How to link main form to subform

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I am doing a PO database and in the PO form, I created a subform Order_Info
to enter the order items. Both forms are linked via OrderNo in the
relationship table. In the subform, I put created a txtbox Sum_Total which
sums all order (LineTotal). I created another txtbox Total in the PO form
which shows the value of Sum_Total, but I get the error #Name. In the Total
txtbox, my Control Source is:

=[Order_Info Subform].Form!Sum_Total

I've checked the sample Order db from Access db. I put the Link Child Fields
and Link Master Fields as OrderNo. Pls. help!!

 

[SusanV]

Take a look here for how to reference subform controls from main forms:
http://cma.zdnet.com/book/masteringaccess/aba13fi.htm#I104