RS-232 on Model 4

I have recently resurrected my model 4 which has been idle for a long time. I ordered a FreHD which will enable me to easily download files from the internet, but whilst I wait for it to arrive I thought it might be "fun" to attempt to see if I could communicate between the Model 4 and an old Pentium IBM computer, which has a DB9 serial port. I made my own cross over cable (null modem). I tested all the pins and they seem to work OK, but still no luck communicating yet.

I have done an absolute stack of reading on the subject and the problem I have is that in my jumble of diskettes the very latest OS I could find was TRSDOS 6.2, and although I can load the RS-232 driver using SET *CL COM/DVR, the command "COMM" does not exist on the disk, therefore I can't effectively run the communication program in order to talk to the PC. SETCOM works to set the word length, parity etc. The idea was to use COMM to stream XMODEM over so I could then use that instead (as per the video done by VintageVolts).

I am using an older version of TeraTerm on the PC using COM2 but I can't get any joy out that - I assume that I won't be able to do anything until I get the RS-232 activated to work on the Model 4? What I have done is to short pins 2 and 3 on the DB25 end of the null modem cable and that does work (when I say "work", I mean I can type stuff in the teraterm window, where-as when the pins are not shorted - ie "no connection" - I can't type anything). So that should mean the cable works at least.

Most of the other disks I own are for my Model 3, so I have NEWDOS80 V2.0, DOSPLUS V3.4 and TRSDOS V1.3. I know I could just ask for a system disk to be mailed to me but given the FreHD is arriving sometime it is a bit silly to do this, I'm really just curious if there is a work around with what I currently have available to me?

I haven't tried it yet, but is it possible to use Matthew Reeds emulator instead - it has RS-232 capability, but again, I need to be able to set the Model 4 to send and receive before I can get any joy? I am a bit stumped.

If your serial cable looks like this one, it is a null modem cable.
DB9 Male Connector DTE (PC)..........................Model 4 Serial
2.......RXD.....Receive Data............in<-------------2 -- TX
3.......TXD.....Transmit Data...........out------------>3 -- RX
4.......DTR.....Data Terminal Ready.....out--|......|<--20 -- DTR
1.......DCD.....Data Carrier Detect.......in<--|......|-->8 -- DCD
6.......DSR.....Data Set Ready............in<--|......|-->6 -- DSR
5.......GND.....Signal Ground-------------SG--------------7 -- SG
7.......RTS.....Request to Send...........out--|......|-->5 -- CTS
8.......CTS.....Clear to Send...............in<--|......|---4 -- RTS
9.......RI......Ring Indicator

For your model 4 running in Newdos, why don't you download Omniterm and give it a try on Newdos.
I seem to remember using it on the Model 3. If you have CP/M for the Model 4 (Montezuma Micro ver 2.23)
I've got a driver for the TR1602 that works with the Modem903.LBR to make Modem 903 Functional.
I also have the .LBR I created (Modem904.LBR) that will work.

Or you can look for Modm700.LBR and use the Model 4 Overlay with Modm700.com to get Mdm700.com.

In terminal modes (on both ends of the RS-232C Cable) you need to be able to type characters at the same
RS-232C settings (300,8,1,N) to verify both computers are transmitting and receiving correctly. Then you can
speed up to 4800 or 9600 baud for your xmodem transfers. You can also unplug the remote end of the RS-232C
cable, short Pins 2-3, and type characters to verify the Characters are echoed from each machine back to
the display.


Once you get everything working your cable should be:
DB9 Male Connector DTE (PC)..........................Model 4 Serial
2.......RXD.....Receive Data................in<-------------2 -- TX
3.......TXD.....Transmit Data...............out------------>3 -- RX
4.......DTR.....Data Terminal Ready.....out---------|-->6 DSR
1.......DCD.....Data Carrier Detect.......in<--|......|-->8 -- DCD
6.......DSR.....Data Set Ready............in<--|----------20 -- DTR
5.......GND.....Signal Ground--------------SG--------------7 -- SG
7.......RTS.....Request to Send...........out------------>5 -- CTS
8.......CTS.....Clear to Send................in<-------------4 -- RTS
9.......RI......Ring Indicator


Let us know how you are progressing.


Larry

Thanks Larry.

