Rev F & earlier CPU, serial upgrade

Introduction

Page last updated Nov 17 2025.This Web page discusses upgrading an 1802 Membership Card CPU from Rev F or earlier to support a power and serial connector like year-2020 M/S CPU and front-panel cards have. A small adapter board is needed with a few components; it's simple to hand-wire. This is from a discussion with a Rev F Membership Card owner and some review by Lee Hart in Nov 2025.

Refer to my Membership Card home page for the current version. A Web link there takes you to developer Lee Hart's Web site for ordering and contact.- Herb Johnson

Add serial & power to an early 1802 Membership Card CPU board

In Nov 2025, one of Lee Hart's Membership Card customers contacted me. He wanted to use his Rev F Membership Card set with a serial port. People run the Membership Card off of a TTL/USB dongle using 5V from the dongle and of course the ground, and serial pins.

But the Rev F model didn't have a serial port. It used the DB-25 connector for parallel communications with the COSMAC - configured for an old PC-compatible parallel port. The Rev F didn't even bring out /EF3, the traditional serial recieve connection to the DB-25; it was on the P1 cross-connector between the CPU and front panel board.

The owner chose to buy a Rev L CPU Membership Card, to obtain access to the serial/power port on that card. But if they chose to build something up, here's what I suggested. Lee Hart looked over my suggestions and added a few alternatives, which I will point out.

Rev F CPU card

Here's the Rev F home page. For the manual and schematic, see this document. The DB-25 on the Rev F was the normal interface to the Membership Card. For serial, pin 15 on the DB25 is the Q signal, that's the serial transmit out. However there's no /EF3 signal on the DB-25 for serial in. So we turn to P1/J1, the 30 pin cross connector.

On the connector between the two boards, P1 and J1, /EF3 is on pin 25. Also Q is on pin 12. Then there's ground on pin 1 and pin 30. Those would be the serial connections, almost. In principle, you could wire up those P1/J1 pins as a TTL serial in and out and ground. Or, if you were experienced, you might be able to rewrire the Rev F CPU's 4-pin power/ground connector, to use its pins 2 and 3 for serial in and out. As available pin 4 is your ground and pin 5 is +5 volts.

Rev L1 CPU card

But modern TTL/USB dongles expect the Membership Card to have some additional circuits for serial. They are shown, for example, in the Rev L1 CPU / Rev L1 front panel manual, look at the Rev L1 manual schematic, on page 18 and 19. I've extracted out the serial components into this schematic image.

On the front-panel board, the Q signal from J1 pin 12 goes to an "inverting transistor" NPN Q5. There's three 100K "pullup resistors" R12 in parallel - 33K ohms total - to the Q5 collector. The output of the transistor is to the P4 power/serial connector as "TX". Note that particular transistor has two internal resistors, 22K. For the /EF3 receive signal on pin 26 of J1, it goes through a 1K resistor R10, to the P4 connector as "RX".

(Those are the RX and TX serial signals. In my schematic grab, I also show the wiring of LEDs for those signals, and the /ON feature to reset the CPU board. If you wanted to add those features, you'd add those parts. They are not necessary for serial communications. - Herb)

To do that work, you'd have to wire up a little transistor and resistor board, with a power/serial connector, and mount it somewhere on the P1 connector of the CPU board. Instead of Q5 with its internal resistors, you could use an ordinary NPN transistor and external resistors instead.

notes

Lee Hart suggested he could bring out wires, but eventually they need those additional components. There's an alternative to the transistor to invert the Q signal: change the logic-sense of any ROM or RAM serial program that expects that inverter. But these mean 1) patching or reassembling current software and 2) a little less hardware protection of the 1802's Q and /EF3 pins. I'd advise making the board.

If I get around to it, I'll make one up and show a photo and schematic. Meanwhile, any Membership Card CPU schematic since about Rev I shows the necessary components to the power/serial connector. The connector's signals mate with common TTL/USB dongle connectors. Photos of use of recent (post 2020) Membership Cards show that dongle in use. - Herb