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See Rockwell Technote Answer ID 33871 Issue: Customer was seeing the "Lack of Memory" status bit being set in Channel 1 Channel Status in a MicroLogix 1500 LRP processor. CH1 was configured for DF1 Half-Duplex Slave. Tried different baud rates, as well as the Comms and Message Servicing Selection bits. Nothing seemed to clear this bit. Details on "Lack of Memory" status: The MicroLogix 1500 slave can buffer up replies for up to 12 master commands. Once it has 12 replies buffered up, the slave will increment the "Lack of Memory" counter with each additional master command received. The receive buffer gets cleared one at a time with each successful master poll. Therefore, this counter is an indication that communications is stronger from the master to the slave than it is from the slave to the master. If the slave can receive the master's commands, but the master can't receive the slave's acknowledgements or replies, then the replies can eventually fill up in the slave's buffer. Note that this is more likely to happen if the master retries is set to zero and the slave's retries is set to one or higher. As long as the master retries is set to one or higher, any time the master transmits a command that doesn't get acknowledged, it will send a poll packet before retrying the packet. That way, even if the slave's buffers are full, when communications is working again and the master sends a new command and doesn't receive an acknowledge, the master will send a poll packet, which will allow the slave to transmit the first reply in its buffer. After acknowledging that reply, the master will retry the command, which the slave can now receive and acknowledge, and the master will poll the slave twelve more times until it receives the reply to that last command that it is expecting. Solution: To minimize the slave buffer from filling up, set the master's retries to greater than zero and the slave's retries to zero (with the slave's duplicate detect left checked).