A couple of more changes:

1- RS232 communication between the original UHU board and the PC is not isolated. When working with High voltage power supplies is is highly desirable to have electrical isolation between both of them, so I replaced the Max232 chip and associated components by a discrete opto-isolated Rs-232 interface that takes the PC side supply from the data lines. On the new design the -12 Vdc supply voltage, for the current limit comparator chip, is not necessary anymore (I also replaced the TL082 op-amp by the LM339 quad comparator).

2- There is no protection on the original UHU board against unconnected Encoder cable. When the encoder is left unconnected the motor will turn at high speed without stopping (once a step command is received from the PC). This behavior is a safety hazard. On the modified board if you left disconnected the encoder cable the board will be under Emergency Stop mode. There is a jumper on the encoder cable that feeds back information about encoder connection integrity. It is necessary to wire that jumper on the motor encoder side of the interface, as shown on the schematics.