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<== Date ==> <== Thread ==>
Subject: StreamDevice response parsing and redirection
From: Josh Fiddler via Tech-talk <tech-talk at aps.anl.gov>
To: tech-talk at aps.anl.gov
Date: Wed, 10 Nov 2021 16:20:10 -0500
Hi EPICS mavens!

I'm having a lot of fun building IOCs left and right. 

Here are some important details:

I've got a HV power supply to charge up a system that comms over RS485. 
We send 13 bytes worth of data to the unit, including 2 bytes for the terminator.
byte are
1 for address, 
2 for command
2 used as header in certain commands: to hold status bytes in some, and is null in others
2 when we want to set a voltage
2 when we want to set the ramping current
2 for checksum
2 for terminator

To pass a voltage, it must first be in tens of kV, sensitive to 0.1 kV. This value is multiplied by 100 then can be converted to hex. Current is similar, but is not scaled (x1) 

The checksum is the sum of the bits in the binary representation of the first 9 bytes. When I send the request, I must include this value. It is to ensure the message is composed of characters, according to the docs (which are very, very minimal). 

So, an example command to set the device to 15kV, with 600mA current is:

A1 61 00 00 00 05 DC 02 58 00 11 00 0D
where 05 DC represents 15kV (hex(15 * 100))
and    02 58 represents 600mA (hex(600))
and    00 11 represents checksum on A1 61 00 00 00 05 DC 02 58
The checksum is computed by summing the individual bits in the binary of each value: 
0b10100001', '0b1100001', '0b0', '0b0', '0b0', '0b101', '0b11011100', '0b10', '0b1011000' 
means 3+3+0+0+0+2+5+1+3=17 or 0x11 in hex. 

When we receive responses from the PS, it has the same structure as the request. The second 'Command' bytes is modified to 0x0A. The rest of the bytes are unique to the request. 

We can request the 'parameters' that are currently set by calling

A1 63 00 00 00 00 00 00 00 00 07 00 0D where the checksum is 0x0007
since A1->3, 63->4 and 7 in hex is 0x07. 

The response would contain the same first 2 bytes, with the 3rd now 0A to show is a response. In the parameter response, the 4th and 5th bytes represent the system state when each is converted to a binary representation, and each bit indicates some status. 

The ask:

How can I extract this information and use it?  Should I use record redirection to put the values and formatters? 

Could someone point me in the right direction? 

Thanks for reading. 

I also have another question here that I could use some help with 
Search: "StreamDevice checksum Methods"
Replies:
Re: StreamDevice response parsing and redirection Zimoch Dirk (PSI) via Tech-talk
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