man my head hurts

[pyles76384]

Back again with another question. I have an SLC500 modular rack with an open slot. I need to communicate with a device(EKM Metering electric meter) via RS485. What module would I need, the 1747-BAS module? How do you send the request to meter for the required information, do you write the program in basic and download with hyper terminal and it will do it's thing.
Thanks for your help
David

 

[harryting]

What's the exact model number of your SLC processor and the meter? What protocol?

 

[pyles76384]

Thanks for thr quick response, the processor is a 1747-L551 series B, I'll try to attach their information.
Thanks David

 

[Lancie1]

It appears that the meter is sending and receiving in Hexadecimal code which is converted to ASCII characters. The EKM Metering Data Format example given is definitely converting 123456 78 ASCII characters to 31 32 33 34 35 36 37 38 Hexadecimal characters.

Example: 123456.78kWh (31 32 33 34 35 36 37 38)

If that example is typical for all data, then a 1747-BAS module (which allows writing and running BASIC language programs) would not help very much.

There are 8 ASCII instructions in RSLogix that allows receiving and transmitting ASCII instructins to and from the built-in ASCII Port Buffer (which receives data from the RS485 port). There are also 6 ACSII string manipulations instructions. With these 14 instructions, you should be able to write a PLC program that will extract the meter data that is received over your RS485 cable.

 

[pyles76384]

Thanks Lancie1 for the reply, if I use channel 0 on the processor(RS232) would I need a 485 to 232 converter??
Thanks David

 

[Lancie1]

No, I don't think you would need any converter. I looked at an old SLC 5/03 program that had DH485 and also a device that was reading and writing ASCII commands. The way its Channel Configuration is setup is like this:

Channel 1 - System: Driver: DH485, Baud: 19200
Channel 1 - System: Driver: DF1 Full Duplex, 1200 Baud, No Parity, 1 Stop bit
Channel 0 - User: Driver: ASCII, Baud 9600, Parity Even, Stop Bits 2, Data Bits 7, Control Line: No Handshaking, Delete Mode: Ignore

The Channel 0 User driver apparently reads the the RS232 signals into the SLC ASCII buffer.

 

[pyles76384]

So you think I can go directly to channel 0 with the RS485
Thanks David

 

[Lancie1]

I think you will need both the Channel 1 System and the Channel 0 User. Channel 1 pulls the comm signals off of the RS485 line, and the Channel 0 seruial port pulls out the ASCII commands and sends to the SLC built-in ASCII Port buffer. The data in the buffer can be pulled out and read using the ASCII instructions, or new data written into the buffer and sent out on the RS485 comm link.

 

[Ken Roach]

Whoah, stop a bit. That's not how Channel 1 and Channel 0 work on the SLC-5/03 controller.

Channel 1 is the DH-485 network port. It can only run the Data Highway 485 protocol; no ASCII, no Modbus, no DF1. It is isolated from Channel 0 both physically and logically.

Channel 0 is the multipurpose serial port. It is RS-232 only, but can be configured for two System mode protocols (DF1 or DH485) or for User Mode, which gives you the use of the ASCII port commands.

If you need to both send and receive ASCII characters, you must use Channel 0. Since your EKM meter uses RS-485, you need an RS-232/RS485 converter.

 

[Ken Roach]

Ack.. and here I go making a similar mistake.

The OP said this is a 1747-L551 controller, which is an SLC-5/05 model. This has a built-in Ethernet port for Channel 1, not a DH485 network port like the SLC-5/03 model.

The basic answer is the same: use a RS-485 to RS-232 converter and User Mode on Channel 0.

 

[Lancie1]

I looked at an old SLC 5/03 program that had DH485 and also a device that was reading and writing ASCII commands. The way its Channel Configuration is setup is like this:

Channel 1 - System: Driver: DH485, Baud: 19200
Channel 1 - System: Driver: DF1 Full Duplex, 1200 Baud, No Parity, 1 Stop bit
Channel 0 - User: Driver: ASCII, Baud 9600, Parity Even, Stop Bits 2, Data Bits 7, Control Line: No Handshaking, Delete Mode: Ignore

The Channel 0 User driver [CORRECTION: does read] the RS232 serial-port signals into the SLC ASCII buffer.

 

[pyles76384]

I will try it tomorow, and thanks to everyone here, just remembered, I need to buy a converter. Can you recommend a RS232 to RS485 converter??
Thanks David

 

[Ken Roach]

If the EKM meter uses "2-wire" RS-485, the Allen-Bradley 1761-NET-AIC is a good converter.

My preferred vendor for serial isolators and converters is B&B Electronics [Link]. Their DIN-rail mounted units are generally very reliable and straightforward to use.

 

[Ken Roach]

I just skimmed over the actual protocol manual.

The protocol doesn't use ordinary ASCII end-of-line bytes, but rather and end-of-transmission byte (0x03) followed by a CRC16 checksum.

This means you won't be able to use the conventional ASCII Read Line (ARL) instruction, but rather will have to use the ASCII Read (ARD), which reads the entire buffer. That's a little more challenging.

Also, some of the data is fairly long, so it won't fit into a single 40-character String data element. You'll have to do a lot of manipulation of the data before even making it into Strings.

The EKM manual is what makes my head hurt; lots of color coding but no key to the color codes, quite a lot of mixing of the words "request" and "read", with no explanation of the difference between them, and lousy formatting. I'm sure it makes sense to the author.

The SLC serial port is the first tool in the toolbox, but you may need to resort to a device like the Prosoft MVI46-GSC generic serial module, or a gateway device like the Anybus Communicator AB7001.

But my first step would be to figure out exactly which commands I'm going to use, and practice sending them using a serial terminal program like RealTerm.

 

[Ken Roach]

One last thing: Even if that's the name your mother gave you, it is confusing to have a Forum username that very similar to a market-leading industrial electronics brand.

Since you don't have a long-established posting history under that username, I strongly recommend that you abandon it and re-enroll in the Forum under your own name or a suitable pseudonym.