TRS-80 Model IV PALs

Lamar Owen lowen at
Thu Dec 5 19:49:36 CST 2019

On 12/5/19 12:39 PM, Eric Smith via cctalk wrote:
> Last year I read most of the PALs and HALs of a TRS-80 Model 16B main 
> board (same as Model 12 main board). I was surprised that none of the 
> PALs were protected, and even more surprised that some (but not all) 
> of the HALs could be read as PALs.
Eric, first, I want to thank you for the excellent M2 ROM disassembly 
you did; while I was helping debug the MAME trs80m2 emulator your 
listing was extremely helpful in tracing down the problem which 
single-stepping in the MAME debugger (it was a bug in the WD1791 
emulation of deleted data address marks).

Second, there is a published European patent on the Model 4 circuitry; 
see for the details.  The 
PAL equations and pinouts begin on PDF page 31.

The 12/16B/6000 mainboard's PAL equations are printed in the service 
manuals for those machines; the 6000 service manual is available on  But it is a bit surprising that the security fuse wasn't 
blown; Jerry Ballard and Frank Durda were both unsure (years ago when I 
asked them both) if the M4's PALs had the security fuse blown or not, 
and at that time I didn't have the equipment to try to read them.

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