Moxa NPort Serial Device Servers, New Zealand

Moxa / NPort serial device servers

The same NPort behaves completely differently depending on one setting. In one mode it appears to a PC as an ordinary COM port. In another it waits for a network connection. In another it makes the connection itself. Fit a replacement without knowing which mode the old one was in and nothing works, even though the hardware is correct.

NPort is how a lot of New Zealand plant keeps serial equipment in service: weighbridges, scales, printers, older PLCs, meters, barcode readers and instruments that were never going to get an Ethernet port.

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The operating mode is the product

Everything else on this page follows from this.

Mode	What it does	Typically used when
Real COM	A driver on the PC creates a COM port that behaves like a local one	Existing PC software expects a serial port
TCP Server	The NPort waits for something to connect to it	A SCADA or PLC opens the connection
TCP Client	The NPort makes the connection outward	The serial device initiates, or the host cannot
UDP	Connectionless, often one to many	Broadcast style data
Pair connection	Two NPorts act as a serial cable extension across the network	Two serial devices, no PC in the middle

Pair connection is the one people forget exists. If a site has two NPorts and no obvious host software, they are probably a virtual serial cable between two devices, and both ends have to be configured to match.

Real COM mode needs the driver installed on the PC. So a replacement NPort on a machine that was working may need the driver reconfigured to point at the new device, and that is a PC job as much as a hardware one.

So the first question is what the old unit was set to. Get that from the configuration file, from the existing unit's web page while it still works, or from whoever commissioned it.

Capture the configuration before you need it

What to record	Why
IP address, subnet and gateway	A replacement at its default address is invisible
Operating mode	The most important single setting
Serial parameters	Baud rate, data bits, parity, stop bits, flow control
Interface type	RS-232, RS-422 or RS-485, and two or four wire
TCP port numbers	What the host connects to
Data packing settings	How the device decides a message is complete

Export the configuration file from every NPort on site while they work. It takes minutes each, the files are tiny, and it converts a replacement from a commissioning job into a hardware swap.

Name the files by location. A folder of configuration files nobody can match to a site is only slightly better than nothing.

Which serial standard

The other half of getting a replacement right, and it decides which model you need.

	RS-232	RS-422	RS-485
Devices on the link	Two. Point to point	Point to point, longer	Many, on a multidrop bus
Distance	Short	Long	Long
Noise immunity	Poor	Good	Good
Wires for data	Separate transmit and receive	Four	Two, or four

Some NPort models are RS-232 only and some support all three, selected in configuration or by wiring. So the model you need depends on the serial standard as much as on the port count.

Check what the field device actually uses, not what the cable looks like. A nine pin connector does not guarantee RS-232, and RS-485 is frequently landed on screw terminals.

RS-485 needs termination and biasing

The single most common cause of an intermittent serial link, and it is free to check.

Requirement	What happens without it
Termination at both ends of the bus	Reflections corrupt data, worse on longer runs and higher speeds
Termination at the ends only, not in the middle	Extra terminations load the bus down
Bias resistors somewhere on the bus	The line floats when nothing is transmitting, and receivers see noise as data
A common reference between devices	Devices at different ground potentials misread each other
Correct polarity on the pair	Nothing works at all, which at least is obvious

The symptom of missing termination or biasing is not a dead link. It is a link that mostly works, drops the occasional message, and gets worse when a drive starts or the weather changes.

So a serial link that has always been slightly unreliable is usually a wiring problem, not a hardware problem. Replacing the NPort does not fix it, and people often replace it twice before looking at the bus.

Many NPort models have selectable termination and biasing built in, which is worth checking before adding resistors in the field.

Serial parameters have to match exactly

Baud rate, data bits, parity and stop bits have to be identical at both ends. Any mismatch produces either nothing or garbage.

Symptom	Usually
Complete rubbish received	Baud rate wrong
Mostly readable with occasional bad characters	Parity or stop bits, or a noise problem
Nothing at all	Wiring, polarity, or the wrong interface standard
Works one way only	Transmit and receive swapped, or a four wire and two wire mismatch

Those symptoms are diagnostic. Garbage means the link is physically fine and a setting is wrong, which is much better news than silence.

