The TMS9918A video project consists of two pc boards, a small modification/addition to the main computer board, and a +5, +12, -5 VDC power supply (an expansion board of some type is necessary). Video board “A” contains the VDP chip, 16K video ram and one support chip for I/O interfacing. Board “B” contains the TMS9918 enhanced BASIC 2764 eprom, a switchable 2764 eprom (8-16K) socket and the logic necessary to decode the system and bank switch the character generator space (for initialization and printing to the 2040 printer).
The computer board modification consists of a diode OR-ing the NMI input to the Z80, allowing the VDP to interrupt the CPU instead of the ULA chip. This is a transparent modification and does not hinder the normal video display when the boards are removed.
The power supply is necessary for the video ram. Although power requirements are modest for -5 and +12 VDC and heat sinking is not required, the +5VDC regulator should have a hefty heat sink especially when using a TS 16K rampack. When using any ram (TS1016, memotech 64K) or other boards that require 9 VDC on edge trace 2B (see edge connector drawing), the following ‘mod’ will allow this supply to power the whole computer:
On the computer end of your expansion board, cut the trace going to the computers’ edge trace 2B. Connect four (4) 1n4148 diodes in series (anode to cathode), then connect the anode end to the lower left trace (looking at it from the computer end) on the expansion board (12 VDC), and connect the cathode end to trace 2B on the expansion side of the cut.
NOW, LET THE FUN BEGIN!
First, install the diode/resistor on the computers main board. a 1N4148 diode and a 4.7K ohm resistor (yellow/violet/red). Check the main computer drawing for your type of computer and board. Make the necessary cuts and connections.
Reassemble your computer and test it to see if it still works. If it does not, then go back over it and check for solder bridges, etc., in the area surrounding your artwork. Do NOT proceed until your computer works with this ‘mod’ installed (ZX80 with a ROM CS not mod installed).
Next, have your power supply ready for hook up. -5 VDC is connected to the top left trace of the expansion board (22AWG wire or larger). This is just left of the TS trace 1A (remember, Johns’ expansion board is a true 50 pin board). Connect the 12 VDC line to the bottom left ‘extra’ trace and the +5VDC line to the 5 VDC Sinclair trace (1B). Connect the power supply ground to the expansion ground (traces 4B and 5B). Hook up the 4 diode connection as previously stated, and you are ready for a cool running, externally powered computer. You won’t have to use that 9 volt plug on the side of your computer anymore. Check all connections and power up your computer. If it does NOT work, then power down immediately and check your connections again. Look for the proper edge connections and shorts to adjacent tracks. Check for proper voltage on each connected line.
You have now made the necessary computer board mod and power supply hookup. Before we begin the video boards, take a moment to relax, get an old issue of BYTE or RADIO ELECTRONICS and look up a source for ths TMS9918A chip and its’ companion the 10.738635 MHz crystal.
The following are sources that we know of:
JDR Microdevices 1-800-538-5000
in Calif. 1-800-622-6279
ACTIVE Electronics (Mass.) 1-800-343-0874
MILGRAY Electronics (with a company PO) is in Wash. DC. and has the VDP chip for $25.00, purchase order only.
JDR and Active has it for $39.95 and $32.95 respectively, but Active is VERY, VERY SLOW!! The crystal is about $4.00
VIDEO BOARD “A” ASSEMBLY INSTRUCTIONS
Assuming that you make your own boards, the full size layout is available for board “A” and “B”. It is beyond the scope of our text in this article to go through the steps required to make PC boards. We can supply those of you who are interested with information on PC assembly, if you supply us with a SASE.
Assuming at this point, that you now have possession of board A, These instructions will get you through its’ assembly.
1. Install all feedthroughs on the board using 30 AWG solid wire, in every location that has a round “doughnut” pad, on the COMPONENT side of the board. DO NOT install feedthroughs in the rectangular pads on U1s’ socket, pins 21-24.
