Date: Fri, 25 Nov 2016 08:42:19 -0800 (PST) From: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> To: Hal Murray <hmurray@megapathdsl.net> Cc: Michael Sperber <sperber@deinprogramm.de>, freebsd-arm@freebsd.org Subject: Re: Can't get 11.0-RELEASE to boot on Banana PI M3 Message-ID: <201611251642.uAPGgJmX022074@pdx.rh.CN85.dnsmgr.net> In-Reply-To: <20161125105751.8F15A406061@ip-64-139-1-69.sjc.megapath.net>
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> > sperber@deinprogramm.de said: > > Ah, thanks ... but that's not standard RS232, right? (BPI homepages says > > "TTL".) If it isn't, what kind of hardware connects to that? > > The normal setup for RS232 is that the transmit and receive signals come out > of a big chip (SOC, or PCI UART, or USB UART, or ...) and then go through a > level converter which is typically a MAX-232 or one of many clones or > variants. The "TTL" is telling you that it doesn't have that level converter > chip. > > You can either add a level converter chip and then plug it into a real RS-232 > port, or find some setup that also doesn't have the level converter and > speaks TTL levels. Adafruit and probably many others sell a USB UART without > the level converter for applications like this. > https://www.adafruit.com/product/954 > > Sometimes, TTL means 3V CMOS levels and 5V from real TTL/CMOS will fry your > expensive chip. Best to check carefully. The above part says 3V. It also > has an extra power wire that you get to ignore. Be SURE to ignore that extra power wire! If your USB/Serial adapter also has a power wire DO NOT CONNECT IT. Many of these embeded boards provide a power pin with the serial interface that can be used to power something external, like a level shifter, and many of the USB/Serial adapters also bring out the USB 5V rail on a wire. DO NOT CONNECT THE TWO! That being said, there are many aftermarket USB/Serial cables avaliable, usually a 3.3V version of these well work everywhere as long as it has 5V tolerent inputs, which most of the newer ones do, check the specs from the vendor. 3.3V outputs well satisfy the input requirements of a 5V TTL/CMOS circuit and not cause it problems, the opposite is not always true. Watch your lead length and wire sizes if you need to do anything funny to get this connected, capacitive loading of any kind on this type of signal can cause character loss, especially at speeds above 9600 baud. -- Rod Grimes rgrimes@freebsd.org
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