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

[Mark Rivers via Tech-talk <tech-talk at aps.anl.gov>]

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