Arduino Forum

Using Arduino => General Electronics => Topic started by: Marciokoko on Apr 01, 2016, 07:32 pm

Title: USB - Serial - TTL - RS232
Post by: Marciokoko on Apr 01, 2016, 07:32 pm
Id like to understand what my usb-ttl converter actually does. 

RS232 is a serial communications standard which runs from +3 to +15V and -3 to -15V.

Whereas the ttl standard runs from +0.8V to 2 & 5V?

Im sure there are other differences but I guess thats the main one?  At least the main difference which requires the use of the usb/ttl converter?
Title: Re: USB - Serial - TTL - RS232
Post by: AWOL on Apr 01, 2016, 07:39 pm
No, there's the whole USB protocol and negotiation stuff, as well as the low voltage differential signalling to high voltage level conversion.
Title: Re: USB - Serial - TTL - RS232
Post by: Grumpy_Mike on Apr 01, 2016, 07:40 pm
RS232 signals TX and RX are upside down compared to TTL. That is normally TTL is high and RS232 is -12V
Title: Re: USB - Serial - TTL - RS232
Post by: CrossRoads on Apr 01, 2016, 08:21 pm
RS232 is full duplex normally, both sides can transmit at the same time, with large signal swing as noted above. Data rate is very slow, 1 Mbit would be fast for RS232.
USB 2 (USB 1.1 not used much) is simplex, one side transmits over the D+/D- wires (like 2-wire RS485 does), the other side can then transmit back over the same 2 wires. USB can also be very high speed, 12 Mbits to 400 Mbits, with small signal swing, 0.4V.
Title: Re: USB - Serial - TTL - RS232
Post by: DrAzzy on Apr 01, 2016, 09:39 pm
Also - RS232 voltage levels are uncommon (not rare) on Arduino projects. Almost everything is TTL. Usually people use a MAX232 or similar to interface between TTL and RS232 voltage levels.

Inaccurate marking (mainly describing TTL serial as RS232) is very common.

TTL and RS232 are otherwise the same protocol.

USB is totally different protocol, uses differential signaling, and has a protocol for negotiating with the host and telling it that it's a serial adapter (or a mouse, or a flash drive, or whathaveyou), and it's much, much faster.