Siemens SIMATIC S7 System Manual page 211

Using character interrupt control to receive data
To allow complete flexibility in protocol support, you can also receive data using character
interrupt control. Each character received generates an interrupt. The received character is
placed in SMB2, and the parity status (if enabled) is placed in SM3.0 just prior to execution
of the interrupt routine attached to the receive character event. SMB2 is the Freeport receive
character buffer. Each character received while in Freeport mode is placed in this location for
easy access from the user program. SMB3 is used for Freeport mode and contains a parity
error bit that is turned on when a parity, framing, overrun, or break error is detected on a
received character. All other bits of the byte are reserved. Use the parity bit either to discard
the message or to generate a negative acknowledgement to the message.
When the character interrupt is used at high baud rates (38.4 Kbps to 115.2 Kbps), the time
between interrupts is very short. For example, the character interrupt for 38.4 Kbps is
260 microseconds, for 57.6 Kbps is 173 microseconds, and for 115.2 Kbps is
86 microseconds. Ensure that you keep the interrupt routines very short to avoid missing
characters, or else use the Receive instruction.
Note
SMB2 and SMB3 are shared between Port 0 and Port 1. When the reception of a character
on Port 0 results in the execution of the interrupt routine attached to that event (interrupt
event 8), SMB2 contains the character received on Port 0, and SMB3 contains the parity
status of that character. When the reception of a character on Port 1 results in the execution
of the interrupt routine attached to that event (interrupt event 25), SMB2 contains the
character received on Port 1 and SMB3 contains the parity status of that character.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Program instructions
7.3 Communication
211