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MS-401 Auto Data Switch

Mike_Z

Mike_Z

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Dec 1, 2013
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Near Milwaukee Wisconsin
This morning I picked up this RS-232 switch at a swap meet for $5. Don't know it if it works, if it doesn't it is a nice box for a future project. BUT, if it does I can switch between RS-232 devices with my 8080A computer. I didn't get any documentation with it, and I'm assuming it is a simple plug and play device. There is a common DB25 plug on the back that I assume would connect to the computer Rs-232 port. Then there are 4 other DB25 plugs where the devices would go. The front panel monitors the common port conditions CTS etal. There is a button that selects the port to use. Does anyone have any experience with one of these? Is it as simple as it seems. I have to find a 9 VAC power supply and give it a try. I've been looking for documentation on it, but I don't even know who made the unit. There is no tag on it.
MS401.jpg
Thanks Mike.
 
The labels on the various lights are suggestive of Serial port usage. The back panel says source and common so it is most likely relatively simple to work. Common goes to computer and the four Source connections go to each serial port device. Plug it in, turn it on, set the connection to the correct source by pressing the service button on the extreme right, and watch the pretty lights show how data gets transferred. Probably is simple enough that swapping cables could result in 4 computers sharing one device.

There should be a manual out there somewhere. I have seen similar before. If there is a manufacturer mark somewhere, searching for a manual will be much easier. While I have found a few better images, nothing tells me who made it.
 
I was thinking just the opposite, one computer port connected to four different devices. I don't think it make a difference. Still looking for info, but maybe I'll not need it. Have to wait on the power supply. Mike
 
Mike_Z said:
..., and I'm assuming it is a simple plug and play device.
Click to expand...
I think there is fair chance that you will need to worry about DCE versus DTE.

Auto port scan

The meaning of 'AUTO' on the front panel may be because the unit has an 'auto port scan' feature (in addition to manual port selection). Picure a serial printer that is shared between four computers. The unit scans each port in sequence until it sees data on one of the ports. The unit then selects that port. After a set period (e.g 60 seconds) of nil data on that port, the unit switches back to scanning all ports.

Computers that desire to send data whilst the unit is busy servicing a different computer, will hold off, because they will see the appropriate hardware handshaking line in a 'do not send' state.

If that is what 'AUTO' is, then perhaps the scanning feature is activated by holding in the DEVICE SELECT button for more than a few seconds.
 
Interesting, both you guys talk about a single device being shared between multiple computers. What I had hoped it would do is to have multiple devices, printer, console, off computer storage, etal, be shared by one computer port. I am thinking backwards? Mike
 
OK, thanks. Yes that is what I'm looking for. Thinking about it would seem that this unit should work both ways. Sometimes my old brain skips a bit once in a while. Thanks Mike
 
Well.... I found a 9VAC power supply and I got this unit to work. I plugged the computer serial port into the common port and my serial printer into port 1 and miracle of miracles, it worked. Although I'm not completely satisfied I completely understand what is going on. The front panel has 7 lights that indicate "Status of Common Port". When a transfer is underway, The CTS and RTS lights are lit, but the READ light is flashes as data to transmitted to the printer. Here is where I can get mixed up. So, the panel lights must be with reference to the switch. The switch is reading what the computer is sending. Well, anyway I can now plug in the peripheral serial devices to this switch and I will not have to plug and un plug stuff to make everything work, pretty neat. Mike
 
So is the consensus that this device can be dealt with as working both ways? I ordered one from eBay last night and I want to use it to switch one terminal between multiple DCE's if possible.

How did most serial switch boxes work - they were straight through and expected the devices they are switching to be DCE/DTE and the "common" port to be the opposite?
 
For anyone interested, this box is configurable on each port for DCE/DTE with jumpers on the inside. If I get a chance; I'll scan the manual and post it.
 
I'd mostly seen these things in two incarnations--multiple terminals connected to a single computer--you could see this in POS applications, for example--mostly with "page oriented" editing terminals where a page is filled in offline and then sent. They were fairly common in the 1970s, and then died out rapidly. If you've ever used a very old CPT word processor, you're probably accustomed to offline editing.

The other type allows multiple computers to share a single device; e.g. a printer.

The trick is in how the "auto" works. For terminals, it's mostly a matter of hitting a key; for a shared printer, it works the same way, but the time-out is considerably longer, as you don't want output mixed up.
 
The manual suggests that hardware handshaking is requires for the auto mode, so I'm not sure what signal it is checking. I plan on using it in manual mode to switch a single terminal between multiple devices, so really the opposite of what it was made for. Luckily they made a way to change each port dce/dte. It is really solid, heavy metal case, very nice.
 
You seem quite knowledgeable Chuck, I have one more question if you have a second. I bought a couple of dec vt420 terminals off of eBay. One works with a modem I hooked it to 9600/19200/38400, but the other only works at 9600/19200. I connected a logic analyzer up to each terminal to see how accurate the signal was and found this. I thought perhaps a crystal was running too fast (from aging), wouldn't the different baud rates be consistently 5% fast, etc., because of the aging? The faster the baud, the more error in both cases.

term2
com1 9600 9794 +2%
19200 19984 +4%
38400 41666 +8%

com2 9600 9803 +2%
19200 20032 +4%
38400 41876 +9%

term2 com1 9600 9708 +1%
19200 19630 +2%
38400 40192 +4%
 
It seems odd to me that the percentage of error also multiplies with the bitrate. One would expect this to be a constant for a given terminal; e.g. for the last "term2" entry, one would expect the rates to be 9708 19416 38832. Are you certain of your logic analyzer accuracy?

I don't know much about VT420s--too late for me; I lost contact somewhere around the VT220 era. Perhaps there's a knowledgeable person on the cctalk mailing list.
 
Found the issue. The LA triggers at 1.4V, not 0V (rs232) and the slew rate looks pretty slow at 38400.
 
Did one search and came up empty; will look again today. I know the manual is with the unit so when I find the unit...
 
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