Datasheet for a NEC Chip in DEC Professional 350

Eduardo Cruz edcross at
Sun Nov 4 07:47:05 CST 2018

A constant pulsing reset is usually a watchdog at play. Hardware watchdogs are usually implemented in systems to reset everything should the system not meet one specific criteria: eg cpu touch one memory address before X amount of time, or pcb voltage lower than X volts, etc.

Watchdogs are also usually found as software routines executed by the cpu also looking for specific conditions. These rarely issue a reset hardware signal, just restar the program.

Sent from my iPhone

> On 4 Nov 2018, at 14:10, Rob Jarratt via cctalk <cctalk at> wrote:
>> -----Original Message-----
>> From: Tony Duell [mailto:ard.p850ug1 at]
>> Sent: 04 November 2018 12:42
>> To: rob at; Jarratt RMA <robert.jarratt at>; General
>> Discussion: On-Topic and Off-Topic Posts <cctalk at>
>> Subject: Re: Datasheet for a NEC Chip in DEC Professional 350
>> On Sun, Nov 4, 2018 at 12:37 PM Rob Jarratt via cctalk
>> <cctalk at> wrote:
>>> I have posted previously about a DEC Pro 350 I am trying to get
>>> working again. At the moment it seems to be constantly resetting the CPU.
>>> I have traced one possible path for the cause of this back to a NEC
>>> chip for which I cannot find a datasheet. It is a 40-pin DIP it is
>>> marked "NEC Japan
>>> 8239K6 D7201C". All I have been able to find is more modern USB host
>>> controllers.
>> Almost certainly a uPD7201 multi-protocol (asynchronous and synchronous)
>> serial chip. I have an NEC data book with it in if all else fails but a google
>> search for 'uPD7201 datasheet' (no quotes) found sites with the data sheet
>> to download as a .pdf file.
>> Quite why that should reset the machine is beyond me....
> I have been trying to find what is driving this path in the logic and this chip was the only one I for which I couldn't identify the pins, but it seems that from this datasheet ( they are all inputs and not outputs. So I need to look again for an output pin that is driving this signal.
> Thanks
> Rob 

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