Hello N.,
What kind of hardware block diagram would you like design by PlantUML?
For starting point, you can see on 'State diagram' or on 'Wire diagram' (feature under development):
1/ State diagram

[See on PlantUML server]
See documentation on:
2/ Wire diagram
@startwire
vspace 100
component Panel10
right:DSRE,SFVC,12VDV,12TRN
bottom:TRIGIN,TRIGOUT
vspace 50
component Panel2 [50x150]
--
component FPGA [100x400]
left:DSRE,SFVC,12VDV,12TRN
right:SDIA, CKIA, +5VDC
right vspace 10
right:SDIA_, CKIA_, +5VDC_
vspace 50
component Reactor [100x200]
--
component MCD_A
vspace 30
component MCD_B
vspace 30
component MCD_C
vspace 30
component MCD_D
@endwire

[See on PlantUML server]
See also documentation:
If that can help,
Regards,
Th.