To install a feedthrough, insert the wire from the boards’ component side, to extend about 1/8 inch outfrom the non-component side of the board. Bend the wire over on both sides, lay the board down flat (non component side up) and solder the wire on this side. Turn the board over, cut the wire off about 1/8 inch above the board, bend the wire over and solder this side also. Repeat this until all the feedthroughs are in place. Clean the flux from the board with some acetone or flux remover and inspect your work for bad solder joints or shorts. Touch up as necessary.
2. Install resistors R1, R2 and R3. Next solder all IC sockets onto the board.
USE CARE in soldering U3-U10 (16K RAM) sockets. The memory array is VERY prone to shorts. Clean the flux off and again check your work.
3. Install transistor Q1 by bending its’ center pin forward (toward flat front) and placing it in the EBC pads with the flat facing the boards’ left. Solder in place.
4. Install C1, C2 just left of R1, in their capacitor marked holes. Install C3 just right of Q1, denoted by the standard polarized capacitor symbol. Be sure the positive pin of C3 goes in the hole marked with the “+” symbol. If C3 is crowded by R2, then it can be laid down with its’ top toward the edge connector. Bend the leads as necessary and solder in place.
5. Install the 10.7386 MHz. crystal by first cutting the leads to 1/2 inch length. Insert the leads only far enough through the board, at its’ location marked with a crystal, to solder them in place. Twist the crystal 90 degrees and tape it down (gently) to the board, shaping its’ leads as necessary. Clean and check your work.
6. If it all looks good, then flip the board over and install the 7 tantalum bypass caps (Cbp) on the non component side (carefully).
Cbp ,PLUS END MINUS END 1. + U1-pin 33 - U1-pin 12 2. + U2-pin 14 - U2-pin 7 3. + U10-pin 16 - U8-pin 1 4. + U3-pin 16 - U9-pin 1 5. + U6-pin 9 - U6-pin 16 6. + U8-pin 8 - U8-pin 16 7. + U3-pin 8 - U4-pin 16
Bend each caps leads to fit over their respective pads, cut lead off 1/8 in. past bend, tin each end and tack down one end to hold it in place. Solder opposite end followed by the tacked end. Repeat for each cap. Inspect, trim and touch up your work.
7. Install jumper wire from U1-pin 33 to U6-pin 9.
8. Prepare a small length of coax cable by stripping each end so that 1/4 in. of the center conductor extends from the center insulation and 3/8 in. of wire braid, twisted together, extends 90 degrees out from the cable. Solder the center conductor of one end to the “VIDEO OUT” hole, and the braid to the ground hole on the right. Tin the braid and center conductor of the other end and connect appropriately to a female RCA (phono) jack. Clamp the cable to the board by forming a ‘U’ from a piece of bare 22 AWG wire, slipping it over the cable and through the two holes on the right side. Bend the wire over and solder, Clip off any excess lead.
9. Jumper U5-PIN 9 to Vcc(+5VDC). Pick up Vcc just above and to the right of the edge connector, at the second edge trace from the right, on the non-component side of the board.
10. Install all ICs on the board. The VDP has pin one to the left and the rest have pin one DOWN. TAKE A BREAK!
Be sure to check the board before going any further. See section on testing the boards.
VIDEO BOARD “B” ASSEMBLY INSTRUCTIONS
1. As with board ‘A’, it is necessary to solder all feedthroughs on the board. Use 30AWG wire in every round ‘doughnut’ pad on the component side. Do not install a feedthrough in the square pad, just left of Eb. Clean the flux from the board with acetone and carefully inspect your work for shorts to adjacent traces and bad solder joints. Touch up anything you see that looks at all suspicious.
2. Install resistor R1 in its’ holes, where marked on the component side, just left of Eb. Be sure to solder its’ right lead on both the top and bottom of the board, at the square pad. Since R1s’ holes are so far apart, take care to shape the resistors’ leads to fit and solder in place.
