6. COMMUNICATIONS ELECTRICAL INTERFACE
6-1
6. COMMUNICATIONS
6.1 ELECTRICAL INTERFACE
6.1.1
ELECTRICAL INTERFACE
The hardware or electrical interface is one of the following: one of two 2-wire RS485 ports from the rear terminal connector or the RS232
from the front panel connector. In a 2-wire RS485 link, data flow is bi-directional. Data flow is half duplex for both the RS485 and the
RS232 ports. That is, data is never transmitted and received at the same time. RS485 lines should be connected in a daisy chain con-
figuration (avoid star connections) with a terminating network installed at each end of the link, i.e. at the master end and at the slave
farthest from the master. The terminating network should consist of a 120 Ohm resistor in series with a 1 nF ceramic capacitor when
used with Belden 9841 RS485 wire. The value of the terminating resistors should be equal to the characteristic impedance of the line.
This is approximately 120 Ohms for standard #22 AWG twisted pair wire. Shielded wire should always be used to minimize noise. Polar-
ity is important in RS485 communications. Each '+' terminal of every SR469 must be connected together for the system to operate. See
chapter 2 INSTALLATION for details on correct serial port wiring.
6.2 PROTOCOL
6.2.1
MODBUS RTU PROTOCOL
The SR469 implements a subset of the AEG Modicon Modbus RTU serial communication standard. Many popular programmable con-
trollers support this protocol directly with a suitable interface card allowing direct connection of relays. Although the Modbus protocol is
hardware independent, the SR469 interfaces include two 2-wire RS485 ports and one RS232 port. Modbus is a single master, multiple
slave protocol suitable for a multi-drop configuration as provided by RS485 hardware. In this configuration up to 32 slaves can be
daisy-chained together on a single communication channel.
The SR469 is always a slave. It cannot be programmed as a master. Computers or PLCs are commonly programmed as masters. The
Modbus protocol exists in two versions: Remote Terminal Unit (RTU, binary) and ASCII. Only the RTU version is supported by the
SR469. Monitoring, programming and control functions are possible using read and write register commands.
6.2.2
DATA FRAME FORMAT
AND DATA RATE
One data frame of an asynchronous transmission to or from an SR469 is default to 1 start bit, 8 data bits, and 1 stop bit. This produces
a 10 bit data frame. This is important for transmission through modems at high bit rates (11 bit data frames are not supported by Hayes
modems at bit rates of greater than 300 bps). The parity bit is optional as odd or even. If it is programmed as odd or even, the data
frame consists of 1 start bit, 8 data bits, 1 parity bit, and 1 stop bit.
Modbus protocol can be implemented at any standard communication speed. The SR469 RS485 ports support operation at 1200, 2400,
4800, 9600, and 19200 baud. The front panel RS232 baud rate is fixed at 9600 baud.
To Next Page
Loading...
