RLV12 Emulator Version 2.0

Except for some small parts, like the resistors, condensators and the voltage regulator, all parts are now through hole parts and you can use sockets if you like. Here is the first PCB fully soldered with sockets ready to accept the ICs. In case of the CPLDs through hole PLCC sockets are required.

The bus receivers have now been replaced with 74HC4049 operated at 3V3 to mimic the thresholds of standard Q-Bus transceivers. The 74HC4049 inputs accept overvoltage up to 15V without damage, operating them at 3V3 volts in a 5V system does not result in any issues. They even accept 15V at their inputs when not powered at all. They have a high-impedance, low leakage current and a very small input capacitance. In other words they match the requirements of Q-Bus receivers. This type of Q-Bus receivers has already successfully been used with the Q-Bus Memory. The High-Level output voltage is high enough to savely drive TTL inputs even when operated at 3V3.

As transmitters I have been using 74S38. Although the output current of 48mA does not fully match the standard requirement of 70mA this is not a problem and many legacy designs use this same IC as transmitter for the Q-Bus. Unfortunately during some time the 74F38 in DIP‐16 packages have been very hard to find. They provide 64mA output, have a very low fan-in and consume less power. They are a better match as a Q-Bus transmitter. It is good to see that they are again readily available.

The two CPLDs are Atmel/Microchip CPLDS ATF1504 and ATF1508 and the MCU is a Atmega1284P. As per the product description on the Microchip home page all devices are still in production (as of 2024).

All the interfaces are put onto an IDC 34 connector. You use a ribbon cable to connect the break-out board mentioned before. The idea is to use one of the BA23 rear I/O distribution panels and mount the PCB and a DB-9/25 connector to the panel with appropriate holes for the JTAG, ISP connectors and the SD-Card slot and as well for the DB-9/25 connector.

DB-25 would be more time accurate but then a DB-9 is much more common today and as in most cases you will need a USB to serial adapter which mostly have a DB-9 connector.

In any case the break-out board only has the JTAG, ISP and SD-Card mounted and for the serial interface the breakout board is prepared to install a MAX232 TTL to RS-232 levelshifter to convert the serial interface signals from the MCU to RS-232 levels. You will then need three wires from the break-out board to the DB-9/25 connector (RX, TX and ground). In other words the two solution only differ in the mounting hole for the serial connection. I’m still using USB to TTL-Serial cables, so no need for level shifters.

The serial connection is used to control the RLV12 emulators firmware and allows you to mount various disk images to the four disk units. It also allows you to show various information and logging as well as to initialise a disk image, i.e. write the bad sector information.

The break-out board

To make all interfaces easily accessible when the emulator is inserted in a Q-Bus backplane I decided to build some general interface that brings ISP, JTAG, Serial Ports and control signals via a single IDC-34 ribbon cable to a break-out board PCB. So you can attach a break-out board with the appropriate interfaces to the back of your computer enclosure. The following shows the schematic used for the RLV12 Emulator V2.0

The signals with names are dedicated for that purpose. So the intention is to have JTAG, ISP, Serial Ports and the WP (write protect) and the CD (card detect) signals fixed. On AVR MCUs the SPI interface used to connect the SD-Card shares some pins with the ISP. There are some user defined pins for future expansion, one has already been dedicated to an activity LED.

The breakout board also allows to add a MAX232 RS‐232 to TTL level shifter. The idea is to use wires from the RS‐232 inputs and outputs of the MAX232 to DB‐9 connectors that are mounted directly to the filler panel. With the appropriate holes in the filler panel you can attach the break‐out board PCB to the panel.

In case where several devices that are programmed using the JTAG interface are used on the Q‐Bus interface, as is the case for the Emulator V2.0, the devices are cascaded. JTAG provides the necessary logic to address the devices and program them individually.

Connecting Q‐Bus Interface and break‐out board

Due to limited space the break‐out board is using SMD devices. However the smallest package is a SOIC‐14/16 package with 1.27mm pitch which is still very easy to solder. As always there is a small error on this board as well. The legend to the connectors SV2 and SV3 at the upper side of the board should read “GND TXD RXD VCC” instead of “VCC RXD TXD GND”

The filler panel

First draft of filler panel

RLV12 Emulator Version 2.0

The emulator in action, not much to see just to show it in a Q‐Bus Backplane, in this case a H9275‐A