Rockwell Automation 1771-SPI,D17716.5.122 SPI PROTOCOL INTERFACE - Manual (Page 20)

Important Information That You Need
to Set Up the SPI Network
Chapter 2
2-6
The device manufacturer should provide a wiring diagram that shows the
RS-485 data connections to the device, or at least a pin-out table of
connections to the 9-pin D-shell connector on the device. Use this
information with the following wiring considerations.
Wiring Considerations
We recommend that you observe the following, because omission of one or
more considerations may prevent the network from working correctly.
SPI Terminal Function Wiring Considerations
A and 0 RSĆ485 Data
Connections
• Daisy chain to each device via the SPI network and secondary connection cable.
• SPI module revision C (and later) has an internal termination resistor.
If replacing an earlier revision SPI module, discard the external termination resistor.
• You must terminate the other end of the SPI network with a 120 ohm resistor.
0 and 1 R1 Pullup Resistor (Optional) Used with pulldown resistor R2. (See notes 1 and 2.)
1 Signal Common Connect it to chassis ground at one end only.
2 Floating Terminal Can be used as a tie point for wiring
3, 6, 7, B Not Used Make no connections to these terminals.
4 and 5 R2 Pulldown Resistor (Optional) Used with pullup resistor R1. (See notes 1 and 2.)
SPI Cable Drain & Shield Daisy chain to each of the primary machine connectors, then to earth ground.
Note 1 Use R1 and R2 to trim the SPI module's output impedance when you need to boost the outputĆdriver signal strength.
Adding pullup and pulldown resistors increases the output current by loading the output. The nominal output impedance
without these resistors is 100K ohms. R1 must equal R2. As an example, we compute the effective impedance for
R1 = R2 = 40K ohms as follows:
R1 = R2 = R = 100K x 40K = 4000K ] 28.6K
100K + 40K 140K
Note 2 Removing or adding a single device on the SPI network may change the network impedance sufficiently to require
computing and substituting a different pair of pullup/pulldown resistors. We recommend that you determine and record
the input impedance of each device. Then, compute the required value of the pullup/pulldown resistor pair for any
desired combination of devices on the network.
Wiring the SPI Network