6805 pinout variations - Page 1
Forum section: electronics / general

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 25, 2021, 04:57:45 am
content: In the process of reverse engineering the Sega SP-400 plotter, I found something weird. It mounts a HD6805V1, which according tothis datasheetis an Hitachi clone of motorola's 6805.The problem is that the datasheet does not match at all what is on the board. The chip on the board has VCC on pin 40, gnd on pin 20, crystal on pins 5 and 6. The closest things I found areMC146805G2, which matches power, but not the crystal, andMC146805F2which matches the crystal as well, but has less pins (I guess less I/O).I would like to dump the content as doneherebut I have no idea on where the ports are located. The plotter board does not give particular clues, other than that pins 25 to 32 are probably the same port.Have you ever seen a version that matches the pinout I have?

author: Benta
author_rank: Super Contributor
timestamp: September 25, 2021, 07:18:30 pm
content: The Hitachi HD6805 is an ancient NMOS MCU. There is no guarantee that an MC6805 equivalent exists for the "V1" version.And what do you expect to do? These are mask programmed parts, you can't read them.

author: srb1954
author_rank: Super Contributor
timestamp: September 26, 2021, 01:01:28 am
content: Closest match would be a Motorola MC6805R3 which is also available in a EPROM programmable version, MC68705R3. It seems to match the Hitachi device pin-out closely except the analog functions on the Port D lines are different.As Benta points out the Hitachi part is a mask ROM device so there is not much chance that you could replicate the functions of that device unless unless you have the source code or binary code to program a new part.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 26, 2021, 04:15:40 am
content: Quote from: Benta on September 25, 2021, 07:18:30 pmI just hoped the NUM pin works as statedhereQuote from: srb1954 on September 26, 2021, 01:01:28 amIt matches the datasheet, but not the actual chip! That is why I am posting in the first place. Have a look the attached pictures.

author: Nusa
author_rank: Super Contributor
timestamp: September 26, 2021, 05:16:00 am
content: Try this version:http://datasheets.chipdb.org/Motorola/6805/146805E2.pdf

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 26, 2021, 05:30:47 am
content: Quote from: Nusa on September 26, 2021, 05:16:00 amThanks. However, this has the crystal in the wrong place, and no NUM pin, same as the MC146805G2 of the first post

author: srb1954
author_rank: Super Contributor
timestamp: September 26, 2021, 07:39:55 am
content: Quote from: fabiodl on September 26, 2021, 04:15:40 amAre you sure that you have traced out the circuitry correctly? NB - some unused inputs may be connected to supply or ground leading you to mistakenly identify the supply and ground pins on the chip.Have you buzzed the supply and ground lines to every pin on the chip with an ohm meter? Check both top and bottom pads on each pin - that's some pretty ugly desoldering work there so you also have to consider the possibility that some plated-through connections between top and bottom of the PCB might have been broken in the desoldering process.If, after thorough investigation, the chip pin-out really doesn't match the datasheet then it must have been a fully customised part and your only option would be to go back to Hitachi and ask them to re-open the factory that they probably closed 30 years ago and do a new manufacturing run of this customised chip.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 26, 2021, 02:08:46 pm
content: Quote from: srb1954 on September 26, 2021, 07:39:55 amI do not own the board, the pictures were kindly provided by another user in a forum. I am pretty sure, just have a look at the pic and follow the thick traces. And he did check with a multimeter, it all matches what I traced by observation.Quote from: srb1954 on September 26, 2021, 07:39:55 amI doubt they had a custom chip made for this subcontractor of sega (if you notice, there's no marking on the board). And we are not interested in cloning it, just in dumping it.

author: amyk
author_rank: Super Contributor
timestamp: September 26, 2021, 11:56:24 pm
content: It wasn't uncommon back then to mislabel/etc. parts to confuse cloners/reverse-engineers. Especially for a company like Sega. I wouldn't be surprised if it was something else entirely, although it's probably still Japanese.

author: woofy
author_rank: Frequent Contributor
timestamp: September 27, 2021, 08:00:52 am
content: I can't see them re-labelling with another manufacturers actual part number when they can just pick a random part number of their own for the chip. Too high a risk of litigation. Its probably real, I suspect the HD6805V1 is a custom plotter driver they have designed or licenced, the HD6805V1 does have its own Rom and Ram. I doubt the code can be retrieved easily, cheaply or from a single sample.The odd mix to use a custom Motorola architecture device along with an Intel MCS48 series device that uses an external EPROM also makes me think its a custom generic part, with the 8035 processor their choice for this particular board.Why are you after a ROM dump, what's the ultimate goal?

author: wraper
author_rank: Unknown
timestamp: September 27, 2021, 08:10:29 am
content: Quote from: Benta on September 25, 2021, 07:18:30 pmDunno about particular chip but mask ROM does not mean you cannot read it.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 27, 2021, 10:42:01 am
content: Quote from: woofy on September 27, 2021, 08:00:52 amNo, the 8035 only does a parallel to serial conversion. We already dumped and disassembled it.Quote from: woofy on September 27, 2021, 08:00:52 amMAME.

author: wraper
author_rank: Unknown
timestamp: September 27, 2021, 11:05:18 am
content: Atari 1020 plotter used the same chip. And it's pinout matches the datasheet. Are you sure you haven't screwed up your measurements/tracing the circuit? Especially considering that xtal pins are right.QuoteDid you check if VCC and VSS pins (according to the datasheet) are actually not connected to the power rails? As you can see from the 1020 schematic, some IO are directly connected to the power rails. And you probably just got confused by IO connected to them.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 28, 2021, 06:51:28 am
content: Thank you, yes I am pretty sure of the connections. Pin 3 is the only other pin connected to gnd, and I suspect that is NUM. I actually checked that atari plotter service manual. The mechanic is the same. The software,though, seems to be different, as they have a different character sets.

author: wraper
author_rank: Unknown
timestamp: September 28, 2021, 07:09:38 am
content: Quote from: fabiodl on September 28, 2021, 06:51:28 amYou said that GND is on pin 20 but it does not look like GND at all, just another data line. But pin 39 is connected to (datasheet GND) pin 1, and both connected to GND in Atari. Pin 40 in Atari is connected to Vcc too, through resistor though. So I doubt what you said. Actual GND trace is on the top side under IC, so I cannot say further. Also marking on ICs in Atari and Sega are almost identical, chance that ICs are any different is miniscule.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 29, 2021, 03:24:56 am
content: I do not have the board, so I am just trusting the continuity checks that I was told about, but I think you may be right.

author: fabiodl
author_rank: Frequent Contributor
timestamp: September 29, 2021, 09:42:47 am
content: yep, I got a pic of the traces under the IC. Anything matches the datasheet. Mystery solved, sorry for asking too early

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 12, 2022, 06:55:49 am
content: Quote from: Benta on September 25, 2021, 07:18:30 pmFor anyone stumbling upon the same chip. It CAN be read. It was done successfully using this methodhttps://seanriddle.com/mc6805p2.html