485 mode :

This configuration, in combination with the example ladder logic referenced below, generates the following memory map:

N10:0 to N10:129 - Slave Response Data
N10:130 to N10:149 - Slave Port Error Table (Contains revision level information also)
N10:150 to N10:199 - Master Error Table (Contains status of commands for diagnostics)

Command List:

The following command list shows some of the commands which were used to test the communications:

The commands which were tested perfomed the following functions ( The Beckwith M-3420 parameter numbers are not offset by 1 as other Modbus slaves are:

1. N7:50 Reads 10 parameter values from the M-3420 unit.
2. N7:60 Conditional write to the M-3420. Any change to the value in register 200 will cause a write of the new value to the M-3420.
3. The Beckwith M-3420 Modbus register map is obtained by translating the BECO2200-M3420 data base Type and Point Number to a Modbus register address. This conversion is straight forward and consists of combining the Type and Point Number in hex and converting to decimal (base 10). The result of this conversion can be used as the address entry in the Command List.

Cable Connection:

The physical connection which was used during testing was as follows:

For ease of installation, termination on the ProSoft module was accomplished using our screw termination adaptor for the DB-25 plug. A similar screw termination plug is also available for the DB-9 required for the 3150-MCM. The part number for these is:

1451-9F : DB-9 to RS-422/RS-485 Screw Terminals (for 3150-MCM)
1452-25M : DB-25 to RS-422/RS-485 Screw Terminals (for 3100-MCM)

Additional Comments and Observations:

1. This unit has several communication ports available. We tested only one of these ports, using COM3 of the unit for all testing.
2. If the MCM module is not slowed down with the RTS On Delay (150 ms), the M-3420 will not respond to the next command issued from the MCM.