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DOSBox – Serial INTERLINK to Another PC

So, I recently had Interlink running between two REAL machines. It occurred to me though: Wouldn't it be cool if we could use a USB-Serial adapter on a newer machine and use Interlink from DOSBox to transfer out files? You could then copy anything into the DOSBox HD and get it onto your vintage unit. Great if you have no network, CD or are just sick of unstable floppy disks!

The requirements

DOSbox happily talks to real COM Ports. We'll want DOSbox running real DOS, so follow a guide like Transmission Zero has here to install DOS to a read HDD image. Firstly, download an SVN build of DOSbox. Then download the 256mb image and get your DOS disk images here. After copying everything into the dosbox folder, grab the dosbox configuration from your user profile (you might have to boot DOSbox up once first!) and bring it over as well. I renamed the disk images to make it easier and then edited the configuration as follows:

[autoexec]
imgmount 2 "hdd-256mb.img" -size 512,63,16,520 -fs none
boot .\D1.IMA .\D2.IMA .\D3.IMA

Run DOSbox and you should be presented with the DOS installation screen. Use CTRL-F4 to rotate through disks. Finally, you should be at the DOS prompt!

Serial Ports

Now for the serial configuration. First up, I'm going to show you a neat trick to get everything tested and working. We're going to use a virtual serial port between two instances of DOSbox to prove that Interlink works in the emulated environment. There's a plethora of virtual com ports available for Windows and they all vary greatly... officially I want data to flow from one COM port to another virtual COM port, so I'll need software that'll do this; you'll find that a lot just want to send data over a network. If you have Windows XP or lower (Windows 7 might still work also), then use com0com. Otherwise, if you need signed drivers then Eltima's Virtual Serial Port Driver is pure magic... but expensive. You get a 14-day trial with the standard version, so plan ahead and use it as much as possible! I've actually sent out a plea for help, asking for a cheaper license for just a single 'null-modem' serial port.

So, you've installed a pair as the above picture. To get here you simply installed the VPSD and chose all default values. You now have a COM1 and COM2... or maybe you have COM20 and COM21... (my laptop has no REAL comports, so COM1 and COM2 were available) regardless, we need to edit the configuration (as per the DOSbox manual) of your DOSbox instance and update the serial1 line to:

serial1=directserial realport:com1

Save it, close it and run dosbox. You should see one connection on one of the virtual ports in the port configuration application...

Whilst DOSbox is running, edit CONFIG.SYS and add the line DEVICE=C:\DOS\INTERLNK.EXE to the bottom. Once this is done, close DOSbox and copy the entire folder to a new folder. Rename this to server, or somesuch. Go in and edit this configuration to point to the second comport of your virtual two.

From here, you can boot up both DOSbox's and see that they connect to either side of the virtual com ports. You can then run INTERSVR from the prompt of one node and INTERLNK from the prompt of the other and you'll then connected! Totally pointless... but we've proven that Interlink via serial port works in DOSbox!

Intercepting data...

Just in-case I wanted to spy on the data, I thought I'd try something quickly with C# and the SerialPort class. I set up two pairs of com ports, connected DOSbox #1 to COM1, DOSBox #2 to COM4 and limited their speeds to 9600. I then used the following code to write a quick WPF app to see if I could get the data to traverse...

	<Grid>
		<Grid.ColumnDefinitions>
			<ColumnDefinition/>
			<ColumnDefinition/>
		</Grid.ColumnDefinitions>
		<TextBox Grid.Column="0" Name="FromText" TextWrapping="WrapWithOverflow"></TextBox>
		<TextBox Grid.Column="1" Name="ToText" TextWrapping="WrapWithOverflow"></TextBox>
	</Grid>

That's the WPF, just two text fields to see the data... then the c# to hook it altogether...

	public string FromData { get; set; }
	public string ToData { get; set; }
	private SerialPort port1 = new SerialPort("COM2", 9600, Parity.None, 8, StopBits.One);
	private SerialPort port2 = new SerialPort("COM3", 9600, Parity.None, 8, StopBits.One);
	public MainWindow()
	{
		InitializeComponent();
		port1.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
		port1.Open();
		port2.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
		port2.Open();
	}

	private void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
	{
		var port = ((SerialPort)sender);
		byte[] data = new byte[port.BytesToRead];
		for (int b = 0; b < port.BytesToRead; b++)
		{
			int fromPort = port.ReadByte();
			data[b] = (byte)fromPort;
		}
		if (sender == port1)
		{
			port2.Write(data, 0, data.Count());
			Dispatcher.BeginInvoke((Action)(() => {
				FromData += " " + BitConverter.ToString(data).Replace("-", "");
				FromText.Text = FromData;
			}));
		}
		else
		{
			port1.Write(data, 0, data.Count());
			Dispatcher.BeginInvoke((Action)(() => {
				ToData += " " + BitConverter.ToString(data).Replace("-", "");
				ToText.Text = ToData;
			}));
		}
	}

