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Assembly of the FP6120	Assembly of the IOB6120

Step 1 - small components Step 2 - "solder in swage standoffs" Step 3 - connectors between FP6120 and SBC6120 Step 4 - connectors between FP6120 and IOB6120 Step 5 - LED bar and the LEDs Step 6 - rotary switch and jumpers JP1, JP2 Step 7 - ICs ... and first activity of the LEDs Step 8 - toggle switches ... and final result

... to be described ...

Introduction

If you have browsed my website, you must have noticed that, besides my PDP-11 collection with their peripherals, I have also developed an interest in one of the predecessors, the pdp8. However, any pdp8 computer, even the most common pdp8/e, is nearly unobtanium stuff to get here in The Netherlands unless I am willing to pay some 1000 euro (approximately US $1350). Buying a pdp8/e on eBay in the USA is not a realistic option either. I could easily pay $1000 for winning the auction, then there would be shipping costs for some 60 kilo (120 lbs), and when the system is finally mine, it will most likely operate on 115 Volts AC, 60 Hz. A transformer to get the voltage up to 230 VAC is easy, the 60 Hz may be a problem as the mains frequency in Europe is 50 Hz.
And if I would get a pdp8/e the next expensive pit would be the search for some nice peripheral equipment like a papertape reader/punch (PC04, almost identical to the PC11/PC05).

After I had build the mini-PDP-11/35 console with the Core and I/O board (see the folder My projects --> Homebrew 'PDP-11') and finding the 6800-based assembler code for a pdp8 emulation, a new project was born. You can read about that in the same folder under Homebrew 'pdp8/e'. Then I was able to buy switches of a PDP-15 which have the size form identical to the switches used on the pdp8/i. That lead to the Fullsize 'pdp8/i' project in which the CPU is a so-called soft core, a processor implemented in an FPGA.

This page describes the SBC6120 developed by Bob Armstrong's "Spare Time Gizmos", see Spare Time Gizmos SBC6120. The SBC6120 is, just as its name says, a Single Board Computer based on the Harris chip HD-6120. "So what?" you can ask. Well, the HD-6120 is the implementation of the pdp8 processor in a single 40-pin DIL chip.
But it even gets better! Bob also developed a "blinkenlight" front panel for the SBC6120, named (you could guess it) FP6120, see SBC6120 Front Panel.

This section describes the assembly of the Front Panel. I can not describe the assembly of the SBC6120, because I have already build the SBC and the PDF manual is very clear in the steps involved. And that is something you can not say of the FP6120 manual, at least that is how I felt. My advice is that you read the FP6120 manual carefully before you plug in your soldering iron. There are a few things that must be done in a specific order, else you run into trouble later on. That is mentioned in the FP6120 manual, but you can read over it easily, and find out the hard way ...

I describe in this step-by-step assembly how I  did it. Bob starts the assembly with soldering the swage standoffs, but I will come to those critters in a minute. The FP6120 manual mentions that you must wait soldering the switches. It does not tell the reason. The problem is that you have to solder a few swage standoffs between the switches, which becomes nearly impossible if the switches are already soldered.

Step 1 - Small components

I started soldering the components which have the lowest height:

1. the few discrete resistors (not the SIL resistor arrays)
2. the two ferrite beads
3. the picofuse F1
4. the zener diode D2 (1N5339)

If you are not sure or can not clearly read the color code of the resistors, grab the Ohm meter and check.
The Front Panel PCB (Printed Circuit Board) is designed to have its own +5 Volt power supply, created from an external simple DC power supply like a "wall wart" that outputs 9 to 12 Volt. The external power supply connects via the picofuse F1, the on-board power ON / OFF switch, the protection diode D1 (1N5820, which protects against wrong polarity connection of the power supply) to the on-board voltage regulator REG1.

This on-board voltage regulator supplies the +5 Volt for all logic, not only the logic on the Front Panel PCB, but also for the SBC6120, the optional IDE hard disk or CompactFlash adapter and the optional IOB6120 module. The higher the voltage output of the external power supply, the more heat must be dissipated by the regulator.

