Designing a Shared-Socket UV EPROM Programmer for 2716 and 2732 – Pin 21 Conflic - Page 1
Forum section: electronics / projects

author: vulkano
author_rank: Newbie
timestamp: May 10, 2025, 04:43:58 pm
content: Hello everyone,I'm currently designing a PCB for a UV EPROM programmer that supports both 2716 and 2732 EPROMs using a shared socket. I've run into a design challenge involving pin 21:On the 2716, pin 21 is V<sub>PP</sub> (programming voltage, 25V)On the 2732, pin 21 is A11 (address line)Since these EPROMs have different functions on the same pin, I need to dynamically control pin 21 depending on which EPROM is inserted.Here's the solution I've come up with:A11 and CTRL1 are digital signals driven by the MCU controlling the programmer.Based on the selected EPROM type, the MCU either:Outputs a logic signal (for A11 when programming a 2732), orEnables a 25V supply (for V<sub>PP</sub> on the 2716)I'm looking for feedback or confirmation that this approach is valid and won’t risk damaging either EPROM type. If you've dealt with a similar issue or have suggestions for safely switching between logic-level and high-voltage control on a shared pin, I’d really appreciate your insights.Thanks in advance!

author: Benta
author_rank: Super Contributor
timestamp: May 10, 2025, 05:00:46 pm
content: Won't work, the transistors will not turn on.I suggest you use a SPDT analog switch instead, eg, DG419 (Vishay/Siliconix) or something like it.

author: chilternview
author_rank: Frequent Contributor
timestamp: May 10, 2025, 05:29:51 pm
content: Use a relay e.g. IM02TS. The programmer I built to handle 2716 and 2732 needed 3, to also control Vpp (25 or 21v) and a couple of the other pins:For a 2716, pin 21=Vpp, pin 20=CS_, pin 18=PGMFor a 2732, pin 21=A11, pin 20=Vpp, pin 18=E_For a 2532, pin 21=Vpp, pin 20=PGM_, pin 18=A11This does everything but the TMS2716 which is more like a 2708, and that is too different to be done by this programmer.

author: Benta
author_rank: Super Contributor
timestamp: May 10, 2025, 05:48:46 pm
content: Same function as a DG419, but clumsier/bulkier. And why choose an expensive RF relay for this?

author: chilternview
author_rank: Frequent Contributor
timestamp: May 10, 2025, 07:01:54 pm
content: Quote from: Benta on May 10, 2025, 05:48:46 pmBecause I happened to have some. And they're DPDT not SPDT so only 3 compared to 6 DG419's.

author: Benta
author_rank: Super Contributor
timestamp: May 10, 2025, 07:20:18 pm
content: Quote from: chilternview on May 10, 2025, 07:01:54 pmWut?It's only a question of switching pin 21 to either Vpp or A11, meaning one SPDT. How do you get 6?

author: chilternview
author_rank: Frequent Contributor
timestamp: May 10, 2025, 07:40:37 pm
content: You need to read what I wrote before. Pins 21, 20 and 18 needed to be switched to handle all 3 types or EPROM. And switch between 21 and 25 Vpp. The devil is in the details.If you can come up with a better design, feel free. There are many ways to skin a cat.

author: vulkano
author_rank: Newbie
timestamp: May 10, 2025, 08:58:05 pm
content: Thanks, guys. I'll go with relays since they are easier to purchase in my country.

author: Ian.M
author_rank: Super Contributor
timestamp: May 10, 2025, 11:15:47 pm
content: No need for relays, just two bipolar TTL open collector buffers (1/3 a 7407 or 74LS07), with outputs with 30V ratings, a P-MOSFET or PNP as a high-side switch for +Vpp control, and a 1K pullup to Vcc with a small signal Schottky diode in series so it doesn't back-feed the +5V supply when Vpp is enabled. One OC output pulls the pin low, and the other turns on the high side switch. If both are high, the pullup takes the pin to Vcc (logic 1). Both low is an illegal state that will destroy the 7407, so add some logic in front of the buffers to prevent that.The various programming voltages are best done by either switching resistors in the feedback network of the regulator or boost converter providing them (e.g. with another 74xx07), or if you want continuously variable, by driving current into the feedback pin of the regulator or boost converter, which must be ground-referenced. This can be as simple as a MCU PWM, low-pass filtered and coupled to the feedback pin by a resistor. When the filtered PWM is equal to the reference voltage it has no effect on the regulator/boost converter. When its above the reference voltage it proportionally decreases the output voltage and when below, proportionally increases. You wont find any great difficulty in getting a 5V to 27V range enabling programming of any 8 bit wide parallel EPROM or EEPROM that doesn't need bias voltages other than Vpp and Vcc.Edit: To avoid confusion, it should be noted that the resistor feeding the feedback pin with filtered PWM is in addition to, not instead of, the normal feedback divider.

author: pqass
author_rank: Super Contributor
timestamp: May 11, 2025, 07:09:17 am
content: See attached for another idea on how to generate discrete voltages (including 0V) for Vpp and Vcc.Using just one high voltage supply (+30V) you can generate any discrete voltage via programming resistor, feedback resistor, opamp and darlington.You'd use one of these circuits for every pin on your socket that needs discrete voltages; ie when Vpp and Vcc pins move location from smaller to larger packages.The circuit can generate a TTL LOW or HIGH too for when it interacts with an ordinary TTL input (eg. A11).You may even get away with driving your MCU GPIO pin LOW or to Hi-Z input and omit the 74xx05/06/07 (open collector part) entirely.For those socket pins with this circuit you'd use one MCU pin to enable (A switch) and one pin per discrete voltage (B, C, D).See simulationhere.

author: chilternview
author_rank: Frequent Contributor
timestamp: May 11, 2025, 08:58:13 am
content: Quote from: Ian.M on May 10, 2025, 11:15:47 pmYes that's the method I use for switching the Vpp supply. It's detailed in the 1977 Intel Memory Handbook.When you have to supply Vpp to different pins, you'd either need multiple circuits like the above, or just one relay or analog switch etc. to switch it between the pins.

author: Ian.M
author_rank: Super Contributor
timestamp: May 11, 2025, 04:55:47 pm
content: Multiple instances of the 7407 circuit isn't much of an imposition - the 7407 has enough gates to handle three pins, and the additional components per pin are only a small diode, transistor and a few resistors. You should probably current limit the variable Vpp supply to make it robust against short-circuits and also clamp the other address data and control pins, otherwise a blown EPROM with a fused Gnd bondout wire would cause havoc, by shorting the relatively high current Vpp supply to some or all of your logic signals! Also to aid survivability, put the resistor + diode pullup direct to the 7407 output with a 270 ohm resistor in series to the EPROM pin and Vcc switch transistor collector or drain. That limits the potential shoot-through (due to propagation delay mismatch) when switching from logic low to Vpp or visa versa to 100 mA, while still allowing a valid (albeit marginal) logic low level if the pin has a TTL input characteristic that sources pullup current.The protection logic can simply be a 7400 NAND driving the Vpp control 7407 input. Its inputs should be the logic control signal going to the pulldown 7407 input and an active high Vpp enable. Again there are enough gates in one IC for all three pins.