Literature
Madelung, "ASI: Das Aktuator-Sensor Interface für die Automation, 2nd Ed.". Hanser Verlag 1999. ISBN 3-446-21065-2 (English version) ISBN 3-446-21064-4 (German version)
Note: an update to this book for AS-Interface version 2.11 is available as PDF from the AS-Interface Association website!
As far as I know, ASi is not based on one of the "common" electrical standards known as RS232, RS422 or RS485. It is a special (but openly defined) standard.
This means that you will find it difficult to create your own ASi communication device, as you have to figure out how to make your own ASi hardware. Consider an ASi "kit" for smallvolume vendors. That could save you a lot of trouble at a reasonable price. Example of ASi OEM kit.