Tom Szolyga’s SBC version C on smallcomputercentral

[David Roper]

Has anyone built this cpm z80 sbc. I have a few Pcb's remaining.

Unfortunately Its showing no output on mine and the sio chip is warm.

I'm using correct rom for this board.

Any help would be appreciated.

[Steve Cousins]

Hi David

If by correct ROM, you mean: SCMonitor-v100-T1-TomsSBC-16k-ROM

then you are possibly having the same problem I experienced.

I had problems with unreliable reset until I figured out that the serial port pins are floating when no serial lead is connected. When SCM starts it does a self test and doesn't like the floating CTS input.

To fix this I put a link of the unused serial port in the position shown in the attached picture. A pull down resistor would also fix it I believe. You would need to do the same to port A if your serial lead does not connect the handshaking lines.

I also made an adapter  as the pin out of the serial port is not the same as the RC2014 and the FTDI serial cable I use. This is also shown in the picture.

Steve

[Richard Deane]

I soldered one up but didn't get it to work, even with Steve C's input. I will debug it eventually or build another as I have an extra pcb. My skills are improving as I've built an rc2014 pro and a number of sc boards which all work.

Mistakes I've made on other builds - plugged memory chip in wrong way round, got very got, but worked when plugged right way.

Also plugged in sio/0 instead of sio/2, now I appreciate the difference ( pinouts, there are more internal connections that want to escape than number of chip pins to support so you pick the desired permutation)
Richard

[David Roper]

I also built the rc2014 without a hitch and worked at power up.

I'll look at the sbc board again. Thanks for your advice.

[David Roper]

Steve I generally make up a new lead for each board and shall incorporate your suggestion into the lead.

There was a whole discussion on this subject at https://groups.google.com/forum/m/#!topic/rc2014-z80/IFkzQh3bHhY

The rom image I've used is sat in an 28c256 on board a pcb adapter of my own making. See pictures below.
With pin 1 which is A14 re-routed to pad 27 and pin 27 of the 28c tied to vcc.

What is the function of P7?

[Steve Cousins]

Hi David

I like your 28C256 adapter. Great chip for development. I have my own adapter but it is a birds nest. Yours is much nicer.

Jumper P7 on Tom's board is to allow the use of a 27C512. The jumper controls the signal to pin 1. The 27C256 uses pin 1 for VPP, while the 27C512 uses pin 1 for A15. 

With the jumper set between pins 1 and 2, pin 1 (VPP or A15) is held high. This is required by 27C256. 

With the jumper set between pins 2 and 3, pin 1 is software controllable. This allows either the top 32k bytes or the bottom 32k bytes of the 27C512 to be selected.

The ROM socket's A14 signal is also software controllable. When combined with A15 there are up to four 16k ROM banks which can be software selected. SCM scans these for additional software.

Steve

[David Roper]

Steve thanks for the compliment. I have 8 of these available for the price of a stamp if you want one.

Thanks for the info on the P7.

With the 3.368 mhz crystal in use the baud rate should be 57800 what of the handshaking? in the settings if the alterations are made to the lead.

I was using one of those cheap ttl-USB converters with six pinout dtr, tx, rx, vcc, cts and gnd.

[Steve Cousins]

Hi David

You can set the terminal software for either no handshake or hardware handshake. However, the CTS input to the SIO must be low for SCM to function correctly.

I believe this is how mine is connected:

Adapter               Tom's SBC

1 - GND   ---------   1 - GND

2 - CTS   ----<----   6 - DTR *

3 - VCC   ---------   3 - VCC

4 - TXD   ---->----   4 - RXD

5 - RXD   ----<----   5 - TXD

6 - DTR   ---->----   2 - CTS *

* These pins are 'swapped' relative to the RC2014 pin outs (shown below)

Adapter               RC2014

1 - GND   ---------   1 - GND

2 - CTS   ----<----   2 - RTS

3 - VCC   ---------   3 - VCC

4 - TXD   ---->----   4 - RXD

5 - RXD   ----<----   5 - TXD

6 - DTR   ---->----   6 - CTS

Could I offer you a board swap? One of my boards (https://www.tindie.com/stores/tindiescx/) for one of yours? 

I also a have a few others that are not listed on tindie, such as SC114 (easy build Z80 motherboard) and an experimental Z180 module.

Steve

[David Roper]

Hi Steve. Which board do you have in mind. My tindie shop is at

https://www.tindie.com/products/Retrogadgets/gamemaker-atmega-328-based/

[Steve Cousins]

Hi David

I was thinking one of your 28c256 adapter boards for your choice of board from my range at https://www.tindie.com/stores/tindiescx/

Wouldn't mind a couple of your adapter boards if you fancy a couple of mine.

Steve

[David Roper]

Steve I'm interested in your backplane 3 minimum cpm system. How many of my boards do you want?

Do you use propeller chips where you need a prop plug? I have a board that adapts a cheap $3 Chinese ttl-USB into a prop plug!

[David Roper]

Steve sent you a PM.

[David Roper]

Thanks to Steve who identified that the rom was actually in binary and not hex format. The board runs great with Steve monitor and Linc80 cf image.

[David Roper]