Starting the app saw the relevant COM ports fire up in the port configurator... then I booted DOSbox:

Hahaha... it's 99% working. I wonder how hard it'd be to disect the entire chat and build an emulator. I had to limit everything to 9600 as my code doesn't auto-negotiate the speed. Without the limits, the client Interlink was speeding up to 115200 BAUD and the server had no idea what was going on.

I'll dig into those bytes in another post... the data looks pretty simple and might be easy to emulate... anyway... what was the point of this post? Oh yeah!

Talking to real metal

Back to that 286 from the previous post about Interlink. Let's talk to it! This time, instead of virtual ports, we're going to talk to real ports. I've got a USB Serial port which will help out a lot here.

I used this recently when talking to the Apple II. It's always good to know that things work before mucking around with them. I also ended up using the real serial port on my main desktop. Either way, any serial port will work.

Install your COM port and make sure your DOSbox configuration is correct as per the hardware port number. Mine is COM1 on this main desktop.

serial1=directserial realport:com1

Then simply boot up as you've done before with DOSbox. This time the client (or server) will be over the real wire.

Crikey! It just-worked(tm). The laptop happily saw the data in DOSbox. Easy file transfers! I wonder if I can make a Windows app that'll just let you share a drive?!

DOS – Serial File Transfers

This was meant to be easy... I was building a 386 for sale on eBay and had a spare 286 laptop hanging around. I was also selling a 4-metre NULL-MODEM cable which I wanted to list as 'tested'.

First, a warning: Don't let the magic smoke out

There's an age-old notion of magic smoke. This smoke is contained in electronic components and, most importantly, needs to stay inside them. Usually the mistakes of an impulsive, rash, impatient user will cause the smoke to escape. Once it has, there's usually no going back... or if there is, then you'll be spending a lot of time/money trying to get back to the spot you were at prior to the damage.

In this case, I did it to the old 286 laptop used below. On the back of the machine, there's two female DB-25 ports. They're not labeled and I thought I was simply in luck! They've already put a correctly oriented serial port (COM2) on there for me!

Turns out not... turns out one is the printer port (duh!?) and the other is for an external floppy drive. Regardless, I plugged my null-modem cable into the printer port and one electrolytic capacitor started cooking. I turned everything off quickly and inspected the damage. Yup, just one black capacitor (it was meant to be orange!) Unfortunately, when I powered the unit up, the capacitor kept cooking! Currently I've just disconnected it... could just be that that cap was shorted or reverse-polarised and fried... could also be that the whole LPT1 port is dead... gonna be hard to test.

Anwyay... I later found the part that labeled the ports... DOH! Where were we?...

InterLink

Would you believe that DOS 6.0+ came with serial transfers as standard? There's an application in-built known as INTERLINK. From here you just set one machine as the server with INTERSVR.EXE and the other as the client with the INTERLNK.EXE application itself.

The basic idea is to load the INTERLNK.EXE as a driver first, in CONFIG.SYS. You can configure extra options here, but if your connection is correct then it should just-work(tm).

Cabling

The best method is to use a specific printer cable, affectionately known as a LapLink cable. This is a cross-over cable for printer ports and is only used for transferring of files. The next best bet is a null-modem cable which does the same thing, but via the serial port, and supposedly slower as there are less data lines.

We're going to use a null-modem serial cable today and you'll find any number of methods for creating one. You can get adapters that'll convert a standard serial cable, entire cables that are 'null-modem' or you can just create your own.

I had actually picked up a 4-metre 'null-modem' cable from a misc eBay auction and wanted to try it out. This had a male plug on one end and a female on the other and didn't quite make any sense. Hence it allowed me to plug it into the wrong port on the laptop (as mentioned above) and wreak havoc. After that little mishap, I decided to just use other spare parts and rig up my own.

I've spoken a bit about serial cables before, but the basic idea is that there's a DTE and DCE end. It's a standard client-server relationship. In our case, our servers need to talk to eachother, so we need to adapt then of of a cable to allow it to connect. The goal is to cross over the transmit and receive lines to have data transfer correctly. There's also some supporting wires to allow data-handling negotiations to take place. You'll find all the wiring methods on the Wiki page.

