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TTY-Connect - FAQ


Q: Do you have parts kits, completed boards, or full systems for sale?
A: Sorry, only the pcb and micro are available.

Q: Do you have dimensions for the PCB and mounting holes?
A: All mounting hole and connector-centers on a 0.1" grid, to make it easy to quickly measure the board and transfer locations to the chassis. Yeah, a dimensioned drawing for chassis holes would be nice. Have not had time for such a thing.

Q: Why did you spec the Sescom chassis (it seems too small)?
A: The Sescom rack-mount box is nice for a couple of reasons: a) it has flat panels that slip into extrusions, so if you want to change a hole layout later, you can just change the panel. b) the panels are 1/16" aluminum (not steel), so it is easy to drill and nibble holes. c) since the panels are flat, you can use a punch press (if available). d) they are available in a variety of sizes -- I happen to be using the 2U-high-by-7-inch-deep version -- if you want more room inside, get a bigger one, like the 10-inch-deep version. But you can mount the unit in anything you'd like. Note that the Sescom box's lower-rear extrusion needed to be notched where the two pcb mounting standoffs are located. This is easy to do since the extrusions are also aluminum.

Q: Why is the assembly drawing cut off around edges?
A: That is all of the assembly drawing that is available in pdf form. It is really only for the pcb components anyway, and the "cut-off" stuff merely shows tentative locations for transformers etc.

Q: I am poking through my junk box for parts, and I have various optoisolators. Can I use them?
A: The places where the 4N37 isolators are shown could use other parts, but I'd rather not get into that. The 4N37s are cheap and easy to find. DO NOT substitute the high-voltage H11D2.

Q: I'd like to use sockets where possible. Do some parts need to be soldered, or can I use sockets?
A: Not necessary, but I would socket all chips, including the 6-pin dip optos. Easier test/repair.

Q: What's the story on the two ways to wire up loops HV1 and HV2, using either 80VDC or 160VDC?
A: The high-voltage loops may be configured for either 80VDC or 160VDC. To use 80VDC, the transformer primaries are wired in series, and to use 160VDC, the transformer primaries are wired in parallel. The power resistor values change too, of course. I really recommend the 80VDC option, since things run much cooler. With the heatsinks listed in the parts list, the unit set to full duplex (which burns additional power), and the loop set for 60-mA (for max power), the heatsink is 175 degrees (F) for a 160VDC loop, but only 125 degrees (F) for an 80VDC loop. In my opinion, 175 degrees (F) is just too darned hot. If you really must use a 160V loop, it should have a larger heatsink and/or fan. If you use 80V, the heatsink listed in the parts list should be fine with vertical mounting and simple convection cooling.

Q: I am curious as to why the transformers run so hot especially in the 160-volt mode? You show the transformers to be 43 VA and I calculate that the current at 115 volts should be 370 ma. Only drawing 60 ma should mean that the transformer is loafing.
A: It is not the transformers that run hot, it is the power-resistor/heatsink that gets toasty. The xfrms are oversized at 43VA, but a smaller xfrm is about the same price, and I like to be conservative, especially on a power design. Which is why I don't like a heatsink running at 175 F (160V loop). With an 80V loop, the heatsink is only 125 F, and the lower voltage is safer as well. Also, to get a worst-case scenario, these measurements were made in Arizona, during summer, in a garage where the ambient air was over 100 degrees.

Q: Depending on the tty-connect chassis size, and what is stacked on top or bottom of it in the rack, will heat be a problem?
A: If you are using 160V loops, a fan (and bigger heatsinks) would be a good idea. With 80V loops it should not be necessary. Good point about what is above/below the unit in the rack. You would want to mount tty-connect at the top of the equipment so it does not heat anything else up, and you would want to leave a rack space or two above/below it for convection air.

Q: I was checking parts list against schematic and found two C10 capacitors. Also U11 and U12 are not on the parts list. Would you clarify?
A: Oops. Yes the schematic should show C11 (not C10) near U11; it is a bypass cap (0.1 uF) for U11/12. The assembly drawing and silk are correct. Actually, the schematic should show U11, U12, and C11 as optional, which is why they are not on the parts list. U11 and U12 are external eeproms (non-volatile memory) -- there are many sizes of eeprom that fit into these sockets. But there is a small eeprom built into the pic, which stores the tty-connect machine configuration. At this time, U11 and U12 are not used.

Q: I thought the pic micro was already programmed, but apparently not -- what's up with that?
A: I programmed the micros with the bootloader, but did not load the firmware. You will need to load the firmware from a PC after your unit is built (it just takes a few minutes). You need a serial cable (DB9-F to DB9-M, straight-wiring) from the PC to TTY-Connect. Get the application file on the website called tc-pic-downloader.exe. This little standalone app runs on winblows xp/98, and allows you to send the actual tty-connect object code to the pic. To use, PIC-downloader.exe, you need to also get the tc_1_0.hex file for download to the pic. If (or when), I make a change to the code, I will upload the latest version to the website, and folks can use PIC-downloader to update their tty-connect system with the latest version. All of this magic made possible by flash memory in these 16Fxxx pics (as opposed to the one-time-programming of the 16Cxxx parts). Details are in an appendix of the firmware doc.

Q: I did not get the micro from you as I already have one -- what do I need to do to program it?
A: For you folks that want to burn your own PICs, and have access to a programmer to do so, you need to obtain a PIC16F876-20/SP chip (mouser part 579-PIC16F87620SP is $5.44 ea). On the website, is a file called bootloader.hex. This small file gets burned into the pic -- it resides in the upper hundred or so bytes, and provides bootload capability over the serial port. This is the only time the pic needs to be in the programmer. Since most folks don't have a programmer, I am doing this step for them when they get the pic from me. Both the bootloader code and the downloader are simple freeware goodies -- I have used them for years, and they perform flawlessly. After burning the bootloader into the pic, put it into the tty-connect board, and refer to the above question for loading the actual tty-connect firmware.

Q: Are you providing source code for the pic?
A: Sorry, no.

Q: The parts list mentions an optional $83 LCD -- what is it for?
A: Well, the Matrix-Orbital lcd is not supported in firmware at this time, but the pcb has a header that provides the signals (it uses I2C, not a simple logic-level uart, like other serial lcds). The objective is to provide a front panel control for the unit, and maybe a monitor mode. But I am not committing to anything at this point (I should not have put the lcd in the docs). It is a nice lcd module, but don't buy one yet. And a front-end on a PC will do the same thing.

Q: Is there a PC GUI for using the system?
A: No. TTY-Connect uses a simple command/message 232 interface for control and programming. It is easy enough to use a terminal emulator like HyperTerminal to enter commands with a few keystrokes. It is also feasible to embed these commands into a PC program, and provide a GUI front-end. Be my guest, if you are so inclined.

Q: I have seen more than a few tty machines that have a ground path on the keyboard side. It's not common to see it on the magnet side but it can happen. It's probably a real good idea to have people ohm out their machines before hooking tty-connect to them. A ground short on either side will smoke the board or at least blow a fuse, won't it?
A: All loop circuitry is floating from chassis/earth ground, and is opto-isolated from the 232/TU section. If you have just a single half-duplex loop to a tty, and one side of the loop (either side actually) is grounded to earth, it will NOT affect operation, will not blow a fuse, nor will it damage the board. But, if you are running two loops in full-dup mode, I suppose certain ground combinations could end up causing problems. However, I DID presume that your tty loops are floating. Checking machine wiring to know what you have is a good idea. Bringing out a pair of wires for the selmags, and a pair for the keyboard contacts is the simplest.



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