What is this?

I searched every possible combination of (mosfet transistor 630 121) with little success, including the datasheetcatalogue. Can anybody point me to the right direction? I think it is used in open collector type of output and is supposed to be a transistor but may be wrong. It has four pins and only says 630 and 121 in two lines. Here is the picture

I suspect they are HCPL-0630 TTL Opto Isolators. Most motion control drivers are usually fitted with high speed opto isolation to protect the parallel port of the computer driving them

The Wolf's idea is good. Isolators should be used here.
I noticed the corner pins (1,4,5,8) have wide traces and go to power and ground. The center pins (2,3,5,6) have thin traces and carry signals.
The HCPL-0630 does fit that pin configuration.