Re: Re: The problem of converting floating-point numbers to 4-byte format (in CDAB order) under StreamDevice

[高振华 via Tech-talk <tech-talk at aps.anl.gov>]

Hi Mark,

Does this Modbus driver use the CRC-16/Modbus check method?

How to set whether CRC uses big-endian or little-endian?

Does this driver only support CRC-16/Modbus check?

If other CRC check methods need to be used, how to set it? (For example: CRC-16/IBM) 

Is there an example of a Modbus RTU IOC with this Modbus driver? 

The parameters of CRC-16/Modbus required for the device I am communicating with are: 

POLY(HEX): 8005

INIT(HEX): FFFF

XOROUT(HEX): 0000

Thanks,

Gao zhenhua

-----原始邮件-----
发件人: "Mark Rivers" <rivers at cars.uchicago.edu>
发送时间: 2026-01-07 20:38:29 (星期三)
收件人: "tech-talk at aps.anl.gov" <tech-talk at aps.anl.gov>, 高振华 <gaozh at ihep.ac.cn>
主题: Re: The problem of converting floating-point numbers to 4-byte format (in CDAB order) under StreamDevice

Hi Gao,

For this type of Modbus communication you might want to use the EPICS Modbus driver rather than StreamDevice.  It will do those conversions for you.  It is also much simpler and more efficient if you want to read or write a number of bits or registers in a single I/O operation.

https://github.com/epics-modules/modbus

Mark

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From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of 高振华 via Tech-talk <tech-talk at aps.anl.gov>
Sent: Wednesday, January 7, 2026 3:53 AM
To: tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: The problem of converting floating-point numbers to 4-byte format (in CDAB order) under StreamDevice

 

Hi all,

I am conducting Modbus RTU communication with a device and need to convert a 32-bit floating-point number into 4 bytes of hexadecimal data and then send it to the device.

For example: 3.15 converted to 4 bytes is 40 49 99 9A (in hexadecimal, big-endian order, ABCD order).

Using StreamDevice, how should I convert to the CDAB order? (For example, the CDAB order corresponding to 3.15 is 99 9A 40 49).

Does StreamDevice have a corresponding conversion symbol?

Best regards，

Gao Zhenhua

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BSRF

Email:gaozh at ihep.ac.cn

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祝好！

高振华

电话：010-88235426

Email:gaozh at ihep.ac.cn