RS232 Maximum Distance for A-B MicroLogix1200's

[xunilbama 0]

Good morning, 

I work as an electronics technician for wastewater treatment and lift stations. We have quite a few RTU's that house a MicroLogix 1200. While I can remotely connect to the MicroLogix 1400's the same can not be said for the MicroLogix 1200's. If a problem occurs at a lift station with the 1200's then I have to connect locally to the 1200 via a 2711-NC21 where one end has a S-video type connector and the other end has a 9-pin serial end with a null modem attachment. I have heard that the maximum distance one can get from a 1200 via a programming/interface cable is 50'. However, there are a few particular RTU's at varous lift stations that are further away than that 50' threshold. Is there any possible way I could exceed this limitation with some kind of interface type device that will take that serial connection and bridge it into another format like bluetooth, LAN, or usb ? I'm really trying to avoid having to be at the mercy of the 6' cable connected to the 1200. There's 1 lift station I can think of where the RTU is probably 60 feet away from the MCC room where the pumps and pump controls are at. 

[DanW 41]

The 50 feet limitation comes from the EIA specification.   It doesn't mean that the signal dies at 50 feet, but that one should expect reliable communications for installations 50 feet and under.  Performance beyond the stated limit can be compromised.  I know of two installations that when I arrived the RS-232 'cable' had already been installed, in one case about 65 feet, in the other about 75 feet away.   The 65 foot run was not even shielded, twisted pair, but it worked at 9600 baud and to my knowledge is still running today (commissioned in the early 2000's).

On a similar note, Ethernet is rated to 100m.  A local town fair ran a CAT 5 cable 500 feet to get an internet connection that ran fine at 10Mb, back in the day before Wi-fi was everywhere (early 2000's).  The signal doesn't 'die' at the limit, but extending beyond the limit affects performance.

My suggestion is to try comm with shielded twisted pair cables, at least 18g, 16g if you've got it to reduce any voltage drop. 

As to converters, USB is more limited that RS-232,short cables only.   There are scads of 232/Ethernet converters.   There are Bluetooth/RS-232 converters but I have no experience at all with them.

[chelton 66]

 Serial device servers work really well. Run ethernet to each RTU and install the serial device server at the RTU. This future proofs by having ethernet at the device, and in the short term serial cable can be short.

[xunilbama 0]

@DanW I appreciate your advice on the area. It has been very helpful

@chelton The same goes for you... 

Thank you

[xunilbama 0]

I picked up a StarTech RS232 to Ethernet Extender hoping to extend the signal to a building where I our switchgear is. I haven't had much luck considering IT has practically ever laptop a technician uses locked down. I've been using my personal laptop as a means of trying to figure out how to get this extender to work properly. I have had no luck thus far. I'm now just limited to using the original programming cable I've been using for the last 3 years and be right in front of the RTU rather than somewhere else. I'm not entirely sure this StarTech extender would work with the programming cable. Do any of you have any experience dealing with these particular extender's 

 

Maybe soon our liftstation's with MDS radios will be updated to RV50 cellular modems, so that I can remote into a PLC without the need of a physical programming cable. 

[pturmel 160]

3 hours ago, xunilbama said:

StarTech RS232 to Ethernet Extender

You are misquoting the product.  It isn't ethernet, just happens to be the same kind of cable.  Ethernet cannot go 1000m either.  It is likely doing some form of RS232 to RS422 transformation (but possibly muxing more than one status signal on certain pairs).  The key to success will be figuring out how the pairs cross between the two units.

[Crossbow 295]

I think it will be just like working with USB to serial adapters.  Some of them do not implement the entire control set, and things like CTS/RTS are not always connected.  I've used a dozen brands of USB to serial adapters, and of those, only 2 worked on all the PLCs I worked with.  Like pturmel said, it comes down to seeing how the wiring is crossed between the two devices.

[DanW 41]

On 9/12/2023 at 3:50 PM, xunilbama said:

using the original programming cable I've been using for the last 3 years

Serial comm is always a project, it is rarely plug-and-play.   You might need to make an adapter cable for the connection between the adapter in the field the RTU field device.

You need to ring out the programming cable that you have to see which RS-232 function is on which pin, going from the laptop to the 'working' end and make a drawing of which pin goes where.  Make sure that the laptop's function match the adapter's functions on each pin.   Things can get reversed going from male to female, or counting from the wrong end.   When I've done stuff like that I always had a print-out of the make DB9 pinout numbering and the female DB9 pinout numbering.

Then you need to match up the pinout on the other end to the RTU that is the field device so that the pin functions end up on the needed pins.

You have noted that you need to use CAT 5 cable that is runs point-to-point between the two adapters.  This is NOT Ethernet and you can't connect the CAT cable to a switch, hub or router.