West Control Solutions N8100 - Manual (Page 27)

3 RS485 SERIAL COMMUNICATIONS
The Controller may be equipped with a two-wire RS485-compatible serial
communications facility, by which means communication may occur between
the Controller and a master device (e.g. a computer or terminal).
3.1 COMMUNICATIONS ENABLE/DISABLE
When Communications are enabled (in Set Up Mode - see Subsection 2.2.36), the
Controller parameters may be adjusted by the master device via the serial
communications link. If communications are disabled, the Controller will not adjust
or change any parameters in response to commands received from the master
device and will send a negative acknowledgement in response to such
commands. Whether communications are enabled or disabled, the Controller will
return the requested information in response to a Type 2 Interrogation message
(see Subsection 3.2.5) from the master device.
3.2 PHYSICAL REQUIREMENTS
3.2.1 Character Transmission
Data format is fixed to be even parity, seven data bits and one stop bit. The Baud
rate may be selected to be 1200, 2400, 4800 (default) or 9600 Baud.
3.2.2 Line Turn-Round
The communications link is operated as a multi-drop half duplex system. When a
device is transmitting, it drives the transmission lines to the appropriate levels;
when it is not transmitting, its outputs are set to a high impedance in order that
another device can transmit. It is important that a transmitter releases the
transmission lines before another device starts transmission. This imposes the
following restraints on the master device:
(a) The transmitter must release the transmission lines within 6ms of
the end of the last character of a message being transmitted. Note
that delays due to buffers such as those used in universal
asynchronous receivers/trans-mitters (UARTs) within the master device
must be taken into account.
(b) The transmitter must not start transmission until 6ms has elapsed
since the reception of the last character of a message.
All Controllers in this range having an RS485 communications facility adhere to this
standard; thus, provided that the master device conforms similarly to the
standard, there should be no line contention problems.
O054-3 3-1
59125