Somehow, I found both a female DB-9 and DB-25 in the junk box and followed the first diagram on the wikipedia page. It was only three wires, GND from 5-7, RX from 2-3 and RX from 3-2.

For the life of me, I couldn't get this to work. Interlink just wouldn't see the other end of the cable. I then tested both ports (just to see if I'd fried more parts in the laptop) with a serial mouse and everything worked fine. From here, it must be the cable, right? Maybe it needs to be more fully-fledged with the other data/signal lines linked up. This site seems to indicate further handshaking is required.

I browsed back to the null-modem page on Wikipedia to work out the other wires to hack together. Something then occurred to me... the DB-29 port has RX and TX the other way around! Why would they do this? Regardless, I simply disconnected my taped and twisted wires and ran 2-2 and 3-3 through directly.... the result? Success!

Configuring Interlink

Before you even think of entertaining this idea, make sure your ports are tested functional with a mouse and that you've gone over your wiring 16 times prior. Make sure you've re-read your serial port pinouts and that you have traced each wire. As mentioned above, the DB-25 RX and TX lines are swapped when compared to the DB-9 pinouts!

Once you're feeling game, edit CONFIG.SYS on both your machines and add the line DEVICE=C:\DOS\INTERLNK.EXE on BOTH server and client! At this stage, you can reboot the server. It'll boot up and will spit out a little 4 line message indicating that it's loaded the driver, what disks are mapped and what printer ports are mapped. It'll also tell you that no connection is available. Once at your command prompt, run INTERSVR.EXE. You'll be presented with a pretty boring screen indicating the current activity.

Now, return to the client and reboot it after saving changes to CONFIG.SYS. When this one boots up, you'll hope to see that it's connected. Initially I didn't and it just tells you in CAPS that no connection is available. At this point, it's actually checked all of your ports for a server. This is both your printer and COM ports, so it actually really means that it couldn't find a server! I had this a lot at the start and began fine-tuning the configuration, attempting to limit it to a single com-port with /COM:2 switches... but it ended up being my cable!

Once you're up though, it's all easy! As per below, you'll be told what drives/ports are mapped and it really is as easy as just switching to that drive and listing directory contents, or copying from them!

I pulled over A-Train (quite quickly, actually!) and tried to run it on the 286 Laptop... it's not VGA, so we didn't get far at all. But it's a 286 without VGA, what was I expecting? Might need to try Railroad Tycoon... I wonder if I can Interlink that over from DOSbox?

Of Pin Straightening and CMOS Batteries

Whilst rebuilding a motherboards thought long dead, I have been required to perform a myriad of tasks. These have been related to anything from re-building PCB traces to fabricating batteries to straightening IC pins. Honestly, there's nothing quite like a good challenge and, recently, there's been nothing short of serious challenges appearing on my workbench!

CMOS Batteries

Some older motherboards have the batteries soldered on... or worse... incorporated into the freaking CMOS RAM chips! Luckily, this latest motherboard had no provisions for an on-board battery. Instead, it just had the usual 4-pin header for an external battery. Of the four pins, the two next to eachother are ground and the single pin to the side is positive. Don't be fooled by the CON1 label thinking that anything with a 1 should be positive!

I ended up at the usual Jaycar franchise and purchased a coin-cell holder and plug socket. It's a little chunkier than a standard header, but it is so much easier to work with!

Solder everything up and determine which way is positive. Note that, for coin-cells, the positive side is the 'bottom'. I had always assumed it'd be the 'inner-ring' (as per AA and AAA and all other batteries)... but it ain't. So be careful!

Once rigged together... install!

Yosh! Worked like a charm. If only all motherboards came with external batteries... the next mobo on the shelf has so much corrosion that I just feel like throwing it out.

Pin Straightening

Whilst doing the previous Recycle-Shopping-Tour-Of-Kumamoto, I stumbling across this 100-yen dream. It's a 386/486 era CPU with an alloy heatsink to obscure its actual identity!

I spent no extra effort breaking it out of the bag... and probably bent more pins... I wonder if works? Or what spec it is?

First step... pop that heatsink off. This required a fair amount of jimmy'ing...

Ooooo! It's a 486 DX2/66. This is seriously my favourite 486 CPU. So much natsukashiiii.

Will have to find a motherboard to slap it into... but before I even have a chance to do that, it's time to break out the tweezers. Actually, I discovered a better method this time around. Use a credit-card/transport-card/library-card to align an entire row of pins at the same time!

Nothing like being an adult and using adult tools!