Choosing an RS-232 to Ethernet converter for a CNC machine

Most shops meet this problem before they've heard the phrase 'DNC software': the machine has a 25-pin serial port, the PC hasn't had one since 2009, and the machine is thirty metres away. Here's what to buy and what to check.

What the converter is actually for

An RS-232-only Fanuc control expects a serial cable to a device sitting close by — the standard is reliable to roughly 15 metres, less in a shop full of electrical noise. A serial device server (the useful kind of converter) plugs into the machine's serial port, takes an Ethernet cable to your switch, and makes that port reachable from anywhere on the shop LAN. The distance limit becomes the network's, not RS-232's, and one PC in the programming room can reach every machine on the floor.

Device server or USB adapter?

• USB-to-serial adapter — one COM port, on one PC, at the end of a short cable. Fine for a single machine with a laptop parked beside it. Useless for a floor.
• Serial device server — the machine's port lives on the network. Any PC can reach it, several machines can transfer at once, and the PC stays in the office. This is what a DNC setup is built on.
• Multi-port device server — 4, 8 or 16 serial ports in one box. Cheaper per port and neater in a panel when the machines are grouped together.

What to check before you buy

• RS-232 support specifically — plenty of industrial converters are RS-485 first, with RS-232 as an afterthought or a jumper setting.
• Baud rates down to 4800 and below. Old controls are often set to 4800 or 9600 and cannot go faster; a converter that starts at 9600 rules those out.
• Configurable parity, data bits and stop bits — 7/E/2 and 8/N/1 are both common on Fanucs, and you need to match whatever the control is set to.
• Flow control: XON/XOFF at minimum, ideally RTS/CTS wired through as well.
• A raw TCP or 'TCP server' mode. Some units only offer a virtual COM port driver, which ties you to one Windows PC and one vendor driver — the thing you were trying to escape.
• 24V DC or wide-range power, so it runs off the machine's own panel supply rather than another adapter falling out of a socket.
• DIN-rail mounting and a metal case. It lives in a cabinet next to a spindle drive, not on a desk.
• A static IP you set yourself. DHCP on a shop network means the address changes and every machine goes missing on a Monday.

Settings must match the control, exactly

The converter does not translate anything — it carries bytes. Baud rate, data bits, parity and stop bits on the converter have to match the control's RS-232 parameters exactly, or you get garbage characters and a transfer alarm. Read them off the machine first, set the converter to match, and change nothing on the control until the link works.

The full parameter reference — I/O channel, baud codes, and the alarms that tell you which setting is wrong — is in the Fanuc RS-232 settings guide.

Testing it before you trust it

• Ping the converter from the programming PC. No reply means it's a network problem, not a serial one — and that's a much easier problem.
• Punch a short program out of the control to the PC first. Receiving proves the wiring and the settings without risking anything in the machine.
• Then send a short program back. If characters arrive mangled, the mismatch is baud or parity, in that order.
• Only then drip-feed something long. A link that transfers a 3 KB file fine can still fail at minute forty if flow control isn't working — test it with a real program, on a dry run, before a real cut.

Common failures, and what they usually mean

• Garbage characters on screen — baud rate or parity mismatch between converter and control.
• Transfer starts, then stops partway — flow control isn't getting through; check XON/XOFF is enabled on both ends.
• Nothing at all, and no alarm — cable pinout. TX and RX are crossed, or they aren't and should be.
• Works for a short file, fails on a long one — the control's buffer overruns because pacing isn't working. This is the failure that only shows up mid-cut.
• Worked yesterday, gone today — the converter is on DHCP and its address moved.

And then you need software on the other end

The converter gets the machine onto the network. Something still has to sit on the PC and send, receive, and pace the programs — that's what DNC software does. ElectronIx DNC talks to device servers over raw TCP, drip-feeds with XON/XOFF pacing, and handles around 25 machines from one ordinary Windows PC. If you tell us your machine list, we'll tell you exactly which converters you need and how many — whether or not you buy the software from us.

Related: Fanuc program transfer over Ethernet, step by step and drip-feeding programs bigger than control memory.

Converter questions

Wiring more than one or two machines? Plan the addresses before you buy — setting up a shop-floor network covers switches, cable routing around drives, and a worked static IP plan.