For the SLC, you want to get the 3015-MCM module from Prosoft, which can act as Modbus master or slave. Dont even think about doing Modbus out the serial port in user mode, the Prosoft card is just so slick. In contrast, I think the 90/30's can talk Modbus out their programming port (without extensive user computation of checksums, etc.), and I think Horner electric has a Modbus card for the 90/30 as well. (Slave mode only, I think, so the Prosoft card would have to be the master.)
Another option if you don't absolutely have to have Modbus is Profibus DP (or if you absolutely must, DeviceNet). Cards are available for both SLC's and 90/30's for both these networks and would provide massively better throughput than Modbus (particularly Profibus).
We are using a ProSoft Modbus interface module to do Modbus communication between an AB PLC5 and an American Automatrix Sage controller. Check out http://www.prosoft-technology.com, they sell all kinds of protocol converters.
You can use a MARC Omnii-Comm to do this. It can talk directly to the SLC using DF-1 protocol and to the 90/30 using SNP protocol. Contact me off list if you need additional information.
You can use Renu Electronics' make Gateway (Model GWY-00)to communicate between GE series 90-30 to AB SLC 5/03 PLC. You can get more information on Gateway at http://www.renuelectronics.com
