I/O ports, serial COM port, timer, 8051

Thread Starter

PG1995

Joined Apr 15, 2011
816
Hi

What is this serial COM port and would it make any difference if it were simply referred as serial port? For instance, in features of the 8051 MCU it says there are four I/O ports and one serial COM port.

Could you please also tell me what a time does in a MCU?

Thank you for your help.

Kind regards
PG
 

Meixner

Joined Sep 26, 2011
117
On the 8051 2 pins of port 3 are the serial port. (TX and RX)

Could you please also tell me what a time does in a MCU?

Are you refering to the timers ? They are timer/counters they can be used as timers or to count input "events"
 

Thread Starter

PG1995

Joined Apr 15, 2011
816
Thanks, Meixner.

But if they call parallel port a parallel one, then why can't they call a serial port a serial one instead of calling it 'serial COM port'?

Regards
PG
 

crutschow

Joined Mar 14, 2008
24,741
A serial COM port refers to a serial port with RS-232 protocol. Serial port is a generic term that can have any protocol, such as USB (although it does commonly mean an RS-232 port).
 

MrChips

Joined Oct 2, 2009
20,911
RS-232 is primarily an electrical standard that defines the voltage levels and transmitter and receiver characteristics. It does not define how the data is encoded. Typically, RS-232 voltage levels are +/- 15V or +/- 12V. However, we have seen voltage levels ranging from +/- 3V to 15V. These voltages are implemented not by the UART but by external RS-232 receiver/driver chips such as MAX232.

Your 8051 does not have RS-232 but a serial transmission mechanism usually known by various acronyms UART (Unversal Asynchronous Receiver Transmitter), SCI (Serial Communications Interface), ACIA (Asynchronous Communications Interface Adapter) etc.

The standard protocol for a UART as implemented on the 8051 uses TTL voltage levels.
When the transmitter is idle, the output is in the "MARK" condition at a TTL HIGH LEVEL, 5V.
A ZERO is transmitted as a "SPACE", TTL LOW LEVEL, 0V and a ONE is a "MARK", TTL HIGH LEVEL = 5v.

This method of data encoding is called NRZ (Non-Return to Zero).

(The MARK and SPACE terminology is a carry over from the telegraph days. When the telegraph key was idle, the circuit was completed. Current flowed to the telegraph solenoid making a MARK at the receiving end.)
 
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Thread Starter

PG1995

Joined Apr 15, 2011
816
Thank you, MrChips.

So, your reply is that serial COM port refers to a serial port with RS-232 electrical standard, not protocol because there exists no such protocol? Please let me know.

Regards
PG
 

MrChips

Joined Oct 2, 2009
20,911
Not quite. This is just a case of semantics. Drop the word COM and everything is still the same.

The word COM is a abbreviation of COMMUNICATIONS. (Every port is a COM port and hence the use of the word COM is redundant). The word is commonly used to refer to the serial port on a PC, which is more a legacy issue since most PCs today do not carry the serial COM port.

(Aside: The SVGA, USB and Ethernet jacks on your computer are all serial COM ports but if we gave it those names it would confuse the heck out of everybody.)
 
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