IEC 62056-21 Mode E Optical Probe Sign-On and Baud Rate Switch Failure

[carlos_mendez]

Field Troubleshooting Case

When connecting an optical probe to legacy IEC/DLMS hybrid meters, the sign-on handshake succeeds at 300 bps (7E1 parity), but switching to high-speed HDLC mode at 9600 bps or 19200 bps fails, terminating the session.

Optical Sign-on Trace (300 Baud, 7E1)

1. Client -> Meter: /?!<CR><LF> (2F 3F 21 0D 0A)
2. Meter -> Client: /LGZ4ZMD4104400.B34<CR><LF>
   • Identification character: Z (Baud rate identification)
   • Mode character: 2 (Mode E - transition to DLMS/HDLC)
3. Client -> Meter: ACK/Option Select: <ACK> 2 5 2 <CR><LF> (06 32 35 32 0D 0A)
   • 2: Normal protocol mode
   • 5: Baud rate 9600 bps
   • 2: Mode E (HDLC)

The Failure

After sending the ACK message, the gateway switched the serial port baud rate to 9600 bps immediately without inserting the required delay $T_a$, causing the first HDLC SNRM frame to be sent while the meter optical receiver was still re-configuring its UART clock.

[elena_rostova]

Also note that during Mode E sign-on, the parity changes: Initial handshake is 300 baud, 7 data bits, Even parity, 1 stop bit (7E1). Once the baud rate switch occurs for DLMS/HDLC, the UART MUST switch to 8 data bits, No parity, 1 stop bit (8N1).

[elena_rostova]

Also note that during Mode E sign-on, the parity changes: Initial handshake is 300 baud, 7 data bits, Even parity, 1 stop bit (7E1). Once the baud rate switch occurs for DLMS/HDLC, the UART MUST switch to 8 data bits, No parity, 1 stop bit (8N1).