Building an ASTM ACK with a Mod-256 Checksum

ASTM frames carry a mod-256 checksum encoded as two uppercase hex characters between the ETX (end-of-block) marker and the trailing CR/LF. QIE's Socket (ASTM) Receiver leaves that calculation and the response framing to the channel, so you produce the response bytes in a mapping or destination script and hand them back through qie.postMessageResponse.

Source-side configuration

On the Socket (ASTM) Receiver, set Response to From Mapping or Destination node. With this option, QIE sends back only what your script explicitly returns; otherwise it would auto-acknowledge with its defaults.

Checksum helper

A standard mod-256 checksum implementation in JavaScript:

function calcASTMChecksum(input) {
    var sum = 0;
    for (var i = 0; i < input.length; i++) {
        sum += input.charCodeAt(i);
        if (sum & 0x80) {
            sum = (0xff - sum + 1) * -1;
        }
    }
    sum = sum % 256;
    if (sum < 0) sum *= -1;

    var hex = sum.toString(16).toUpperCase();
    return hex.length === 1 ? '0' + hex : hex;
}

The function returns a two-character uppercase hex string suitable for splicing into an ASTM frame between the ETX marker and the CR/LF terminator.

Sending a response

Build the response bytes in a mapping or destination script and call qie.postMessageResponse(bytes) to send them back over the open socket. The exact frame layout (STX, frame number, content, ETX/ETB, checksum, CR LF) depends on the analyzer's ASTM implementation. Consult the device's specification, then use the helper above to compute the checksum over the bytes between STX and ETX.

Caveats

• Mapping or destination response only. When the source is set to respond from a mapping or destination node, QIE does not send any reply unless your script calls qie.postMessageResponse. If the script throws before that call, the analyzer may time out.
• Mod-256, not CRC. Some lab integrations call their checksum "CRC" colloquially, the ASTM standard uses a plain mod-256 sum, not a CRC polynomial. Use this helper, not a crc16-style function, unless the analyzer documents a non-standard variant.