Overview Of The Rs-232 Serial Interface - Isotech TTI-7-R Operator's Handbook Manual

Communications Interface

7.3 Overview of the RS-232 Serial Interface

This conforms to specification ANSI/EIA/TIA-232-E-1991 Interface
Between Data Terminal Equipment (DTE) and Data Circuit-
Terminating Equipment (DCE) employing serial binary data
interchange.
Signal levels; MARK (logical "1"); -3V to -15V
SPACE (logical "0"); +3V to +15V
Data is transferred using the TXD (transmit data) and RXD (receive
data) lines.
Hardware handshaking for each character transfer uses the RTS
(request to send) and CTS (clear to send) lines. RTS is an output
from the instrument that indicates its receiver status. When asserted
(low), it indicates that it is ready to receive another character. When
negated (high), the instrument receiver buffer is full and cannot
receive another character until the buffer is processed (if the PC
sends one, it may be lost). As soon as space becomes available in
the receiver buffer, RTS is re-asserted to allow the PC to send the
next character.
When the instrument detects receipt of a command terminator
character, RTS is negated whilst the command line buffer is read
and validated. This is to prevent the PC sending further characters
whilst a command is being validated. Following validation, RTS is re-
asserted to permit the next command to be transmitted by the PC.
CTS is an input to the instrument and controls the transmission of
characters. If the PC asserts CTS (low), then the instrument will
transmit the next character from the output buffer (if one is waiting).
If the PC negates CTS (high), then the instrument will not transmit
the character, but will wait until CTS is re-asserted. TXD will remain
in the mark (low) condition whilst CTS is negated. Note that
changing CTS during transmission of a character will not disrupt the
transmission of that character.
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