Personally, I do not like hot components mounted on a circuit board. If you want to install a 2½" IDE hard disk instead of a CF card, you may also need +12 Volt. In that case, a small PC power supply is a good choice. As the PC power supply outputs +5 Volt, you must remove the protection diode D1, because of its voltage drop, and solder a wire instead. This means that you loose the protection against wrong polarity connection, so better check twice before turning on the power! Further, you do not need the regulator and solder a wire jumper from pin #1 to pin #3. Make sure (for example by a short piece of insulation tube) that the wire jumper can not make contact with pin #2. While at it, now is a good moment to decide if you want to use the on-board ON / OFF switch S1. If you mount the FP6120 in an enclosure, this switch is probably never used, so solder a wire jumper for S1 instead.

To conclude this step, solder the sockets for all ICs. The sockets are not included in the kit from Spare Time Gizmos, and my advice is that you buy good quality machined pin sockets. After all, the SBC6120 and FP6120 are not cheap, so why saving one dollar on some cheap sockets of dubious quality?
As Bob suggests in the FP6120 manual, solder machined pin sockets for the LED resistor arrays, RP1, RP1, RP3 and RP4. All 4 arrays are located above the LED bar. The machined pin sockets will make a few experiments later on easy to determine the light output of the LEDs to your personal taste and the environment where the Front Panel will be set.
You can solder the 4 SIL resistor arrays RP5, RP6, RP7, and RP8 (value 4k7) directly on the PCB. Three are located below the switches, the fourth SIL array is left of the rotary switch. Make sure that the common pin #1 (often indicated by a small dot on the SIL package) goes into the square pad on the PCB. If you are not sure which pin is pin #1 of the SIL package, get the Ohm meter. Connect one lead to pin #1 and check on 3 other pins if the measured resistance is always approximately 4700. If you measure a value approximately 9400, you have not pin #1, but the pin on the opposite side!
Finally, solder all decoupling capacitors next to the IC sockets, the tantalum and electrolytic capacitor.
Make sure you get the polarity of the tantalum and electrolytic capacitor correct!

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Step 2 - The "solder in swage standoffs"

The swage standoffs were difficult things for me. I knew what standoffs are, but I had never heard of the term "swage".
So I looked it up in the dictionary, but buying these critters (here in The Netherlands) was not possible. I asked Vince, and he was so kind to buy some 30 "swage standoffs" and ship them to me. Vince got the swage standoffs from Jameco, the part number is 1582428. NOTE for non-US citizens: all mounting material is "inch stuff", not metric, so if you live in a European country, get the metal and nylon screws and spacers with threading from a US-based supplier. Shops here only sell the metric stuff.
Again, read the FP6120 manual very careful, as some of the "solder in swage standoffs" are placed on the component side of the PCB (and soldered on the solder side of the PCB), and some of them are placed on the solder side (and thus soldered on the component side of the PCB).

• 7 swage standoffs for the LED bar
  The plastic LED bar is mounted on the component side of the FP6120 Front Panel PCB with 7 metal #4-40 countersunk screws. See figure 4 in the FP6120 manual. The 7 swage standoffs for the LED bar are placed on the component side of the PCB, thus soldered on the solder side of the PCB.
  You need a soldering iron that has sufficient heat capacity, I used a soldering iron of 75 Watt to solder the swage standoffs. A soldering iron used to solder electronic components like resistors etc. has not sufficient heat capacity. Make sure that the rim of the swage standoffs rests on the component side of the PCB, that is, that the screws will mount in an angle of 90 degrees to the PCB. I used a piece of painter's tape to keep the swage standoffs in the holes of the PCB and, after turning the PCB over to the solder side, the swage standoffs rest on a flat piece of wood. Do not let the swage standoffs rest on your desk. To solder them you must apply quite some heat, and you do not want burn marks on your desk!
  Blow gently over the just soldered swage standoff and let the soldered swage standoff cool down before you proceed to the next one to prevent to much heat in the PCB.

• 5 swage standoffs for the SBC6120
  The SBC6120 is mounted on the rear (solder) side of the FP6120 Front Panel PCB with 5 nylon #4-40 spacers. See figure 5 in the FP6120 manual. The 5 swage standoffs for the SBC6120 are placed on the solder side of the PCB, thus soldered on the component side of the PCB.
  Read the tips given in "7 swage standoffs for the LED bar". First solder the 3 swage standoffs that are "in line" at the left side (seen from the solder side), near the rotary switch, then solder the other 2 swage standoffs.

