CHAPTER 3: MAINTENANCE AND TROUBLESHOOTING
SNAP PAC S-Series User’s Guide
41
Communicating on a Serial Network
If a SNAP PAC S-series controller fails to connect using either an RS-232 serial interface for PPP host
communication or an RS-485 serial interface for I/O unit communication, check the following:
Indicators
—Depending on the port(s) being used on a SNAP-PAC-S1 controller, LED S0, S1, or S2 should be
red (for receive data) and green (for transmit data). On a SNAP-PAC-S2, there are separate LEDs for Transmit
and Receive for each port. Green indicates activity.
Power
—Make sure that both the controller and the device it is connected to have been turned on.
Physical Connections
—Check that the connectors on each end of the serial link are firmly attached to the
controller and the other connected device.
RS-232 Configuration
—RS-232 communication on a SNAP PAC S-series controller is configured using PAC
Manager software, and ports must be configured correctly for their use, or they won’t work. See “Configuring
Direct Communication to Serial Devices” in the
(form 1704).
NOTE: Port 0 is set by default to communicate using PPP. If you’re connecting to serial devices with Port 0, you must
change its configuration in PAC Manager.
RS-485 Configuration
—On a SNAP-PAC-S1, RS-485 serial network bias and termination are configured
using DIP switches on the top of the unit. Confirm that DIP switches are set to the correct positions for the
controller’s location in the RS-485 network. See
“RS-485 Connections” on page 18
for information. On a
SNAP-PAC-S2, all port configuration is performed using PAC Manager. For details, see the
(form 1704).
Timeout Errors when Communicating with a Serial Brain
•
To avoid timeout issues when communicating from a controller to a serial brain, you should configure
matching turnaround delays in both the controller and the brain. (Turnaround delays are typically
required at 9600 baud—and sometimes needed at higher baud rates when your RS-485/422 bus is using
repeaters.)
–
Configure the controller's turnaround time in PAC Manager > Tools > Inspect > Status Write button
> Turn-Around Delay for Port <#> (msec).
<#> represents the port number that the controller is using to connect to the brain.
–
Configure the same turnaround time for the brain in PAC Manager > Tools > Inspect > Status Write
button > Turn-Around Delay for Port <#> (msec).
The amount of turnaround time you'll need depends on the RS-485 cable connecting the devices
(turnaround time increases with the length of the cable). Start with a 1 millisecond delay, and then, if
you're still experiencing issues, increase the turnaround time until the connection is satisfactory.
•
If you get a timeout error when trying to establish communication between an S-series controller and a
serial brain (for example, a SNAP-PAC-SB2), the problem could be configuration-related.
–
If you get the error when trying to run a PAC Control strategy, make sure that the baud rate on the
brain's Baud Rate Switch is the same as the baud rate configured in your PAC Control strategy (File >
Strategy Options > Serial I/O Ports tab). For example, if the brain's baud rate is set at 115,200 Kbs,
then the serial I/O (RS-485) port in your strategy must also be configured for 115,200 Kbs.
–
If you get a timeout error when using PAC Manager to communicate with the brain via a controller:
a.
Check that the baud rate setting on the brain is the same as the baud rate configured in PAC
Manager (Tools > Inspect > Communications > PPP Configure > Serial Port Speed).
b.
If that doesn't solve the problem, then make sure the brain's baud rate setting is the same as the
baud rate in the controller's strategy (File > Strategy Options > Serial I/O Ports tab).
When you've configured the same baud rate for the brain, the strategy, and PAC Manager, you then
need to cycle power to the controller. Cycling power releases a lock previously put on the
configured communication port, and allows the new connection to succeed.