3. Install all IC sockets onto the board. Again, as with board ‘A’, take care in soldering the two 28 pin sockets in place. Clearances are tight. Clean the flux from the board, inspect and touch up your work once again.
4. Install SW1 in its’ mounting hole, just left of Ea. These holes are marked on the boards component side with an ‘x’ between them. Install the switch so that it is closed (on) when it is up, and open (off) when it’s down. It should work like a normal room light switch.
5. Install the two tantalum type bypass capacitors in their locations on the boards’ non-component side, in the locations detailed below. Install the capacitors in the same manner explained in step 6 of the board ‘A’ assembly instructions. Remove all flux with acetone and Q-tips, and check your work once again for shorts and bad solder joints.
CBP1 + U4-pin 14 - U4-pin 7 CBP2 + Eb-pin 28 - Ea-pin 14
6. Insert all ICs into their sockets. All four logic chips mount with pin 1 DOWN, and the eprom(s) mount with PIN 1 TO THE RIGHT. Eb is optional and not needed for the project to work. It may be left empty. Ea will contain the TMS9918A Video enhanced BASIC for the project.
Be sure to check the board before going any further. See section on testing the boards.
The machine code necessary for this project will be covered in depth in the next issue. If you have no facilities for burning eproms and/or just don’t want the hassle, we can supply you with an eprom with the previously noted (SWN 1/4) improvements and enhanced BASIC for $20.00. Please specify your printer interface, Timex, Memotech, or JLO (only one per eprom). This project is compatible with Memotech I/F only with additional decoding on the video board, and an eprom with the I/F code patched in. If you are in need, please get in touch with me in the evening. Tom Bent.
John supplies a tape for $6.95 that contains all the necessary MC for those of you who have an eprom programmer and 21 VDC power supply.
Those of you who still have the OLD 8K rom that was updated two years ago, will have to update before the machine code will work properly. Those of you who want to go it alone, you will need a 2764 eprom programer, a 21 VDC power source, a disassembler such as, Hot Z or another that allows block transfers of memory, and enough memory to hold the disassembler and 8K of machine code (0000 to 1FFF). Another help may be access to an eprom eraser, in case it’s not quite right the first time.
While you are at it, it is time to get a monitor, there are several sources for these. A good 12″ green screen composite video (75 ohm) for $80 to $90 is a good buy. Since you have a color computer now, a better bet would be a color monitor. Panasonic makes one for about $225, but there are several better ones. If you ever switch to a 2068 or some lesser computer, then the monitor can be used with it as well. We do not recommend using a TV modulator, because the rf interference caused by the high frequency of board ‘A’ (>10 MHz), may be intolerable. Sheilding would probably help though.
Now, you are ready to ‘power-er-up’. Insert boards ‘A’ and ‘B’ with Ea installed, on your expansion board. Plug your expansion board into the computer and connect your monitor to board ‘A’s video jack. With no other boards attached, at all, plug in your new power supply. Good Luck!!
THE FINE PRINT
If for some reason, you try all of the tests and cannot get your computer to recover or the cursor does not appear, then regress and start undoing some of your work and see if you can repair it. You can usually find the problem if you look long enough. We at SWN and many other people have this upgrade running (although I sent my board back to John for repair- a short in the 16K memory array), we can’t be held responsible for your errors. We will however help you in any way we can. John services what he sells. Send boards ‘A’ and ‘B’ (completely assembled), with your eprom and a check for $15.00 to him and he will check it out. This will eliminate the video boards from the problem. If your computer still does not work, then try checking the power supply, expansion board connections and any other add on circuitry you have added.