• 5 swage standoffs for the IOB6120
  The IOB6120 is mounted on the rear (solder) side of the FP6120 Front Panel PCB with 5 nylon #4-40 spacers. See figure 5 in the FP6120 manual. The 5 swage standoffs for the SBC6120 are placed on the solder side of the PCB, thus soldered on the component side of the PCB.
  Read the tips given in "7 swage standoffs for the LED bar". First solder the 3 swage standoffs that are "in line" at the right side (seen from the solder side), at the short edge of the PCB, then solder the other 2 swage standoffs.
  At this moment (February 2010), a new I/O board is being developed, so I do not have the board yet. I assume that the form factor will be identical to the original IOB6120 board, at least as far as mounting concerns.

• 4 swage standoffs for the 2½" hard disk
  The 2½" IDE hard disk is mounted on the rear (solder) side of the FP6120 Front Panel PCB with 4 metal #4-40 spacers. See figure 5 in the FP6120 manual. The 4 swage standoffs for the hard disk are placed on the solder side of the PCB, thus soldered on the component side of the PCB.
  Read the tips given in "7 swage standoffs for the LED bar". First solder the 2 swage standoffs that are "in line" at the left side (seen from the solder side), at the short edge of the PCB, then solder the other 2 swage standoffs.
  At this moment (February 2010), I have not yet mounted the hard disk, but I suspect that you can not mount the hard disk directly onto the spacers. That may depend on the screw holes in the chassis of the hard disk. Perhaps some sort of bracket must be installed on the spacers, and the bracket will hold the hard disk.

• 4 swage standoffs for the CompactFlash adapter
  The CompactFlash adapter is mounted on the rear (solder) side of the FP6120 Front Panel PCB with 4 metal #4-40 spacers. See figure 5 in the FP6120 manual. The 4 swage standoffs for the CF adapter are placed on the solder side of the PCB, thus soldered on the component side of the PCB.
  Read the tips given in "7 swage standoffs for the LED bar". First solder the 2 swage standoffs that are "in line" at the left side (seen from the solder side), at the short edge of the PCB, then solder the other 2 swage standoffs.
  I do not have a CF adapter and I suspect that there are CF adapters with mounting holes that do not fit on these spacers. In that case some sort of bracket must be installed on the spacers, and the bracket will hold the CF adapter.

Even if you have not yet decided whether your mass-storage device for the SBC6120 will be a hard disk or a CompactFlash card, you can solder the 4 swage standoffs for the hard disk and the 4 swage standoffs for the CF adapter. As the device that you select in the end will be mounted on spacers, the not-used swage standoffs of the other device do not pose any problem. If you solder the 4 swage standoffs of the device you select now, and do not solder the other 4 swage standoffs, and you change your mind regarding the mass-storage device after you finished the assembly of the FP6120, it is nearly impossible to mount the other 4 swage standoffs. In that case you will have to kludge some bracket to be mounted on the "wrong" 4 swage standoffs. I did not solder the swage standoffs for the CF adapter. If I will ever want to use a CF card, I will put a piece of aluminum on the spacers for the hard disk and mount the CF adapter on that aluminum plate.

Swage standoffs mounted on the component side, and the ones soldered on the component side.

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Step 3 - The connectors between the FP6120 and the SBC6120

The SBC6120 is mounted on the rear side of the FP6120 PCB and the two circuit boards connect to each other with two connectors. On the FP6120 the 2-pin connector is labeled J5, the 50-pin connector is labeled J1. As the FP6120 manual explains, it is important that the distance is correct. Too short pins may result in a flaky connection, too long pins will damage the SBC6120 or FP6120 PCB. Bob tells how the female headers and the male connectors are installed. As I only had two 50-pin female headers with long soldering pins (they were included in the SBC6120 kit) and I want to install the SBC6120 and the IOB6120, I did not follow Bob's instruction. Bob's construction needs two 50-pin female headers. The long pins of one female header are used as connector pins that plug into the other 50-pin female header. That is, the connector side of the 50-pin female header on the component side of the FP6120 is not used. Soldering a precious 50-pin female header just for the long pins seemed a waste to me, so I soldered a standard (short) pin male header on the solder side of the FP6120 PCB. That automatically implies that you must mount a 50-pin female header on the component side of the SBC6120 PCB, and solder the pins on the solder side. To get the height correct, put the 50-pin female header on the already soldered male connector on the FP6120, and then put the SBC6120 on the long pins of the 50-pin female header and let the SBC6120 PCB rest on the nylon spacers. Doing so, you get the correct height. Solder a few of the outer pins of the female header on the solder side of the SBC6120 PCB, and pull the SBC6120 off the male connector. Now you can solder the remaining pins of the female header. FYI, in my case, the distance between the SBC6120 and FP6120 was such that the female header was snug against the SBC6120 PCB!

