Hi There
I do not think that you can use a Cisco 2520 as an access server. Also I do not think that there is an interface on the back of the 2520 which will allow you to connect a octal cable.
The asynchronous serial ports on a 2520 AFAIK are classed as low speed serial ports and would normally be used to connect to a CSU/DSU.
Normally the Cisco routers that are used with octal cables as access servers are 2509 (proivdes 1 interface for a single octal cable, which provides access to 8 pieces of equipmnet) or 2511 (provides 2 interfaces for two octal cables, and provides access to 16 pieces of equipment).
There are also NM-16A and NM-32A interface cards, which can be added to a Cisco 26xx/36xx router to make it into an access server. But IMHO these router models are too good to be used as access servers in a lab.
If you look at this Ebay posting
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=300246898823
you will see a photo of an NM-32A card. Notice that the interface to which the Octal cable connects resembles a SCSI interface, and is not the standard 68 pin Serial interface normally found Cisco equipment.
The RJ45 end of the octal cable is connected to the console port of the router you want to access through the access server and not to the Ethernet or serial port, so usually no adapters are required. However there are some pieces of Cisco equipment (The LightStream 1010 comes to mind) where the console port is indeed a DB25 interface, in which case an Rj45 to DB25 adapter will be required.
HTH,
Best Regards,
Michael