RMS Technologies R325 DRIVER W/PDT - Manual (Page 20)

RMS Technologies Page 20 Version 1.20
R325I Single Axis Controller/Driver Manual 02/13/2006
11. RS485 Communication
1. The
Interface
The EIA specification RS485 defines an integrated circuit that is to be used to connect up to
32 nodes to a two-wire party line bus that does not exceed 4,000 ft. in length, and for use
with data rates up to 10M Baud.
The two-wire bus must be terminated at one-end for short wire runs and at both ends if the
runs exceed 20 ft. One of the two wires must be biased positive with respect to the other by
approximately 700 millivolts.
A single 5VDC supply can be used to power the interface IC, and this same supply can be
used to satisfy the bias and termination requirements. A 681 ohm 1% resistor is connected
between the +5VDC supply and the positive line. A second 681 ohm 1% resistor is connected
between ground and the negative line, and a 220 ohm 1% resistor is connected across the
two lines. The transceiver A terminal is connected to the negative line and the B terminals to
the positive line.
For wire runs over 20 ft, twisted pair cable with a characteristic impedance of approximately
100 to 200 ohms, and the far end of the run should be terminated by a 150 ohm resistor
across the line pair. For runs under 20ft almost any wire can be used.
2. The Protocol
One node on the bus is designated ‘Master’ and all other nodes on the bus ‘Slaves’. The
Master only initiates communication, and does so by sending a message that includes the
address of a specific Slave. All Slaves read the message, but only the addressed Slave
replies.
The outgoing message from the Master is ‘framed’ by always starting the message with the
‘#’ character (0x23) and ending with the linefeed character (0x0A). The reply from the Slave
is framed by always starting with the ‘*’ character (0x2A) and ending with the linefeed
character (0x0A).
The Slave address is the first character after the ‘#’ in the outgoing message, and the first
character after the ‘*’ in the reply. For ease of use RMS Technologies restricts the range of
address characters to the range of capital letters ‘A’ to ‘Z’, with ‘A’ being the default.
Again for ease of use RMS Technologies restricts the other characters in the message to
ASCII printable characters. This enables the default Windows terminal emulation program
HyperTerminal to be used for configuring and testing modules. However this restriction and
the restricted address range are not an official part of the protocol. Any of the 8 bit character
values other than the framing characters can be used for the address and as any other part
of the message.
3. Messages
Messages should be transmitted as a continuous character stream with less than a half
character time between characters.
Messages are classified as either ‘Commands’ or ‘Queries’. Commands instruct the
designated Slave to do something. Queries request the designated Slave to provide
information.