If you have used the SBC6120 before you bought the FP6120, the CPREQ connection already has a short 2-pin male connector on the SBC6120 with a jumper installed on it. That implies that you must mount a female header for J5 on the solder side of the FP6120 (and solder the 2 pins on the component side), or remove the 2-pin male connector on the SBC6120 and mount the 2-pin female header instead. The second option is risky, because you might damage the SBC6120 PCB. So, I removed the jumper (and kept the 2-pin male connector) and mounted a 2-pin female header on the FP6120 using the same procedure to get it at the correct height.

Step 4 - The connectors between the FP6120 and the IOB6120

As I do not (yet) have the IOB6120, I did not install the 2-pin connector labeled J4, nor the 50-pin connector labeled J2 on the FP6120. That's an easy step for now, we'll see about this later ...

Step 5 - The LED bar and the LEDs

The installation of the LED bar and the LEDs on the FP6120 PCB is straight forward, and clearly described in Bob's FP6120 manual.
Get the polarity and the soldering height of the LEDs correct!  Personally, I did not like the red LEDs that other builders used to complete the Front Panel. It just does not seem right, but that is my opinion, my taste. I also saw a few movies of front panels of running pdp8 systems on YouTube, and that strengthened me even more to buy "warm-white" LEDs instead. If the light is "too white", or you want to approach the warm white, yellowish light of an incandescent bulb you can always put a thin foil of (appropriately) colored plastic between the LED bar and the Front Panel. To experiment with the light output it is mandatory that you install machined pin sockets on the FP6120 PCB for the SIL resistor arrays, as I described earlier. That is important, because it is amazing how little current the white LEDs need. The movie clip (see step 7) shows the LEDs with series resistance of 2200 Ohm!

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Step 6 - The rotary switch and jumpers JP1 and JP2

The end of the assembly is in sight. There are two 2-pin male jumpers that must be installed on the component side of the FP6120 board (thus soldered on the solder side), jumper JP1 (called 30Hz ENABLE) and JP2 (called SLU BREAK ENABLE). I soldered holding them in place, neatly vertical, with a piece of painter's tape.
It is not mentioned in the FP6120 manual, but you must put a jumper on the pins of 30Hz ENABLE, otherwise you will not see much "blinkenlight" effects on the LEDs.
Finally, the last soldering action (for now...). Check that the "tang washer" of the rotary switch is in the correct position. If it is, the rotary switch has 4 positions. Read paragraph 2.5.6 of the FP6120 manual. Do the following steps to determine the length of the shaft. You need to have the knob you want to use for this switch.

• Put the knob completely on the shaft of the rotary switch. Use a pencil to mark the end of the knob (the side where the knob will be against the Front Panel). The length of the mark (let's call it "A") till the end of the shaft is the length that must protrude from the Front Panel.
• Remove the nut and the lock washer from the rotary switch. Put the Front Panel on the FP6120 PCB. The Front Panel should be resting with its hexagonal spacers on the PCB.
• Put a marker line (with the pencil) on the shaft of the rotary switch at the surface of the Front Panel. Let's call this line "B".
• Add the length of marker "A" to the end of the shaft to the marker line "B". That is where you cut off the excess length of the shaft.

Before you solder the rotary switch, make sure that you cut the length of the shaft correctly. Put the pins of the rotary knob in the PCB and put the Front Panel on the PCB. Now, if you slide the knob over the shaft, the knob should rest or almost rest on the face of the Front Panel. If the distance between the rear end of the knob and the face of the Front Panel is too much to your liking you can file the shaft a little shorter. After you removed the Front Panel and took the rotary switch from the PCB!  It is wise to keep a tiny distance between the knob and the Front Panel to prevent scratches on the Front Panel caused by the rim of the knob while you rotate it.

