How different is SHA-512/224 when compared to SHA-512?
As stated by FIPS 186-4 (6.6) The only two differences between SHA-512 and SHA-512/224 are the eight 64-bit words used for initialization, given by FIPS 186-4 (5.3.6.1); and result truncation to the first 224 bits of the 512-bit result: $H_0^{(N)}$, $H_1^{(N)}$, $H_2^{(N)}$ each 64-bit, and the high-order 32 bits of $H_3^{(N)}$.

I suggest to start by checking the SHA-512 implementation pass test vectors; then implement the truncation, and test; then change the initialization value, and test.

[epilogue: that was the test vectors, see this]