What is this?

[mvoltin]

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

 

[tunedwolf]

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

 

[ronsimpson]

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.