ML1100 measue negative voltage on analog input

Please can anybody recommend a way of measuring a negative voltage (-40 to -60V dc) on the analogue input of an Allen Bradley Micrologix 1100 PLC.

The manual says that the common terminal is connected to ground so I cant just use a voltage divider and reverse the analogue input connections. I guess I will have to use an opto isolator of some sort. The negative voltage is from a battery.

Al

Yes, you will want a signal isolator with the input signal wired so that the device sees the battery voltage as a positive signal, and converts it to 0-10vdc or 4-20ma for your PLC

something like this should work:
http://www.actionio.com/products/actioniq/dc/q408_spec.html