How to calculate checksum (three ASCII values) when using streamdevice

[LiangChih Chiang via Tech-talk <tech-talk at aps.anl.gov>]

Hi, EPICS mates,

I'm using streamdevice to communicate with the following device:

Vacuum gauge controller: Pfeiffer OmniControl 200 

gauge model: Pfieffer PKR361

I don't know how to handle the checksums of commands/responses.

The command/response format is of all ASCII values.

The checksum is of three ASCII values.

The calculation method is: sum of all ASCII values, then modulo 256

For example: to get the vacuum pressure value:

Command: 1220074002=?110

122: address of the component gauge.

00: means this is a query command

740: means parameter pressure

02: means the length of the following data is 2

=?: means to query something

110: checksum

Response: 1221074006539212045

122: same as in command

10: means this is a response of query command

740: same as in command

06: means the length of the following data is 6

539212: -> 5.392x10^(20-12) hPa = 5.392x10^-8 hPa

045: checksum