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  SY/MAX PLC Communication Primer


NOTE: This primer isn't finished yet.

The Square D SY/MAX PLC has remarkable communication features that can allow for quite complicated messaging. There are four message ladder elements used by the SY/MAX PLC:

  • TREAD - Read register data from a remote device.
  • TWRITE - Write register data to a remote device.
  • TALARM - Write a single word of data to a remote device.
  • TPRINT - Send ASCII messages to a remote device.

TREAD Message Structure


|  Trig         +--------ROUTE--STAT--LOCAL--REMOTE-COUNT-+
|--| |----------|TREAD1 001 101 0501  0050    1001    20  |
|               +-----------------------------------------+

The TREAD1 at the start of the message indicates the PLC port for the read. The possible values for this number depend upon the specific SY/MAX PLC.

  • Port 1 is always the PROG port and is usually on the front of the PLC. This RS-422 port defaults to 9600 baud, 8 data bits, EVEN parity, 1 stop bit, and SY/MAX mode on power-up but may be overridden by the ladder code. Placing the PLC in HALT and cycling the power will always set the port to the default values listed above.
  • Port 2 is the COMM port and is sometimes on the bottom of the CPU. This RS-422 port also defaults to 9600,EVEN,8,1, SY/MAX but will remain in a programmed state after a power cycle.
  • Port 3 is the Ethernet port on units with Ethernet (450, 650). The Ethernet port on these CPUs is 10Base2 coax. Model 700 PLCs may have port 3 as an RS-422 port on the COMMs module.
  • Port 4 is valid only on Model 700 with the COMMs module.

The ROUTE fields may include up to 8 drops. The first drop is the number of the network port to which the PLC is connected. This drop number will be in the range of 000 through 199 or the special drop number 201 is sometimes used. 201 is the "don't care" drop number which tells the network card to substitute its own drop number in the route. 201 is handy in redundant CPU configurations. The memaining drops direct the message to its final target.

The STAT field is an internal register that the PLC uses to keep track of the status of the outstanding message. Each communication message must have a unique STAT register. There are a number of bits used by the PLC in the STAT register but the ones of interest are as follows:

  • Bit 1 - Error Reply has been received by the PLC.
  • Bit 16 - Successful data reply has been received by the PLC.
  • Bit 22 - Input Ladder Logic Solved True.
  • Bit 23 - SY/MAX Timeout has expired.

The STAT bits are all cleared when the logic that turns on the TREAD is opened. Thus it is important to trigger any operation off of the STAT bits before the TREAD is opened. Bit 1 comes on if an error was returned by a network module, remote device, or the PLC itself. Bit 16 comes on when a good reply is received from the remote device. Bit 22 is normally used to latch the TREAD once it is triggered. Bit 23 is used as a failsafe in case an error reply never comes.

The LOCAL field is the internal starting register where the remote data is to be stored in the PLC. This value must be within the valid range of data registers for the specific PLC.

The REMOTE field is the starting register in the remote device. This value must be within the range of 1-8192.

The COUNT field is the number of registers to be read. The maximum value depends upon the limitations of the remote device but must be 128 or less.

TWRITE Message Structure


|  Trig         +--------ROUTE--STAT--LOCAL--REMOTE-COUNT-+
|--| |----------|TWRITE1 001 101 0501  0050    1001    20  |
|               +-----------------------------------------+

The TWRITE1 at the start of the message indicates the PLC port for the write. The possible values for this number depend upon the specific SY/MAX PLC.

  • Port 1 is always the PROG port and is usually on the front of the PLC. This RS-422 port defaults to 9600 baud, 8 data bits, EVEN parity, 1 stop bit, and SY/MAX mode on power-up but may be overridden by the ladder code. Placing the PLC in HALT and cycling the power will always set the port to the default values listed above.
  • Port 2 is the COMM port and is sometimes on the bottom of the CPU. This RS-422 port also defaults to 9600,EVEN,8,1, SY/MAX but will remain in a programmed state after a power cycle.
  • Port 3 is the Ethernet port on units with Ethernet (450, 650). The Ethernet port on these CPUs is 10Base2 coax. Model 700 PLCs may have port 3 as an RS-422 port on the COMMs module.
  • Port 4 is valid only on Model 700 with the COMMs module.

The ROUTE fields may include up to 8 drops. The first drop is the number of the network port to which the PLC is connected. This drop number will be in the range of 000 through 199 or the special drop number 201 is sometimes used. 201 is the "don't care" drop number which tells the network card to substitute its own drop number in the route. 201 is handy in redundant CPU configurations. The memaining drops direct the message to its final target.

The STAT field is an internal register that the PLC uses to keep track of the status of the outstanding message. Each communication message must have a unique STAT register. There are a number of bits used by the PLC in the STAT register but the ones of interest are as follows:

  • Bit 1 - Error Reply has been received by the PLC.
  • Bit 16 - Successful data reply has been received by the PLC.
  • Bit 22 - Input Ladder Logic Solved True.
  • Bit 23 - SY/MAX Timeout has expired.

The STAT bits are all cleared when the logic that turns on the TWRITE is opened. Thus it is important to trigger any operation off of the STAT bits before the TREAD is opened. Bit 1 comes on if an error was returned by a network module, remote device, or the PLC itself. Bit 16 comes on when a good reply is received from the remote device. Bit 22 is normally used to latch the TWRITE once it is triggered. Bit 23 is used as a failsafe in case an error reply never comes.

The LOCAL field is the internal starting register where the data for the remote device is stored in the PLC. This value must be within the valid range of data registers for the specific PLC.

The REMOTE field is the starting register in the remote device. This value must be within the range of 1-8192.

The COUNT field is the number of registers to be written. The maximum value depends upon the limitations of the remote device but must be 128 or less.

Triggers

The T at the beginning of each element indicates that it is a transitional operation. Transitional operations in a Square D PLC only happen on the transition from off to on. So if a TREAD is tied to the left rail in a ladder then it will only send the read once and never again. Therefore, some sort of trigger input is required to control the power flow to the communication element.