From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: designing COM port interface without UART
Date: Sat, 26 Oct 2002 01:58:20 GMT
NNTP-Posting-Date: Fri, 25 Oct 2002 18:58:20 PDT
John B wrote:
> 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?
> Voltage levels:
> 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.
> Many thanks,
This smacks of homework, however, some of your questions are answered
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org