Quick Answer: Serial and Ethernet answer different questions. RS-232 is a point-to-point byte stream with no addressing, practical to about 15 m at 19.2 kbit/s; RS-485 is a differential bus that reaches roughly 1200 m at low rates. Ethernet is a framed, addressed network that runs 100 m per copper channel at 1 Gbit/s on Cat5e or Cat6. Use serial for one legacy device, deterministic control and two-wire wiring; use Ethernet when more than two nodes, remote diagnostics or routing are involved. Most panels already speak Ethernet, so the question is often the last few metres.
The two interfaces are often compared as if one replaces the other. They sit at different layers: serial defines voltage levels and a bit stream, Ethernet defines a frame, an address and a collision domain. A protocol such as Modbus can run over either, which is why the same register map appears on a DB9 port and on TCP port 502.
One Carries Bytes, the Other Carries Frames
Serial: timing instead of structure
A UART shifts bits out at an agreed rate with a start bit, optional parity and one or two stop bits. There is no address and no enforced checksum; if two devices talk at once on a two-wire RS-485 bus, both ends get corrupted data. RS-232 is single-ended, with swings of plus or minus 3 to 15 V against a shared ground that drifts with distance, the usual reason a long link drops characters.
Ethernet: structure first
An Ethernet frame carries a 48-bit destination and source MAC address, a type field, a payload and a 32-bit frame check sequence. Switches learn which MAC sits on which port, so unicast traffic is forwarded rather than repeated everywhere. 1000BASE-T is transformer-coupled and differential on all four pairs, which is why a Cat6 run tolerates ground differences that would destroy an RS-232 link. Pinout conventions for the 8P8C end are covered in our RJ45 to DB9 adapter notes.
Reach and Cabling
Distance claims need the rate attached to them. A 15 m RS-232 figure assumes 19.2 kbit/s; halve the rate and roughly double the run, raise it to 115.2 kbit/s and 5 m is realistic. RS-485 trades speed for reach because the differential pair rejects common-mode noise, but it needs 120 ohm termination at both ends and one continuous trunk, not a star of stubs.
| Property | RS-232 | RS-485 | 1000BASE-T |
|---|
| Signalling | Single-ended | Differential pair | 4 differential pairs |
| Topology | 2 devices | Bus, 32 unit loads | Star via switch |
| Working distance | 15 m at 19.2 kbit/s | 1200 m at 100 kbit/s | 100 m channel |
| Typical rate | 9.6 to 115.2 kbit/s | Up to 10 Mbit/s short | 1000 Mbit/s |
| Addressing | None | Node ID in protocol | MAC, then IP |
Determinism and Timing
Serial wins on predictability at the low end. A 9600 baud poll of one register takes a fixed number of bit times, so a control loop can bound its worst case by arithmetic. Switched Ethernet has store-and-forward delay that varies with queue depth. Priority queues and industrial Ethernet profiles close most of that gap, but TCP retransmission can still stall a poll for hundreds of milliseconds. If a machine cycle cannot tolerate that, keep the loop on serial.
Bridging the Two
A serial device server wraps the byte stream in TCP as a virtual COM port or raw socket. Settings on both sides must match baud rate, data bits, parity, stop bits and flow control. Half the failed retrofits we see are a parity mismatch or an RTS that the converter asserts differently. Where a laptop has no native port, a USB to serial adapter with an FTDI chip is more stable than a generic bridge cable. Our RS232 cable guide covers null-modem pinouts for direct device-to-device links.
Practical Scenarios
- Single VFD in a cabinet: keep RS-485 on a 120 ohm twisted pair with termination at both ends. An Ethernet card in one drive buys nothing unless it is monitored remotely.
- Twelve meters from panel to scale: RS-232 at 9600 is fine on shielded cable, but check the ground reference. If the enclosures sit on different earths, convert to RS-485 instead.
- Legacy PLC that must join a plant network: use a serial device server on the programming port, put it on an isolated VLAN, and never expose port 502 directly. The protocol has no authentication.
- Bench testing before installation: prove the link with a terminal program at the final baud rate and cable length. A cable that passes at 1 m can fail at 20 m once capacitance is in play.
FAQ
Can I connect an RS-232 device directly to an Ethernet switch?
No. The voltage levels and framing are incompatible, and a passive RJ45 to DB9 adapter only maps pins. You need an active converter or device server that terminates the serial protocol and speaks IP. A passive adapter will not damage most hardware, but nothing will pass.
Is Ethernet always faster than serial in practice?
On raw throughput yes, by orders of magnitude. On round-trip time for a single small read, a 115.2 kbit/s serial link can answer in a few milliseconds while a TCP stack may take longer to establish and recover. For polling one register, latency matters more than bandwidth.
Do I still need shielded cable for serial runs?
In a cabinet full of drives, yes. Shielded twisted pair with the shield bonded at one end reduces induced noise on the signal lines, and a separate ground conductor avoids using the shield as a return path. In a quiet office, unshielded three-core cable at short length usually works.