Can you please confirm that 7 goes to 5 and 8 to 4? On my wiring I have 7 to 4 and 8 to 5 (RTS to RTS and CTS to CTS)? The only actual crossover I have is RX to TX and TX to RX.

I will only be able to download Omniterm and run it once I have the FreHD, but currently I have no means to do so because I cannot communicate with the Model 4, and I do not have a PC type 5.25 " drive to create a TRS80 disk. I do not have CP/M either.

Greg.

Yes, on the PC DB9, RTS, Pin 7 goes to the Model 4 DB25, Pin 5, CTS.
On the PC DB9, CTS Pin 8 goes to the Model 4 DB25, Pin 4, RTS.

RTS (Request to Send) is an OUTPUT from the DB25 & DB9 to control CTS (Clear to Send) INPUT to the other machines Port.
RTS is a HIGH Voltage when ASSERTED. You can measure this with a DC Volt meter to Signal Common (Pin 7 on DB25 & Pin 5 on DB9).
When not ASSERTED there is a Negative Voltage on RTS.


Larry

OK, thanks for that Larry. I have downloaded the omniterm manual, thanks for the link.

Greg.

Since I spent the last week or so hacking on the serial port in my M4, this is fresh in my mind so I'll throw in my $0.02. Here are the steps I follow whenever setting up a new serial connection configuration:

1. Pick a terminal program on your PC/Mac.
2. Pick your serial settings ie. baud=300,parity=even,stop=1,word=8. Start with 300 baud.
3. Connect pins 2 & 3 on your PC/Mac serial cable.
4. Test your PC/Mac terminal program to make sure you are seeing what you type (make sure echo is turned off). Don't proceed further until you get this working.
5. On your TRS-80

In LDOS 5

SET *CL RS232T
SETCOM (BAUD=300,WORD=8,STOP=1)
LCOMM

In LS-DOS 6/TRSDOS 6

SET *CL COM
SETCOM (BAUD=300,WORD=8,STOP=1)
COMM

6. Connect pins 2 & 3 on your TRS-80 serial cable.
7. Test your terminal program (COMM or LCOMM) to make sure you are seeing what you type. Don't proceed further until you get this working.
8. Connect your PC/Mac to your TRS-80 using a null modem cable or adapter. If using multiple cables/adapters, make sure there is only one "null conversion" in the chain.
9. Typing in one terminal should now be mirrored on the other and vice/versa.


Oh, and why was I hacking the serial port all week?

Getting TRS-Box to work using serial-to-TCP/IP.

If I can chime in a suggestion here....

Having gone through this with a Kaypro 4 and a Raspberry Pi running Ubuntu on it, so that the Kaypro 4 became simply a terminal to the Raspberry Pi, the one thing that will keep your sanity is to get a RS232C tester and put that on your cable. Without it, you are literally in the dark trying to find luck that you get the right pins matched up, etc. The tester will give you visibility as to what lines are active, what communications handshaking is needed (if any), software vs. hardware handshaking, and so on.

Eventually you will get the right combo here, but a $20 tester that you can plug onto your cable and see what is going on is priceless when doing this sort of thing.

My $0.02 worth based on my own hairloss.

Myles

Thanks Pete, that's a good simple starting point. Unfortunately I don't have LDOS so i can't use LCOMM and as mentioned on my TRSDOS 6.2 copy the COMM function in not recognised, I don't know why.

Good luck with the project.

Greg.

That's good advice Myles, thanks.

Greg.

The great news is that Gregga's FreHD was sent today and should arrive tomorrow.

In the package I also included two diskettes:
RS Service Centre Diags (the same one I use in my M4 RS232 repair video)
MAL/CMD on a LDOS formatted disk

Between them you should be able to test your Model 4 RS232 and get connected to your PC more quickly.

Cheers,

Ian.

That's awesome, thanks Ian. I watched your RS-232 repair video so it will be nice to know if it is in working order. I want to try and use my Kleinschmidt teletype as a printer output, plenty of food for thought there.

Greg.

FreHD arrived today and works a treat. So many programs pre-loaded, and fast execution. I checked the serial port, and using the diagnostic disk that Ian included, it shows that I get exactly the same error as shown in the video: "PE, OE or FE on receive", and yes I did short pins 2 and 3 (and rechecked to make sure).

I need to check a few pins on chip U81 to see if it is faulty, but I'll need a logic probe for this first.

Greg.