From c917b77952aaaaee899576de20fedb73373b1f36 Mon Sep 17 00:00:00 2001 From: Zack Buhman Date: Tue, 19 Nov 2024 21:47:47 -0600 Subject: [PATCH] commentary --- commentary.txt | 154 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 154 insertions(+) create mode 100644 commentary.txt diff --git a/commentary.txt b/commentary.txt new file mode 100644 index 0000000..e5ac814 --- /dev/null +++ b/commentary.txt @@ -0,0 +1,154 @@ +The most significant challenge in this project is easily the Dreamcast serial +connector. + +As a comparison, Dreamcast "AV connectors" can be purchased at a unit price of +roughly $1.70 USD on Ebay or $1.00 USD on Aliexpress. These are quality +connectors that appear to use a copper alloy for the conductive pins. The +dimensions and fit appear to be reasonably perfect, and the mounting style is +suitable for direct soldering to a 2.0mm thick PCB. + +In contrast, Dreamcast "Serial connectors" only exist either as Sega-original +parts (and therefore not available in consistent bulk quantities), or in the +"Dreamcast SD adapter". + +I asked many sellers whether they were able to provide the raw connector (not +yet soldered to a printed circuit board), and in all cases the vendors informed +me this was not available. The best-advertised price for a complete "Dreamcast +SD adapter" appears to be about $7.00 USD. + +The Dreamcast serial connector from the "Dreamcast SD" product is also vastly +inferior to the Dreamcast "AV connectors", despite being a very similar +connector differing only in width and pin count. + +At least the following issues exist: + +- the outer dimensions of the Dreamcast SD serial connector are too small + +- the retaining holes that are intended to "latch" with the female connector's + detents are slightly misaligned + +- the injection molding is significantly lower quality compared to the Dreamcast + AV connector + +- the conductive pins appear to be a plated aluminum alloy, a downgrade from the + AV connector's copper alloy + +Despite this, $7.00 USD is still a decent price. I then investigated the +viability of salvaging the Dreamcast serial connectors. However, I was +discouraged when I found desoldering to be too difficult. The issue is a +combination of: + +- the pads on the Dreamcast SD PCB are too narrow (both as a general statement, + and as it applies to desoldering specifically), which increases the + difficultly of applying copper braid + +- a high-temperature solder was used--this means excessive heat is required, + which risks deforming the injection-molded plastic + +I found the best approach to salvaging "Dreamcast SD connectors" is to instead +cut the PCB, first separating the Dreamcast serial connector from the bulk of +the PCB, then cut the PCB between each conductive pin. The pins can then be + removed individually, which makes cleaning the pin much easier. + +However, all of this considered, I felt that surely it would be easier to just +make a completely new connector, rather than undertake this scavenging +process. This would have an additional advantage that already-cleaned connectors +could be installed at a PCB-assembly factory, no doubt more cleanly, quickly, +and cheaply than I can do myself. + +My original idea was to attempt to directly copy the design of the Dreamcast SD +adapter's Dreamcast serial connector verbatim. + +In my discussions with rapid prototyping factories, the most immediate issue is +the outer metal shield. This metal shield must fit between a small gap (roughly +≤0.5mm) inside the female connector. At that size, it effectively means the only +practical material that could be both the appropriate thickness and +simultaneously durable would be sheet metal. The Dreamcast SD's serial connector +uses 0.3mm aluminum sheet metal. + +After re-creating a technical diagram for the reverse-engineered dimensions of +the serial connector's shield, I was correctly informed by rapid prototyping +factories that this type of sheet metal bending/tolerance requires a mold, and +the minimum order quantity would be somewhere around 10,000 units. + +I don't think I can sell 10,000 units, so the idea seems too risky. + +I then redesigned my 3D model to be appropriate for SLM 3D-printing. Compared to +sheet metal, printed metal is significantly more brittle. To counter this, the +shield needs to be thickened and reinforced significantly to prevent cracking +and excessive fragility. + +I decided given the overall maximum outer dimensions of the connector, the +placement and size of the conductive pins, and the consequences this would have +on the durability of the interior plastic, that the maximum shield thickness +would would be around 0.7mm. + +This thickening has a further compromise, in that 0.7mm will not fit in the +≤0.5mm gap inside the Dreamcast serial connector. Taking inspiration from the +RetroOnyx SLS 3d-printed connector, I decided it would be acceptable to remove +that area of the shield entirely. + +After ordering SLM 3d-printed aluminum shields for a fit test, I found the +results simultaneously impressive and disappointing. + +- the outer dimensions, particularly after light sanding, are far superior to + the Dreamcast SD connector tolerances + +- my slightly-corrected detent-receptacle positioning appears to be perfect; the + connector mates with a Dreamcast with a satisfying "snap" feeling. + +- I don't know exactly what happened, but during assembly testing at the rapid + prototyping factory, the shields were about 0.5mm too long. The factory then + corrected this by grinding the parts by hand. This grinding was not done + carefully, and many parts are visibly no longer square as a result. + +Most importantly, SLM-printed aluminum (and aluminum in general) very strongly +resists naive attempts to plate it with tin. In my discussions with factories, I +was surprised to learn that they apparently have never tried to make custom +connectors like this, and don't formally support any surfacing options that +would be explicitly usable for soldering. + +This causes two problems: + +- the solder joint between the PCB and the connector shield is the primary means + that the connector is mechanically joined with the PCB + +- not being able to provide a solid electrically-conductive path from the ground + plane of the PCB and the shield causes the shield to lose some of its "shield" + properties. + +After thinking about this for many days, I decided that it would be "ok" to join +the connector to the PCB with a precise application of (non-conductive) glue, if +I were not able to find a factory able to provide tin-plated 3d-printed aluminum +(or an equivalent surfacing option that makes the part usable for soldering). + +The next issue is in price: + +- the factories are able to provide fairly nice laser cut conductive pins, but + these are still very expensive: about $1-2 per pin, depending on + material/surfacing. I could "save" up to $4 per connector by simply not + installing the unused 5V and 3.3V pins, but this feels "too cheap" for me to + be comfortable with. + +- overall, the material cost for a rapid-prototyped Dreamcast serial connector + appears to be around $21 to $31 USD. + +I personally think these are quite nice, but I doubt most people think the +"satisfyingly tactile snap" is worth +$31 USD to the cost of a Dreamcast +USB-UART adapter. + +The next option is to look at higher-volume production. This would mean +commissioning: + +- molds for sheet metal bending for the shield + +- molds for injection molding for the pin guides + +- cutting stamps for the conductive pins + +All of these have minimum order quantities around 1,000-10,000 units. The goal +would be to get the price of the connector down to somewhere on the order of +$2-$4/each at 10,000 units. + +Is it just me, or is anyone else interested in crowdfunding "a better Dreamcast +serial connector" into existence?