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Each such relay board requires one USB port. The more USB ports you have the more Relay boards you may connect. Each such usb relay board is shown as separate virtual com port (VCP) on your PC. If you are software developer - be sure "Synchronous Bit-Bang Mode" is selected!
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.