ASL INTERCOM F200, Quick Start Manual

Page: 44 / 73

8. Communications Interface
8.1 Introduction
The F200 is fitted with an RS-232 serial communication interface as standard.
8.2 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.
Data flow control is by the RTS (request to send) and CTS (clear to send) lines to prevent data loss
due to an instrument receiver buffer overflow. 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.
F200-14-001 Page 8-1
ASL 2003