Message framing catches people on newer installations

Worth knowing, because it explains a class of fault that appears after a change.

Serial data is a stream and the network sends packets, so the NPort has to decide where one message ends. It does that on a delay, a character count, or a specific end character.

Symptom	Often
Host receives half a message, then the rest	Framing settings
Two messages arrive joined together	Same
Worked for years, broke after a network change	Timing changed and the framing no longer suits

A weighbridge or scale that started returning odd readings after a network upgrade is a good candidate for this, and it is a setting rather than a fault.

Choosing a replacement

Decide	Notes
Port count	One, two, four, eight or sixteen
Serial standards supported	RS-232 only, or all three
Connector style	Nine pin, screw terminal or block
Mounting and power	DIN rail or desktop, 24 V or mains adapter
Temperature rating	Extended range versions exist and are specified for a reason
Isolation	Isolated serial ports where earth potentials differ

Isolation is worth mentioning to us. On a link between buildings, or between a plant and a weighbridge in the yard, isolated ports prevent earth potential differences from destroying the port. If the original was isolated, match it.

Before ordering a replacement

Symptom	Check first
Link intermittent, mostly works	RS-485 termination, biasing and common reference
Garbage characters	Baud rate and parity. The link itself is fine
Nothing at all	Polarity, wiring and interface standard
New unit fitted, host cannot find it	IP address and operating mode
PC software cannot see the COM port	Real COM driver pointing at the old device
Messages split or joined	Framing settings
Port failed after a storm	Isolation and surge protection, before replacing again
Two units on site, neither talks to a PC	They may be a pair connection

What to send us

The model number.

The operating mode, or what the serial device connects to.

The serial standard. RS-232, RS-422 or RS-485, and how many wires.

The port count and connector style.

Whether the port is isolated.

Whether you have the configuration file.

Where it is installed, if the location is outdoors or harsh.

What we can supply

Device servers. NPort across the port counts and serial standards, in DIN rail and rack formats, current and obsolete models.

Isolated and extended temperature versions.

Serial cards. CP multiport cards where the connection is into a PC rather than a network.

Accessories. Power supplies, mounting kits, adapters and surge protection.

Related gateways. Where what you actually need is protocol conversion rather than a transparent serial link, an MGate may be the better answer and we will say so.

Serial to Ethernet or protocol gateway

Worth being clear about, because the two get confused and they solve different problems.

	NPort	MGate
What it does	Carries serial data across a network unchanged	Translates one protocol into another
The host must	Understand the original serial protocol	Only understand its own protocol
Use it when	Existing software speaks the device's language	A PLC needs to read a device it does not understand

So if the plan is for a PLC to read Modbus registers from a serial meter, that is a gateway job. If the plan is for existing PC software to keep working with the device moved onto the network, that is a device server job.

Tell us what is at each end and we will say which you need.

We buy Moxa NPort

NPort resells steadily, including obsolete models that sites are standardised on.

Keep power supplies and mounting kits with the units. See Sell To Us.

Common questions

Can you get a part that is not listed on your site?
Usually. The site shows what we hold, not what we can source.

My new NPort does nothing.
It arrives at a default address with no configuration. The operating mode and serial settings have to be restored.

Why is my serial link intermittent?
On RS-485, termination and biasing. That accounts for most of these and it is free to check.

I get garbage characters.
Baud rate or parity. The wiring is fine or you would get nothing.

What is pair connection?
Two NPorts acting as a serial cable across the network, with no PC involved.

Should I use an NPort or an MGate?
NPort carries serial data unchanged. MGate translates protocols. Tell us what is at each end.

Do you supply used equipment?
Yes, tested before it ships. On obsolete models it is often the only route.

What is the lead time?
Ex stock items ship the next working day within New Zealand. We give you a firm lead time with the quote.

Do you quote in NZD?
Yes, in NZD including GST for New Zealand customers.

Send us your model numbers

And tell us what the serial device is and how it is wired. On this product the mode and the wiring decide whether a correct part works. Quotes in NZD including GST.

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Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with Moxa. Moxa, NPort and MGate are trademarks of Moxa Inc. Modbus is a trademark of Schneider Electric. All trademarks, brand names and model numbers are the property of their respective owners and are used here for identification purposes only.