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CHAPTER 2: OVERVIEW
2.1 INTRODUCTION
Use this compact, isolated USB to RS-232 or RS-422/485 converter to add an RS-232 or RS-422/485 port to your USB-only PC. Built
to rugged specifications with 2 or 15 kilovolts of isolation, it protects your PC from hazardous ground loops. It’s ideal for use in
instrumentation and laboratory environments.
One USB port is required for each installed device. The port can be native to the PC or it can be a USB port from an installed USB hub to
the PC. The USB port supports full speed (12 Mbps) USB 2.0.
NOTE: The devices work with USB 1.1 or 2.0 ports, but have a maximum USB data rate of 12 Mbps.
The RS-232 port is a DB9 male connector wired as a DTE with full hardware handshake support. It handles RS-232 data rates up to
460.8 kbps.
The RS-422/485 port is a 5-pin pluggable terminal block that also handles RS-422/485 data rates up to 460.8 kbps.
Model SP385A-R3 supports RS-232 communications, and is ideal for shorter distances (the maximum cable length is 50 feet [15.2
m]). In fields such as laboratory automation or surveying, RS-232 devices may continue to be used. Some types of programmable logic
controllers, variable-frequency drives, servo drives, and computerized numerical control equipment are programmable via RS-232.
Model SP390A-R3 is a USB to one port RS-422/485 converter. Supporting 2-wire RS-485 or a 4-wire RS-422/485 communications, this
device is great for any application that requires long range or multi-drop capabilities. The converter uses pluggable terminal blocks on
the RS-422/485 side and a pair of LEDs that indicate data being transmitted or received. Model SP390A-R3 includes special circuitry
that adds 2000 volts isolation protection against ground loops and voltage spikes. The product draws power from the USB port so no
power supply is required.
Simply plug the converter into an available USB port on your computer or USB hub and the device will show up as an additional COM
port in the Windows Device Manager. Configured as an additional COM port, the converter is now compatible with your Windows
applications.
RS-485 Control
No special software is required to control the RS-485 receiver or transmit line driver. The driver is automatically enabled during each
byte transmitted in RS-485 mode. The transmitter is always enabled in RS-422 mode. The receiver is tri-stated during each byte
transmitted in the echo-off mode. The receiver is always enabled in the echo-on mode. There are 4.7 kOhm pull-up/pull-down resistors
on the RDA and RDB lines. A termination resistor may be added to R16 if needed.
2.2 FEATURES
SP385A-R3 Provides 2-kV isolation
SP390A-R3 provides 15-kV surge protection
Protects against ESD (4 kV contact, 8 kV air)
Features a compact in-line case
Compatible with USB 2.0 full-speed (12 Mbps) devices
Supports RS-232 or RS-422/485 data rates up to 460.8 kbps
Powered by the USB bus, so no power supply is needed