Step 7 - The ICs

Take the necessary precautions before handling static-sensitive devices like ICs.
The installation of the ICs is straight forward. Bend all pins on either side of the DIL IC package so that the IC fits nicely on the machined pin socket. Then push the IC gently into the socket.
Take notice of the correct orientation! All ICs have the same orientation, the notch or indent on the package must be at the side of the LED bar.
The image on the right shows the result with all electronic components soldered, and the SBC6120 mounted on the rear side. If you click on the image a .mpg movie starts (size is 3.5 Mb). It shows the LED activity after powering up, the entry of the "B" command to boot OS/8 and then the result after entering the ".DIR" command.
You can see moving a part of my hand to press the "Shift" key to enter the ".DIR" command in capitals.

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Step 8 - The toggle switches

The soldering of the switches is not difficult, but for the best result you must choose a "reference" against which you solder all  switches to get them perfectly aligned. Of course, you will make sure that the momentary switches (the spring-loaded switches that return to their resting position when released) are in the correct place on the PCB. To get the alignment as good as possible you might solder just one pin to start with. Then align all switches. Check that the switches are positioned "centered" in the opening of the Front Panel. If everything looks fine, solder the remaining pins.
BUT ...   You must decide before you solder the switches whether you like the color of the switches. If you want to change the color of the levers of the switches after they are soldered on the PCB, you will have to remove the levers from the switches, with all the risk of damage to them!

Painting the levers of the switches
The problem here is to find paint that matches the color of the Front Panel as closely as possible. May be you can take the Front Panel to a shop where they can measure the color and then produce the color, but it will probably be quite expensive. I did find a shop capable of this job, but one color would cost some 70 to 80 euro (that is approximately US $100) and you need two colors ... However, they could supply "standard" colors for just 18 euro per spray can. This paint is for example used on cars.
Painting the levers is not different of painting any other surface. If you follow these steps the result should look good.

1. First of all, divide the switches in two groups of which the levers must have the same color.
   The switches of the Switch Register (SR0 - SR11) and the HALT are "ON/OFF" switches (having two positions).
   All other switches are momentary switches (having one position, if pushed to the other position and released, the switch returns to the initial position). 6 "ON/OFF" switches and 4 momentary switches must have the darker brownish color.
2. Degrease the levers.
3. Use fine-grained sandpaper to make the surface of the levers a little "rough".
4. Blow the dust from the levers, then degrease the levers again.
5. Using the pins, mount all switches that must be painted in the same color on a piece of foam or cardboard.
   
6. Spray a thin layer of primer on the levers. The primer will increase the bonding strength of the paint.
7. After the primer has dried (10 to 15 minutes), spray a thin layer of the paint.
   Do not try to cover the entire surface of the lever with paint in this first layer.
8. After the first layer of paint has completely dried, apply a second layer. Again, just a thin layer!
9. Repeat step 8 for a third thin layer. If the color coverage is OK, you are done else an other thin layer is needed.
10. Repeat steps 2 to 9 for the other lighter color, but that is obvious.

before the primer is applied	after the dark paint is sprayed	after the light paint is sprayed

Due to the light, especially the color of the dark painted switches seems wrong, but here is the final result after soldering the switches on the PCB. One tip: If you have not already soldered the rotary switch: don't! It is easier to solder the switches when the (long) shaft of the rotary switch is not resting on the desk. If you painted the switches put a soft cloth on the desk. The cloth prevents the first scratches on the top of the levers when the PCB is turned over to solder the pins. I soldered one switch at a time, and only the center pin. For alignment I pushed every switch "upward" in the direction of the LED bar. It does not matter in wich direction you push the switches as long as you are consequent. This will give the best alignment. Actually, I did not need to re-align a single switch. They were all perfectly "in line".
The nut on the rotary switch makes the total height just too much, so that the hexagonal standoffs of the Front Panel are not resting on the PCB. I removed the nut, but then a small space remains between the tang washer and the rear side of the Front Panel. I have cut a small ring of felten that fits over the threading of the rotary switch. That keeps the tang washer in place and can never make "noise" against the Front Panel.

Somehow the darker tan color of the Front Panel seems lighter and more like a red color, even though the picture is taken without flash light, using daylight only. Anyway, you can see that the colors of the switches nicely match the colors of the Front Panel.

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This section describes the assembly of the I/O Board IOB6120. Actually, it is a redesign of the original IOB6120 which had a few issues ...

... to be continued ... soon !

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