ROM CS not NOT ALLOWED
This upgrade will work with several other addons. The one restriction is the ROM chip select not line (trace 23B, the bottom right trace on the back of the computer). Many boards try to control this line (character generators, Memotech I/F, 64K memory boards). To eliminate the problem, simply cut this trace behind the first expansion slot on your expansion board. Insert the eprom board (B) into the first slot and always run it there. If later you want to return to the regular Sinclair video and have another board control ROM CS not, then plug that board into the first slot or into the back of the computer, with the expansion board following. However, if you own a JLO 64K board, then remove ic, U5, bend out pin 15 and reinstall it in the socket. It’s now ready for use. The video board must swap out the character generater in order to initialize and show lower case letters on the screen (without using any additional memory).
ZX-80 owners need only cut the trace going to pin 20 of the ROM, jumper the cut with a 680 ohm resistor and run a jumper wire from pin 20 out to edge trace 23B.
PARTS LIST BOARD B
U1 74LS00 Quad 2-input NAND gate U2 74LS27 Triple 3-input NOR gate U4 74LS32 Quad 2-input OR gate Ea, Eb(opt.) 2764 eprom 300ns or faster R1 10K Ohm 1/4 watt resistor Cbp1, Cbp2 12 MF/6V axial tantalum cap. SW1 SPDT ultra-sub mini PC mount slide switch(cut 1 lead off) MISCELLANEOUS 4 14 pin soldertail IC sockets 2 28 pin soldertail IC sockets 12" 30AWG solid bare wire (for feedthroughs) Bare board A -Main VDP I/F $14.95 Bare board B -Firmware/bank select board - $11.95 Both together - $24.95 Parts kit A - $22.95 Parts kit B - $5.95 Parts kits together - $27.95 All of the above - $48.95 (save $6.95)
TESTING YOUR WORK
BOARD ‘A’
After assembling board ‘A’, completely, we can now partially (smoke) test it. Remove U2 (74LS32) and bend out pin 8. Reinstall the IC the the socket with pin 8 extending over the side, not entering the socket. Now, insert board ‘A’ into your expansion board and power up the computer. With the regular TV output from the computer, the cursor should appear as usual. With the monitor connected to the output jack on the video board, you should get a blank screen. If either of these do not occur, then power down and remove your ‘A’ board and start hunting for your shorts (keep your pants on please) or other errors. The most likely place for a short or solder bridge, is in the memory array (I found three in mine-ed.). Be sure the tantalum bypass caps are installed correctly (especially Cbp 4).
Now remove U2, bend in pin 8 and reinstall the IC in its’ socket. Be sure all pins make contact correctly.
BOARD ‘B’
The initial test for this board is done by removing any eproms that are installed, and also by removing U3. Insert this board into your expansion board and power up your computer. With the video taken from the normal Sinclair source, you should see the cursor appear on the screen as usual. If not, then power down and start looking for the problem.
PARTS LIST BOARD A
U1 TMS9918A Video Display Processor** U2 74LS32 Quad 2-input OR gate U3-U10 4116 Dymanic Ram, 150ns. Y1 10.7386 Megahertz Crystal** Q1 2N3904 NPN transistor R1 470 Ohm 1/4 watt resistor (yel/vio/brn) R2 30 Ohm 1/4 watt resistor (or/blk/blk) R3 75 Ohm 1/4 watt resistor (vio/grn/blk) C1, C2 33 Picofarad ceramic capacitor C3 50 Microfarad/6V electrolytic capacitor Cbp1-Cbp5 12 MF/6V axial tantalum cap. Cbp6, Cbp7 10 MF/16V tantalum cap. MISCELLANEOUS 1 14 pin soldertail IC socket 8 16 pin soldertail IC sockets 1 40 pin soldertail IC socket 1 female RCA (phono) type connector 8" coaxial cable 2.5" 22AWG strand wire (for jumper) 2" 22AWG solid bare wire (for cable clamp) 12" 30AWG solid bare wire (for feedthroughs) ** not included in parts kit.
[Editor’s note: printed page 17 of this issue is missing from the only scan available, so any text and figures from that page are not included here.]