Experimental Physics and
Industrial Control System

Dirk Zimoch <[email protected]> · Fri, 3 Aug 2018 09:38:05 +0200

Hi David,

At the moment, StreamDevice cannot calculate the checksum of something

that is not in the current I/O line.

In your case I see three possibilities. But before I show them, one

correction seems to be necessary. According to the "MeCom Protocol

Specification" document

(https://www.meerstetter.ch/de/category/19-mecom-communication-protocol-specifications),

"The Frame Checksum is represented by four HEX digits." Your example

uses binary format (2 bytes). You need the 0 flag: "%0<xmodem>". Now

that all bytes are ASCII, I would put the "\r" in the Terminator. This

saves you from relying on timeout to detect the end of the input line.

Now the three possibilities:

a. Pre-calculate your checksum. As the output line is constant (does not

contain any formats), the checksum is constant as well.

    IN "#0115AA?IF257D";

b. Ignore the input checksum. The sequence "\?" stands for "any byte".

    IN "#0115AA%\?\?\?\?";

c. Post-process the string. First use the string to calculate the

checksum, then after the checksum modify the string with regsub:

    IN "#0115AA?IF%0<xmodem>%#/?IF//\r"

However, according to the "MeCom Protocol Specification", this is not

what you need. In the example on page 13 it says that the reply checksum

is 342D, not 257D. Thus I guess the checksum is indeed calculated from

the current input (which is unfortunately incomplete in the example, so

that I cannot confirm this). Also the reply in the example starts with

"!", not with "#".

I guess what you really need is this:

Terminator = "\r";
OUT "#0115AA?IF%0<xmodem>";
IN  "!0115AA%20c%0<xmodem>";

If you see something starting with "#", can it be that the device echos

the request before sending the reply? In that case you would need:

Terminator = "\r";
OUT "#0115AA?IF%0<xmodem>";
IN  "#0115AA?IF%0<xmodem>";
IN  "!0115AA%20c%0<xmodem>";

Also the document recommends to use a changing "sequence number". You

can use a calc record (e.g. with the name "SEQ") to count the sequence

(FLNK to it from any I/O record) and reference it using redirection:

Terminator = "\r";
OUT "#01%(SEQ)04X?IF%0<xmodem>";
IN  "!01%(SEQ)=04X%20c%0<xmodem>";

The "=" in "%(SEQ)=04X" means that the value of record "SEQ" should be

formatted and compared to the input rather than parsing the input and

storing the value in the record.

Here is the counting calc record:

record(calc, "SEQ")
{
    field(VAL,  "0x15AA")
    field(INPA, "SEQ")
    field(CALC, "(A+1)&0xffff")
}

Or with recent EPICS versions:
record(calc, "SEQ")
{
    field(VAL,  "0x15AA")
    field(CALC, "(VAL+1)&0xffff")
}

Best regards,
Dirk

On 03.08.2018 02:44, David Vine wrote:

Hi all,

i'm writing device support for the MeCom protocol for a TEC driver from

Meerstetter Engineering.

In order to parse the response correctly I need to calculate a checksum

thats not included in the response. This is because the response

includes the checksum of the command and not the response.

For example:

Command format:
# address sequence_number command CRC16 terminator
# 01 15AA ?IF 257D \r

Response is:
# address sequence_number CRC16 terminator
# 01 15AA 257D \r

So to parse the response I need to calculate a CRC that's not included

in the response. For example:

OUT "#0115AA?IF%<xmodem>\r"
IN "#0115AA%<xmodem>\r" but the CRC is for the OUT not the IN.

Any ideas would be much appreciated.

Thanks,
David

--
Best regards,
David Vine
(925) 326 7503
[email protected] <mailto:[email protected]>

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System