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Dave Platt (dplatt@ntg.com) mentioned that although the Toshiba EPROMs are 
compatible with the EPROMs used in the ZyXEL in _most_ ways, and would work 
properly in _most_ circuits.  Toshiba EPROMs are not 100% pin-compatible and 
won't work in the modem without a small hardware modification.

To modify the Toshiba EPROMs to work in ZyXEL modems, Peter Nitezki 
(Nitezki@NiDat.sub.org) wrote:
The behaviour of Toshiba EPROMs is well known.  They differ from Texas  
chips slightly, but the way ZyXEL uses them it's crucial.  Toshiba chips  
(and many other) require Vpp to be at Vcc during normal operation.  But  
ZyXEL leaves this pin open (no connector in the socket) as Texas chips
don't need this.  The solution is to solder a bridge from Vpp to Vcc pin,  
that has to be removed every time you want to program the chip.
(IMPORTANT!!! Leaving the bridge in could damage the programmer circuit!)