Serial communication interfaces · RS-232 versus RS-485 electrical limits, data rate against cable length, unit loads and termination, failsafe biasing, isolation and ESD, converters and adapters
A serial communication interface is the hardware, a UART with line drivers and receivers, connectors and, where needed, isolation, that turns a device's serial data into RS-232, RS-422 or RS-485 signals on a cable, either built into equipment or supplied as a converter, adapter or interface module. Ask three things first: which electrical standard and topology the link uses, what data rate and cable length it must reach, and what isolation and surge or ESD protection the site needs.
Serial interface hardware: RS-232, RS-422 and RS-485 transceiver interfaces, interface modules and boards, and USB- or TTL-to-serial converters and isolators. Ethernet serial device servers, serial hubs and splitters, and routers with serial ports are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
TI's RS-485 design guide describes RS-485 as an electrical-only standard for balanced multipoint lines: a single 5 V supply, −7 V to +12 V common-mode range, up to 32 unit loads, 10 Mbps maximum at 40 ft and 4,000 ft maximum cable length at 100 kbps. Drivers must give at least 1.5 V differential into 54 Ω and receivers detect down to 200 mV. [1]
TI's TIA/EIA-232-F design notes give RS-232 logic levels of ±5 V to ±15 V driven and ±3 V to ±15 V received, pins that must survive a short to any other pin, a 3–7 kΩ load, a 30 V/µs maximum slew rate, a 2,500 pF maximum capacitive load equal to about 15–20 m of cable, and a 20 kbit/s maximum data rate in the standard, with practical links reaching around 120 kbit/s over short cables. [2]
Editorial note: RS-232 suits a point-to-point link of a few metres; RS-485 suits multidrop buses and long runs. Higher protocols such as Modbus RTU sit on top; confirm which one the equipment speaks.
Give the supplier the electrical standard (RS-232, RS-422, RS-485 two- or four-wire), host side (USB, TTL UART, PCIe, internal board), data rate, cable type and length, number of nodes, connector or terminal type, isolation voltage, ESD and surge level, supply voltage, operating temperature, driver support and the protocol the link carries.
02
Check cable, termination, loads and protection
The TI guide recommends 120 Ω twisted-pair cable of 22–24 AWG, termination at each cable end with 120 Ω resistors matching the cable impedance, stubs as short as possible, failsafe biasing so receivers hold a defined state during open, short or idle bus conditions, and control so that only one driver is active at a time. [1]
It lists maximum stub lengths for a cable with 78% velocity: 7 ft for a 100 ns rise-time driver at 1,000 kbps, 19 ft at 250 ns and 250 kbps, and 38 ft at 500 ns and 200 kbps. [1]
Guangzhou Dtech lists a USB 2.0 to RS-232 converter cable with full signal opto-isolation rated up to 6,000 Vrms to block ground-loop currents and surges. [3]
Editorial calculation: a bus at 1,000 kbps with 100 ns rise-time drivers limits each stub to 7 ft, so a node 3 m (about 10 ft) from the trunk would exceed it; either route the trunk past the node or choose slower-edge drivers and a lower rate. [1]
03
Set Modbus RTU framing, speed and bus rules on RS-485
Modbus.org's MODBUS over Serial Line Specification and Implementation Guide V1.02 (December 2006) fixes the settings a serial device must support. In RTU mode each byte travels as 1 start bit, 8 data bits, 1 parity bit and 1 stop bit; even parity is the required default, odd or no parity may be offered, and no parity needs 2 stop bits. Frames are separated by at least 3.5 character times, and a gap over 1.5 character times inside a frame marks it incomplete; above 19,200 bps fixed timers of 750 µs and 1.75 ms are recommended. Rates of 9,600 bps and 19,200 bps are required, with 19,200 bps the default. Addresses run from 1 to 247, and 32 devices are always allowed on an RS-485 Modbus bus without a repeater. At up to 9,600 baud with AWG26 or heavier cable the trunk may be 1,000 m, branches no more than 20 m, each end terminated with 150 Ω (0.5 W) or 120 Ω (0.25 W) in series with 1 nF, and any line polarisation uses 450–650 Ω pull-up and pull-down resistors at one point, which cuts the device limit by 4. [4]
Editorial calculation: one RTU character is 11 bits, so at 19,200 bps it lasts about 0.57 ms and the 3.5-character gap about 2.0 ms; at 9,600 bps the gap is about 4.0 ms. Ask an interface or converter supplier to confirm it handles these gaps without inserting delays of its own, and whether it supports even parity, 2 stop bits for no-parity mode, and the termination and bias options above. [4][1]
Suppliers linked to this page list serial interface hardware; each card links to the page reviewed.
Listings include USB-to-RS232 and RS-485 converters, isolated converter cables and RS-232-to-TTL adapters alongside serial hubs and Ethernet serial servers.
05
Trace the interface components
This is a functional reference built from the sources cited; it is not one maker's design.
Fill the specification fields with values for your link
A serial interface quotation needs values for your link. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
Field
What it means and published values
Ask the supplier for
Data Rate (kbps)
RS-485: 10 Mbps at 40 ft, 100 kbps at 4,000 ft; RS-232: 20 kbit/s in the standard. [1][2]
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Compiled and source-reviewed by the CNFX editorial team.
Published product pages show what a company offers, not tested performance. Confirm the legal seller, factory, isolation test reports and scope for your order. How we source and review
· Page last updated 13 October 2026
Buyer questions
How far can RS-485 run?
TI's guide gives 4,000 ft at 100 kbps and 10 Mbps at 40 ft over 120 Ω twisted pair. [1]
How long can an RS-232 cable be?
The standard limits load capacitance to 2,500 pF, about 15–20 m of cable. [2]
The Modbus serial line specification requires 8 data bits with even parity and 1 stop bit by default, 9,600 and 19,200 bps support with 19,200 as default, and allows 32 devices on RS-485 without a repeater. [4]
Sources and what still needs a direct check
Sources checked 2026-10-12. TI's application notes summarise the RS-485 and TIA/EIA-232-F standards; Dtech describes its own product. Limits for your link must come from the transceiver data sheets and the supplier's specifications.
