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If you have a special application, we may already have a solution for you.
Give Ron a Call at 513-874-4796 or 513-638-0228 for additional information.
USBA4: USB to RS485 2 Wire Half Duplex Serial Converter


USBA4 with Box

USBA4 Label

"Let me spin a very large
RS485 network for you!"


Product Features:

  • This auto transmit unit features RS232 to RS422 4 wire full duplex conversion with an intermediate CMOS/TTL tap point.

  • This auto transmit unit features bi-directional serial conversion from USB to RS485 in 2 wire half-duplex communications mode.

  • Optimized for non-isolated office/indoor point to point or multi-drop systems.

  • Transparent to all asynchronous data, with any combination of data, parity, and stop bits, at data rates up to 230.4 Kbps. No Jumpers or settings required.

  • Fast auto turn-around, from transmit to receive mode, requires no software delays and eliminates timing problems and data collisions. A special jumper(J5) can further shorten the turn-around time to accommodate fast responding target devices. We pioneered and perfected this leading edge technology.

  • The RS485 driver, with built in electro-static protection, allows 32 standard loads at 4000 ft. at data rates up to 230.4 Kbps.

  • Jumpers permit the RS485 termination and bias resistors to be user configured. Jumper J4 will place the unit in Loop-Back mode for self test.

  • Edge mounted LEDs display power and data activity at the RS485 interface.

  • Broad environmental range. Temperature: 0C to +70C(32F to +158F). Humidity: 10 to 90% RH(non-condensing)

  • Four(4) easy mount through hole standoffs, which accept #6 hardware, are provided on the circuit board.

  • User connections are a USB type B receptacle, and a 3 position removable screw terminal strip for the RS485 port. There is no external power connector. Device power is obtained through the USB port.

  • The required "Virtual Com Port" USB driver, for the host computer, is available from www.ftdichip.com under "Drivers". This web site provides drivers for most host operating systems. Some OS's will automatically search for, find, and install the appropriate software/driver.

  • Accessories available are an enclosure, and a USB cable(6 ft USB type A plug to USB type B plug).

  • Conformal Coating, cable, and connectors, are factory options. Contact us for options help.

  • R E Smith Inc makes a wide variety of isolated converters that address the electrical noise problems that contribute to unreliable communications, and product failure. If what you see here is not exactly what you need, don't hesitate in calling us at 513-874-4796.

  • We also have a new series of Converters and Repeaters that extend the RS485 communications distance far beyond conventional limits. Contact us for information on these leading edge problem solvers.

  • WARNING: Do NOT connect this device to a telephone system under any circumstances. Damage could result to the circuit board and/or your telephone system. The RS422 drivers allow 32 single unit loads at 4000 ft. and data rates up to 115.2 Kbps on the port.

About R E Smith, Inc.:

R E Smith, Inc. engineers and manufactures a comprehensive line of isolated and non-isolated converters, repeaters, multiport repeater hubs, fiber optic based products, and I/O boards. All our products are designed, built, tested, and shipped from our facility in the Greater Cincinnati Ohio area. We ship small quantities of most products from stock the same day, and larger quantities in a few business days.

We listen to you. We are customer driven. Many of our products were developed and improved due to suggestions and feedback from our word-wide customer base. They are being used at the South Pole, at the bottom of the sea, at the edge of space, and everywhere in between. Typical uses are very large networks, monitor and control of elevators, machinery, lighting, security, fire and alarm, access, cameras, military ground support, and much more. We provide free support, no-argument warranty repairs, and recommendations for your application. Challenge us with your communications problem. We may already have the solution. If your system keeps failing, or is unreliable, we have isolated, doubly isolated, noise resistant, cost effective, and proven solutions for difficult environments.

Related Files:

pdf icon USBA4revB.pdf Converter schematic/physical Rev. B

Physical Properties and Features:

USB: USB type B receptacle
RS485 3-position removable screw terminal strip.
Power Provided by the host device through the USB port.
LEDs Power and RS485 data activity(TX: red, RX: green)
Bias 620 Ohm pull-up and pull-down (jumper selectable)
Termination 120 Ohm (jumper selectable)
Protection SN65LBC184N Driver with built in ESD tolerance


Distance Up to 4000 ft. (1220 meters)
Operation USB to 2 wire half-duplex RS485
Format Asynchronous data with any combination of bits, parity, and stop (auto mode)
Speed Up to 230.4 Kbps
Environmental: 0C to +70C ambient, 10-90% RH(non-condensing)
Supply Voltage: +5VDC Provided by the host device through the USB port
Power Under 1/2 Watt (typical) + external load
Size 2.50" X 2.00" X 0.9" (PC Board), with connectors: 2.50" X 2.15"
Mounting 4 Holes at 2.00" X 1.50" (built-in standoffs for #6 hardware)

Options and Ordering Information:

USBA4: $99.00 ea. (Circuit board only)
Enclosure: $8.00 ea. (small)
Cable: $4.75 ea (6ft USB type A Plug to USB type B Plug)


This is the complete order number for the USBA4 with all available options:

USBA4 with all options (USB cable and enclosure): Order Number: USBA4-B6AB @ $111.75 ea. (Qty. 1-9)


Need a different configuration or more information? Give Ron a Call at 513-874-4796 or 513-638-0228(Cell)

Webstore: USBA4

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