UART vs 422 vs 485

Thread Starter

MikeJacobs

Joined Dec 7, 2019
198
I am a bit confused

As usual i suppose

Compare a UART and 422?

is a UART also 422?
Is a UART something that passes over 422?

I understand Rs232 very well but not following 422/485 and uart
Thanks
 

SamR

Joined Mar 19, 2019
3,091
UART (Univeral Asynchronous Receive Transmit) is the actual chip used by the transmit/receive device. The original PC serial interface used an 8250 UART chip and the RS-232 protocol. The UART is a microprocessor for the specific protocol in use.
 

Marley

Joined Apr 4, 2016
404
The UART generates a serial stream of data bits.
RS232, RS422, RS485 etc. describe the voltages to be used and the wiring arrangement between the transmitter and receiver.
The UART only deals with the serial data. It knows nothing about the actual voltages used to transmit the data.

So there are at least 3 parts to transmitting and receiving serial data:
  • The actual voltage levels and wiring arrangement: This is RS232, RS422, RS485, etc.
  • The serial data: Baud rate, bit sequence and parity, start and stop bits. "8-N-1", etc. This is handled by the UART.
  • The protocol: How is the data formatted and sequenced. Flow control. Half duplex, full duplex, etc. This is handled by software.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
198
See and i think i would of agreed with some on the fact that a UART is the device that generated the serial data right?
And the others are protocols that dictate wiring and voltages.

But here is why i am confused. Some of those protocols are differential some are not.

Does that mean a UART can be differential?

I thought a UART was just RX and TX.
Which means a UART also defines its wiring not just generates serial data
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
198
That's just two different methods of sending the same signal.
Differential or single-ended has nothing to do with the UART protocol.
See and that is what i dont understand. Just looking at some pages on the interest, a UART is defined by a single tX and single RX? isnt it? that is what i have seen.

So if that is the case, then a UART does define the physical or electrical interface
 

crutschow

Joined Mar 14, 2008
26,436
So if that is the case, then a UART does define the physical or electrical interface
No.
The single TX or single RX signal can be transmitted by a single (single-ended) line or a double (differential) line.
In either case the information in the signal is the same.

Don't confuse the signal with different methods of transmission of that signal.
 

MaxHeadRoom

Joined Jul 18, 2013
22,403
See and that is what i dont understand. Just looking at some pages on the interest, a UART is defined by a single tX and single RX? isnt it? that is what i have seen.

So if that is the case, then a UART does define the physical or electrical interface
UART does not define the method of transmission and data format etc.
For example a RS23 format originally was operated through a 25pin COM port, later reduced to 9 pin, the specs of the port included handshake conductors, later, this was often achieved by RX/TX only with software handshake, (XON/XOFF) if needed.
The UART just handled the incoming/outgoing data stream NOT the format.
Max.
 

SamR

Joined Mar 19, 2019
3,091
A lot of the confusion seems to come from the chinesium description of small data transmission modules as UARTs when the actual UART is a single chip on the particular module that is designed to handle a specific protocol. The modules are not UARTs but they do contain a UART processor chip.
 
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