Why does sending SNRM frame 7E A0 0A... result in timeout with no response over optical probe?

[carlos_mendez]

️ Technical Redaction & Safe Harbor Warning:
Proprietary utility keys, serial numbers, and meter addresses have been neutralized. Frame traces use standard DLMS User Association public examples.

Hi everyone, testing a new optical probe with an electricity meter (IEC 62056-46 Clause 6.4) to read logical device name 0.0.42.0.0.255.
I connected the serial debugging tool and directly transmitted the following SNRM (Set Normal Response Mode) frame:

7E A0 0A 00 02 00 23 21 93 18 71 7E

The optical probe LED blinked once, but the meter returned absolutely nothing, causing a 5000ms timeout.
Is there an error in this HDLC frame structure or did I miss something fundamental in physical layer setup?

[elena_rostova]

Looking closely at this issue, this is a classic beginner trap where physical UART framing and server addressing are missing.

Here is the step-by-step diagnostic tree per IEC 62056-46 Table 2:

1. Parity and Baudrate mismatch:
   Per IEC 62056-21 and IEC 62056-46, standard optical probe ports default to 9600 bps, 8 Data Bits, Even Parity (8E1), 1 Stop Bit (or 300 7E1 initial optical wakeup). If your serial tool is configured with 8N1 (No Parity), the meter's hardware UART parity check will silently drop the incoming frame.

2. Server Addressing (Logical & Physical Device SAP):
   In your frame 00 02 00 23, the Server Address is 0x0023 = 35. If the meter's physical address is different, the MAC layer filter will discard the packet.

3. Check CRC FCS:
   The FCS bytes 18 71 correspond to CRC-CCITT (polynomial 0x8408). If baudrate is unstable, CRC calculation fails on meter MCU.

[carlos_mendez]

Actually, changing the parity bit solved the entire problem!
My serial port was set to None parity (8N1). Once I switched to Even parity (8E1), the meter immediately answered with the expected UA response:

7E A0 1F 21 00 02 00 23 73 D5 F2 81 80 14 05 02 00 80 06 02 00 80 07 04 00 00 00 01 08 04 00 00 00 01 54 A7 7E

Per IEC 62056-46 Clause 6.4, the UA frame contains the negotiated HDLC parameters. Thank you for the quick guidance! Marking this issue as resolved.