HackMac Posted October 14, 2015 Share Posted October 14, 2015 Hello TIer, I can't realize the sense of the wiring of HOLD and HOLDA lines of the TMS9900 in the TI-99/4A. Which sense makes it to bring out the HOLDA line (or'ed with IAQ) to the I/O port, when HOLD (act. low) is alway high? The HOLD input signal is pulled up via a resistor to +5V and not connected other ways. Is it possible to apply a wire to that pin which pulls it down through an other circuit, so we can use the DMA feature? Quote Link to comment Share on other sites More sharing options...
Willsy Posted October 14, 2015 Share Posted October 14, 2015 I don't think so. IIRC, the very simple bus decoding on the TI motherboard means that some types of special cycles are mis-interpreted. This is why we are told in the Editor Assembler manual that we cannot use the CLKON and CLKOF instructions, for example. Quote Link to comment Share on other sites More sharing options...
+mizapf Posted October 14, 2015 Share Posted October 14, 2015 The HOLDA wiring seems to be meaningless, indeed. Is it possible to apply a wire to that pin which pulls it down through an other circuit, so we can use the DMA feature? You mean a wire to the HOLD* pin? The HOLD feature is indeed used in the TI-99/8. The Mapper circuit puts the 9995 on hold until it has downloaded (or uploaded) the map settings from (to) the SRAM. This is why we are told in the Editor Assembler manual that we cannot use the CLKON and CLKOF instructions, for example. We can let the CPU execute them, but they don't have a useful effect ... at least they make nothing kaputt. :-) Only RSET and IDLE have some visible effect; IDLE is indeed used sometimes. Quote Link to comment Share on other sites More sharing options...
HackMac Posted October 14, 2015 Author Share Posted October 14, 2015 You mean a wire to the HOLD* pin? Yes. HOLD* is connected to a pull up resistor. So there could be a connection (of an external circuit) to a collector of a npn transistor i.e. to pull down the signal and activate HOLD. Or is that pull up resistor only for that case, that inputs of TTL or CMOS IC must not be open? Quote Link to comment Share on other sites More sharing options...
Tursi Posted October 15, 2015 Share Posted October 15, 2015 The externally available line is actually routed to READY on the 9900, not to HOLD*. READY halts the CPU but does not release the bus, so it can't be used for DMA purposes. (It is used for a ton of things in the 99/4A though, including the multiplexer, GROMs, Speech...) I thought last time I'd looked at it that HOLD* was tied high with such a low value resistor that overriding it might not be reasonable... but since you'd have to change the board anyway it'd be nicer to change the resistor to something more friendly. Quote Link to comment Share on other sites More sharing options...
Tursi Posted October 15, 2015 Share Posted October 15, 2015 (Ah, I see you were asking why HOLDA was brought out in that case... yeah. All we can assume is maybe the engineer who did it hoped that HOLD* would be exposed too, so people could use it.) Quote Link to comment Share on other sites More sharing options...
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