Choose message-based communication mode if you want to use MSG instructions in user programming to communicate with one station at a time. If your application uses satellite transmission or public
switched telephone network transmission, consider choosing
message-based. Communication to a slave station can be initiated on an as-needed basis.
Also choose message-based mode when a redundant PLC-5 system is being used as a master station. Connect both PLC-5 processor serial ports to the master station modem through an RS-232 modem splitter and precondition all MSG instructions with the Primary Processor status bit.
With message-based mode, you do not have an active station file that you can use to monitor station status. Also, you cannot implement slave-to-slave messaging or slave programming over the telemetry network.
To configure the processor for a master station using message-based communication, place the processor in program mode and follow the steps below using RSLogix 5:
1.Double-click on the Channel Configuration file to bring up the Edit Channel Properties interface.
2.On the Channel 0 tab, choose System (Master) for your Communication Mode.
3.Configure the Serial Port, Options, and Polling parameters according to Table 2.3.
Use Table 2.3 to help you understand the communication parameters you need to specify on the Edit Channel Properties screen.
Use Worksheet 2.2 (page D-6) for an example configuration and to record your station’s configuration.
Table 2.3 Communication Parameters for a PLC-5 Master Station Using Message-Based Communication Mode
4.Configure Options parameters according to Table 2.3.
5.Configure the Polling parameters according to Table 2.3.
6.When all parameters are set, click OK.
RSLogix 5 Tab Parameter Selections
Channel 0 Diagnostic File Select an unused integer file to store channel status information. You must define a diagnostic file in order to be able to view channel 0 status. See Table 2.2 on page 2-8 for description of what’s in this file.
Remote Mode Change Check enable remote mode change if you want to switch the configuration of the channel during runtime. Leave the parameter set at the default (unchecked) if you are not using this feature.
Mode Attention Character
Select a character that will signal a remote mode change. Leave the parameter set at the default if you are not using remote mode change.
System Mode Character Select a character that will signal the channel to switch into system mode. Leave the parameter set at the default if you are not using remote mode change.
User Mode Character Select a character that will signal the channel to switch into user mode. Leave the parameter set at the default if you are not using remote mode change.
Serial Port Baud Rate Select a communication rate that all devices in your system support. Configure all devices in the system for the same communication rate.
Bits Per Character Match the numbers of bits per character to the devices with which you are communicating.
Serial Port Control Line This parameter defines the mode in which the master driver operates. Choose a method appropriate for your system’s configuration:
• If you are not using a modem, choose NO HANDSHAKING.
• If the master modem is full duplex and the slave modem is full-duplex, choose FULL-DUPLEX MODEM.
• If all the modems in the system are half-duplex, choose HALF-DUPLEX MODEM WITHOUT CONTINUOUS CARRIER.
Parity Parity provides additional message packet error detection. To implement even parity checking, choose Even. To implement no parity checking, choose None.
Error Detect With this selection, you choose how the processor checks the accuracy of each DF1 packet transmission.
BCC: This algorithm provides a medium level of data security. It cannot detect:
• transposition of bytes during transmission of a packet
• the insertion or deletion of data values of zero within a packet
CRC: This algorithm provides a higher level of data security.
Select an error detection method that all devices in your system support. When possible, choose CRC.
Options Station Address Define the octal address of the processor on the DF1 half-duplex link. Each station on a link must have a unique address. Choose an address between 0 and 3768.
Station address 3778 is the broadcast address, which you cannot select as a station’s individual address.
DF1 Retries Define the number of times a master station retries either a message before the master station declares the message undeliverable, or a poll packet to an active station before the master station declares the station to be inactive.
RTS Send Delay RTS send delay is the amount of time, in 20 millisecond increments, that elapses between the assertion of the RTS signal and the beginning of the message transmission. This time allows the modem to prepare to transmit the message. The Clear to Send (CTS) signal must be high for transmission to occur.
RTS Off Delay RTS off delay is the amount of time, in 20 millisecond increments, that elapses between the end of the message transmission and the de-assertion of the RTS signal. This time delay is a buffer to ensure that the modem has transmitted the message but should normally be left at zero.
ACK Timeout Define the amount of time, in 20 millisecond increments, that you want the processor to wait for an acknowledgment from a slave station to its transmitted message before retrying. This timeout value is also used for the poll response timeout.
Reply Message Wait Define the amount of time, in 20 millisecond increments, that the master station will wait after receiving an ACK (to a master-initiated message) before polling the slave station for a reply.
Choose a time that is, at minimum, equal to the longest time that a slave station needs to format a reply packet. This is typically the maximum scan time of the slave station. MSG Application
Timeout
The application timeout of the message is the number of 30 second increments within which the reply message must be received before the error bit is set on the message. The timer starts when the ACK is received.
Polling Polling Mode If you want to:
• accept unsolicited messages from slave stations, choose MESSAGE BASED (ALLOW SLAVE TO INITIATE MESSAGES).
Slave station-initiated messages are acknowledged and processed after all master station-initiated (solicited) messages.
Note: Slave stations can only send messages when they are polled. If the message-based master station never sends a slave station a message, the master station will never send the slave station a poll. Therefore, to regularly obtain a slave station-initiated message from a slave station, you should choose to use standard communications mode instead.
• ignore unsolicited messages from slave stations, choose MESSAGE BASED (DO NOT ALLOW SLAVES TO INITIATE MESSAGES).
Slave station-initiated messages are acknowledged and discarded. The master station acknowledges the slave station-initiated message so that the slave station removes the message from its transmit queue, which allows the next packet slated for transmission into the transmit queue.
Master Message Transmit
If you want the master station to:
• send all of the master station initiated MSG instructions to the remote stations before polling the next remote station in the poll list, choose Between Station Polls.
This method makes certain that master station initiated messages are sent in a timely and regular manner (after every remote station poll).
• send only master station initiated MSG instructions when the master’s station number appears in the polling sequence, choose In Poll Sequence.
With this method, sending master station initiated messages are dependent on where and how often the master station appears in the poll list. To achieve the same goal as Between Station Polls method, the master station’s address would need to appear after every remote station’s address.
Normal Poll Node File Enter an unused integer file that will store addresses of the remote stations you want in the normal poll list.
Active Station File Enter an unused binary file that will store the status of all the stations in your network configuration. The file stores one address per bit. 0=inactive, 1=active.
Priority Poll Node File Enter an unused integer file that will store the addresses of the remote stations you want in the priority poll list.
Normal Poll Group Size Enter the quantity of active stations located in the normal poll list that you want polled during a scan through the normal poll list before returning to the priority poll list.