Strange request possibly

[sbasham]

OK, I have a challenge for someone. I have a proface Sp-550 WA WXGA connected to a DM BOARD. Because of ,let's just say, an uncooperative company, I cannot get any information about the DM board and the Proface is made explicitly for this company . I need to monitor the 3x addresses that the Proface is reading from the DM board. It is talking Modbus and I am using a AB1400 micrologix PLC to try and accomplish this. I uploaded the HMI program to check what 3x addresses i need to monitor. I am pretty novice when it comes to messaging and do have very little knowledge of setting up or even wiring RS485. I do think I have figured out how to setup the messaging instruction, what I want to know is can I just wire my serial port on the 1400 parallel to the 2 wire connection that the Proface and DM board is using to communicate with, and what pins are the TX and RX on the serial port of the 1400? I tried using the ethernet port on AB to connect to either ethernet ports on the Proface since they weren't being used and could only see one of the comm lights turn on, I believe they possibly are disabled in the software program because they aren't needed for comms to the DM Board.

 

[drbitboy]

https://literature.rockwellautomati...ure/documents/um/1766-um001_-en-p.pdf#page=62

It sounds like you are trying to snoop the Modbus protocol. That should not be too difficult, if it is indeed Modbus.

Two wire RS-485 is pretty forgiving; even if you get it backwards nothing bad will happen, but make sure you do not connect to any of the RS-232 pins in that mini-DIN.

See also this link, which should provide some background, even if that is not what you are doing.

I would start with an RS-485 USB converter to a PLC, and watch the protocol go by using PuTTY or even Python (in WSL, if necessary).

The protocol itself is pretty simple; see this and this, and any other references you can find with a search engine.
-

 

[drbitboy]

For cables, I suggest you talk to forum member @geniusintraining.

The mini-DIN connector needs to be one without the dimples - you'll know what I mean if you get one with the dimples; I was able to flatten out the dimples I had from the Radio Shack connector with a pair of needle-nose pliers.

 

[sbasham]

For cables, I suggest you talk to forum member @geniusintraining.

The mini-DIN connector needs to be one without the dimples - you'll know what I mean if you get one with the dimples; I was able to flatten out the dimples I had from the Radio Shack connector with a pair of needle-nose pliers.

I guess i should clarify, I need to use the 9 pin D type connection of the PLC but i can't find a pinnout of the tx-rx pins. I have RX logix 500 to program the 1400 and want to use the message read instruction to do my snooping. I have AB1400 configured for modbus slave in the instruction setup

 

[sbasham]

I guess i should clarify, I need to use the 9 pin D type connection of the PLC but i can't find a pinnout of the tx-rx pins. I have RX logix 500 to program the 1400 and want to use the message read instruction to do my snooping. I have AB1400 configured for modbus slave in the instruction setup

 

[drbitboy]

If the 1400 is the Modbus Slave, what device will be the Modbus Master?

Also, you referred to 9-pin D-sub and Tx and Rx pins, which suggests RS-232, but earlier you referred to RS-485. It is a Bad Idea™ to mix the two electrically, and RS-232 is for point-to-point communications, and having three devices communicating via RS-232 would be a configuration that is pretty difficult to implement.

 

[sbasham]

Judging by the proface software in the comms settings I think the DM board is master and the proface HMI is the slave but am not completely sure, the settings of RS485 (2 wire) is selected instead of RS232. I'm new to this forum so I haven't figured out how to include a snapshot of my screen on the proface software program

 

[sbasham]

There are two wires running from the DM board to the proface HMI to communicate. Can I parallel those to the 9 pin d connector of the AB 1400 PLC and do a read MSSG instruction with the rslogix 500 on the 3x registers the HMI is reading from the DM board?

 

[Steve Bailey]

I think the DM board is master and the proface HMI is the slave

That's the opposite to the usual configuration for an HMI panel. If the Proface is indeed the Modbus slave, I would expect that fact to be clearly stated in the configuration of the HMI port. Almost everyone who sets up Modbus in an HMI will assume that by default they're setting up a Modbus master.

 

[sbasham]

OK the HMI should normally be the master, possible it is, i just thought it's reversed because it appears that way in the HMI set-up and it was because of proprietary reasons

 

[Steve Bailey]

I need to monitor the 3x addresses that the Proface is reading from the DM board.

That statement suggests that the Proface is the master. 3xxxxx addresses in Modbus are read-only. The only way the DM card could write data from 3xxxxx addresses to a slave would be to first copy the 3xxxxx address to a 4xxxxx address and write the value from the 4xxxxx address.
Regardless of which device is the Modbus master, simply adding another Modbus slave on the RS485 cable won'y gurantee that the second slave will be able to get the data. The Modbus master could be sending the data to a specific Modbus ID and not broadcasting it.

 

[sbasham]

Thank you Steve Bailey for that Master/slave info, so I must be able to read the 3x values also with my AB 1400. I guess I just need to know how to wire the 9 pin D connector(serial port of AB 1400) to the two wires running from DM board to the HMI and do a read MSSG instruction to obtain the values of the 3x addresses I need to monitor?Correct?

 

[Ken Roach]

The pinouts of both Channel 0 (round Mini-DIN8) and Channel 1 (DB9) on the MicroLogix 1400 are in the User Manual (1766-UM001).

Channel 0 includes both an ordinary RS-232 port and a non-isolated RS-485 port (pins 1,8 and 2).

1 RS485 Data B(+)
2 GND
3 RTS
4 RXD
5 DCD
6 CTS
7 TXD
8 RS485 Data A(-)

Channel 1 is a conventional RS-232 port with Data Terminal Equipment (DTE) pinout.

1 DCD
2 RXD
3 TXD
4 DTR
5 GND
6 DSR
7 RTS
8 CTS
9 RI

Please clarify if you plan to disconnect the ProFace HMI and use the MicroLogix to read those Modbus registers from the DM Board, or if you essentially want to "sniff" the data or otherwise replicate it in the MicroLogix ?

Modbus is a pure master-slave protocol. It would take some very careful and nonstandard work to have two "master" devices polling the same "slave" device.

 

[Steve Bailey]

There can only be one Modbus master on a network. I don't see a way to add your ML1400 to the existing Modbus network and get the data you need. Is there a second port on the Proface HMI that can be set up to communicate with the ML1400? And is there a way for the Proface to periodically write the Modbus data it reads from the DM card to the ML1400?

 

[drbitboy]

There are two wires running from the DM board to the proface HMI to communicate. Can I parallel those to the 9 pin d connector of the AB 1400 PLC and do a read MSSG instruction with the rslogix 500 on the 3x registers the HMI is reading from the DM board?

It is very unlikely: two wires (probably, as in very likely) indicates RS-485; as far as I can tell, the 9-pin D-sub connector on the AB 1400 PLC is RS-422; to avoid damaging any hardware, I strongly suggest you do DO NOT CONNECT any pins from that D-sub to the two wires going between the DM board and the HMI, at least until you know more.

Do you have an oscilloscope? That would be the best way to determine what those two wires are. If they are RS-485, which seems likely, then there are two pins on the mini-DIN connector of the 1400 PLC that may be able to "sniff" the data passing between the HMI and the DM board.

A few more queries:

• Can you update the application running on the HMI?
• Can you update the application running on the DM board?
• Do you know what protocol is being used between the HMI and the DM board?
• Do you have any USB-serial converters that you could hook up to either the HMI or the DM board?

Strange request indeed.