Reply-To: "John B"
From: "John B"
Subject: designing COM port interface without UART
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Fri, 25 Oct 2002 23:54:06 GMT
NNTP-Posting-Date: Fri, 25 Oct 2002 16:54:06 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
I'm reviewing the EIA-232 specification (formerly known as RS-232C). It
describes hardware requirements, such as voltages, slew rates, etc. I have
also read a basic primer on serial communiation, which says that EIA-232
does not specify the type of pulse train used. That is, the asynchronous
protocol is undefined by EIA-232. The same primer suggests two protocols
for this purpose: "Two methods in which a data signal is self-timed are
nonreturn-to-zero and biphase Manchester coding." I suppose there are
I'm trying to build a DTE serial communication device that will communicate
with the ordinary serial port of a computer or dumb terminal. (I realize
I'll have to install a null-modem(i.e., criss-cross) in the cable, as
computers and dumb terminals are themselves DTE devices.) A UART is not to
be used. I would like to include RTS and CTS.
I understand the pertinent standard to be EIA-232, but that alone doesn't
tell me how to shape the asynchronous pulse train. Can anyone advise what
protocol applies? If this is variable, what is the default?
Suppose it's NRZ. Can you further advise what that means? May I speculate
that if a subsequent bit is a "1" then there is necessarily NO transition
through 0 volts?
What is biphase Manchester?
This is a bit confusing. Suppose we describe the voltage levels that are on
the cable, half-way between two boxes that are connected by EIA-232 serial
Please confirm or correct.
1) I have read that a "mark" corresponds to a negative voltage, such as -12
VDC. Conversely a "space" is +12 VDC.
2) A quiescent cable carries a "space" condition.
3) A start bit is necessarily a "mark."
4) The voltage levels of the information bits (e.g., 8 data bits) is
by the protocol; e.g., NRZ.
5) If there's a parity bit, then I'm clueless as to whether its voltage
follows from the protocol of the data bits, or if a "1" is always one
voltage or the other.
6) Ditto for stop bit(s).
7) And can someone clarify if a "1" is a "mark" or a "space"? I suppose
doesn't matter, until I try to explain my system to someone else.