or, let’s start yet another thing!
I did make a note about increasing the content, and technical difficulties combined with attention span errors caused me to back-burner re-reviewing a certain PDF file. 😉
Enter the Imagewriter II localtalk option card, and what I’ve learned about it so far. Let’s begin.
The card is a network adapter for Apple’s Imagewriter II series printers, allowing a direct connection to an appletalk or localtalk network. This gives a lab the ability to share the printer with all systems on the network without a switchbox or other trickery, while also allowing faster data transfers into the printer: Normally the printer communicates at 9600bps while appletalk runs at 230.4kbps.
I’d had curiosity about the card for quite a few years and spotted one on our favorite second hand website for an inexpensive sum. To this, I thought to myslef “why not? It looks like it shouldnt be too bad to figure out.”
Pre-exam before arrival
When examining the board in any images I could find online I got hints of a few key IC’s that indicate that the network adapter is in fact an intelligent device. Clearly visible in online images are the z8530 one would expect since Appletalk is generally driven by the Zilog SCC, as well as 8k of sram, a 65c02, and a rom-sized IC with an Apple copyright date.
This gives me the impression that the card in general is in fact an independent SBC, and the copyrighted IC is likely a mask or OTP ROM that provides the network services for the printer. In fact, I’d seen images of these cards with a ceramic DIP and paper sticker over a circular indentation, indicating that it is in fact compatible with an EPROM.
Then I considered some requirements of the 6500 series microprocessor family: Ram at pages 0 and 1 (unless you like a gelding system), and ROM at the end of memory (unless you like a comatose computer). Given the SRAM was easily identifiable as a 6264 in the auction page I viewed and the EPROM socket had the same number of pins, I guessed that the firmware was 8k in size.
The other chips aside from the oscillator weren’t well photographed in the auction page so other guessing must be done to distract myself from the waiting process while the card made its journey to my hands. For example, what sort of memory map might this processor see?
Remember again the need for ram at the bottom of memory and rom at the top, and that they’re 8k in size? A likely solution for decoding this easily is by using a 3-8 demultiplexor, likely our friend the 74LS138. Bets are that it’s involved, so lets keep our scope to eight devices.
So far we have four devices. I could see a few chips in the original pictures, a trio of 74ls374 8-bit latches, and a 74ls245 bus transciever. That gives us four more, leaving us at six. Add the SCC and we’re at seven, and some possible control logic regarding controlling data flow to get to eight. No rule says all areas have to be used, however.
The board arrived in great condition, and I proceeded to take some good pictures. Here’s a top shot for reference:
When the board arrived I spent no time giving it a good examination. The 65c02 is rated at 2mhz, and a 3.6864mhz crystal clocks the system. A small non-volatile memory IC was also found, adding another device. The remainder are all standard 7400-series logic IC’s, one of which is a 74ls138 as predicted.
Since I can’t remove the CPU as I did for the IIEasy Print board, I’ll have to use a less automated method of finding the memory map. Time to get the continuity checker out!
Mapping the network (card)
If the 74ls138 is indeed used as a memory mapper, a few assumptions could be made. The first is that it’s enabled continuously, and the second that it’s controlled by the CPU’s clock to ensure correct write timing on the SRAM and latches. A quick check on /g1 and /g2 show them to be grounded. G3, the active high input however was not grounded or tied high. An intelligent guess led me to the phase2 line on the CPU, and my beeper agreed. Next is the blocksize decoded by the LS138. The I3 input on the 138 was quickly traced to A15 on the 65c02, I2 to A14, and I1 to A13. Eight 8k blocks of memory. With a single CPU and a 74ls138 the memory map is decoded.
Now let’s follow some outputs. Given the address bit connections, O0 should lead to the RAM chip, and as expected it did. O7 to the rom? Correct. Now where’s that SCC? I parked one probe on my meter on the /cs pin on the SCC and walked my way across the remaining outputs on the ‘138. Connected to O5 on the ‘138, the SCC starts at $a000 and ends at $BFFF.
All but two of the other outputs are traced so far. The remainder are a bit more complex and will be covered in a future update. A detailed memory map is below.
U9: 74ls138 pin name dest 7 /o7 [ e0-ff ] u3:20, u3:22 (8k eprom /oe, /ce) [$e000-$ffff] 9 /o6 [ c0-df ] nc? 10 /o5 [ a0-bf ] u2:33 (scc /cs) 11 /o4 [ 80-9f ] u7:1 (ls374 /oe) 12 /o3 [ 60-7f ] u5:1 (ls374 /oe) 13 /o2 [ 40-5f ] u6:11 (ls374 /load) 14 /o1 [ 20-3f ] nc? 15 /o0 [ 00-1f ] u4:20, u4:22 (8k sram /oe, /ce)
But what about the printer?
The Imagewriter II isn’t incredibly well documented, but a SAMS manual does have something resembling a schematic, in which a port for an ‘optional device’ is pinned out. It has 26 pins like the localtalk card, and has vcc and gnd in the same locations.
The pins are pretty straightforward: eight ‘AD’ pins for a multiplexed address/data bus, an ALE line to latch the address during an access, a DE line to enable the data transfers, and other various control lines.
AD0-7 head straight for the bus transciever to keep the network card off the imagewriter’s ADbus unless there’s a valid transaction, and then head into the three ‘374 8-bit latches. Likely, the latches are used for Address, Data in, and Data out (respectively). There’s some twisty logic to be traced so that this can be worked out.
And the software?
The firmware on the card is still hidden as I’ve not taken time to dump its contents and examine it. This may or may not be covered later as well, but I do find it interesting to find out how the card communicates with the printer. Perhaps other communications cards could be created- perhaps a parallel interface or other networking option?
To be honest, I didn’t really intend on adding this to my Imagewriter II, since printers aren’t really my thing these days. I just find vintage hardware interesting, and enjoy the process of mapping simple systems to feed my imagination of what could have been done with them had we known what they really were made of.
(Proabaly) more later. 😉