FT245 board to replace 6850

[Mark T]

When I saw Ben Chong's 6402 board I thought about making a version that would be software compatible with the 6850. Then I thought it might be better to use a PIC18F to go from USB direct to RC2014 and skip the serial link but this would have needed extra register and buffer for data and was a bit of a tight fit in 100 x 50mm. Finally I've arrived at using an FTDI FT245 after considering also the FT240 as it seems to be more readily available. There are a number of breakout boards available that could avoid dealing with surface mount components but I thought it might be fun to try soldering an SSOP.

I panelised the board with a CTC board in a 102mm x 100mm panel, only silkscreen marks out the division, so I'll need to separate the boards myself, but this avoids the charge for panelisation by the manufacturer.

I use a 74ls273 as a control register, and a 74ls245 to read the status and try to map these as close as possible to the 6850.

If there is an attempt to read an empty Rx buffer or write a full Tx buffer then IC2A and IC2B should hold the wait line. This is attempt to allow use of z80 INIR instruction for a simple bootstrap program. The data sheet of the FT245 was a bit unclear about attempting to read empty or write full buffers so I included IC3 and IC4 in case they are needed, with option links if not required.

Links are included to connect to either the z80 interupt or to a CTC to allow mode 2 interupts.

If anyone can see any problems please let me know.

Mark

[ZO...@gladucalled.com]

Hi Mark,

I am not up to speed with he 6850, which is a compatible UART for the 6502 or 68000 chips?  So your board is the result of a serial port that ending up using an FTDI part, yes?  The final destination being a 6502 w/ USB, no?

Let me know. Sorry for being slow.

Also, there seem to be five channels. What are these?  USB is four pins.  Please advise.

Thnx =Steve.

[Peter Willard]

6850 is from the 6800, 6809 family... the 6551 is the ACIA from the 6502 series.

[Mark T]

Hi Steve,

As Peter said, 6850 is from 6800 family, but was used in Grant Searle's Z80 CPM boards which inspired the RC2014. Its only recently that the SIO has started to replace the SIO in RC2014 systems.

I was looking for an easy way to get CPM running without sourcing the 650.

Standard USB is 4 pins, but micro and mini connectors have 5 pins, the 5th is usually used as an ID pin to identify On The Go devices or special charging devices.

Mark

[Mark T]

I just spotted a stupid mistake in my FT245 board. I thought A0 should be low for data and high for control and status, but it should be high for data and low for control and status. I can use different addresses, but kind of breaks compatibility with the 6850 software. I guess I could always wire mod a board to get it working.

[Mark T]

I finally built up one of my FT245 boards. Hand soldering the USB connector and the FT245 was a bit of a pain using a fine tip iron and 0.3mm solder, but seems to have worked OK.

I haven't tried using the interupts yet, not sure if there is much point as it runs too fast and interupt handler is just going to slow it down.

Tried measuring the speed by echoing characters back to the pc and sending text files using teraterm. In order to avoid the time taken by the pc leaving the z80 in single step until the file is sent and then switching z80 to run. 15k byte text file is too fast to measure. Serial interface at 115200 would be about 1.3 seconds.

Trying with larger files it seems to slow down and appears to run in bursts. Suspect this might be something in the usb protocols when both directions are busy.