# crc method for a serial driver for fiscal cash register

Bogdan Radu Bolchis bolchisb at gmail.com
Wed Mar 15 14:18:19 EDT 2017

```hi,
i'm developing a Point Of Sale app in python for my company, and we need to integrate the fiscal cash register driver in python. The original driver is only for windows and no longer supported, we want to switch to Linux. We managed to find the serial protocol, but the sent data is followed by a CRC.

Can someone help me building a CRC function according to the manufacturer data?

Here is what i have:

ALGORITHM FOR CRC CALCULATION

The two CRC bytes are calculated according to the formula x^15 + 1. In the calculation are included all data bytes plus the byte for block end. Every byte passes through the calculation register from teh MSB to LSB.
Three working bytes are used - S1, S0 and TR
S1 - Most significant byte from the CRC ( it is transmitted immediatelly after END)
S0 - Least significant byte from the CRC ( It is transmitted after S1)
TR - the current transmitted byte in the block.

The CRC is calculated as follows:
1. S1 and S0 are zeroed
2. TR is loaded with the current transmitted byte. The byte is transmitted.
3. Points 3.1 and 3.2 are executed 8 times:
3.1. S1, S0 and TR are shifted one bit to the left.
3.2. If the carry bit from S1 is 1, the MSB of S1 and LSB of S0 are inverted.
Points 2 and 3 are executed for all bytes, included in the calculation of the CRC - from the first byte after BEG up to and including byte END.
4. TR is loaded with 0 and point 3 is executed
5. TR is loaded with 0 and point 3 is executed
6. Byte S1 is transmitted
7. Byte S0 is transmitted

ALGORITHM FOR CRC CHECK ON RECEIVING

Three working bytes are used S1, S0 and RC

S1 - Most significant byte from the CRC ( it is received immediately after END)
S0 - Least significant byte from the CRC ( transmitted after S1)
RC - the current received byte in the block ( beginning from the first byte after BEG and ending 2 bytes after END).

The CRC is obtained as follows:
1. S1 and S0 are zeroed