HP LaserJet 4-5-6 family of printers is a real engineering landmark in sense of reliability. Beside the aging rollers, the particular design is built rock solid and will last eternities. It is usual to meet these old workhorses on the flohmarkets for 10-15€. The space of your home office permitting, it is really cost effective to refurbish and use these gerontological printers, especially networked models like Laserjet 5Si, LaserJet 2400TN and LaserJet 8000. The network interface (albeit 10MBit/sec) provides the easy standard connectivity and extra SNMP monitoring. It even is possible to set up Google Cloud print for these devices. Pro: cost and reliability, network connectivity, monitorability and size A3 (for certain models); Cons: size and weight.
A particularly interesting topic is printer memory (RAM) upgrade up to the maximum amount supported by the design. In case of my 5Si printer, the upper limit is 128Mbyte, plus some 4MB available on the motherboard itself.
Ancient memory modules had a challenge while presenting itself to the printer. 20 years ago, the memory SIMMs mostly had no service ROM available, from where the host device could read the type and size of the memory stick. Thus various methods were used to recognize memory sticks and the one HP loved much, was through the “presence pins”. It means, on a 72-pin memory stick, certain pins (67,68,69,70) were used as control lines to communicate memory specifications to the printer. These pins are possibly grounded against pin 72.
The table depicting the encoding scheme was obtained from http://ps-2.kev009.com/eprmhtml/eprma/h871.htm , which, in turn, was linked from http://www.keycruncher.com/blog/2003/12/14/making-standard-simm-s-work-memory-upgrade-on-the-hp-laserjet-6mp-5mp/ :
In my particular case, the LaserJet in my possession and waiting to be upgraded, was a 5Si. The repairability of the unit is amazingly good, thus I only needed to remove some dust and choose better fuser unit out of two I had. Printer brains will become servicable right after opening two big screws and sliding the module off.
My 5Si had 4 memory modules, each one of 8MB. Notice the soldering area used to communicate the “presence” to the printer!
This is from the test printout (Menu, Test, PCL):
I had some extra 32MB memory sticks of the same 72-pin standard available, but the printer was not recognizing these.
Looking at the Table above, the reason was evident – these sticks were wired/soldered as 60nS modules while the particular printer is only recognizing slower sticks starting from 70nS. The solution was easy – to solder a deception wire into position two, claiming the sticks are of 70nS speed. In other words, according the Table above, I did ground Bit 2, thus changing the presence code combination from 1101 to 1001. One can notice the code 000 on the SMD resistor which means – 0 Ohm, thus a thin wire will do.
[Added 2016-05-18] The source at http://ps-2.kev009.com/eprmhtml/eprma/h871.htm uses negative logic: X’s to indicate the vias and .’s for the lack of a via. This way I departed from the code 1101 (..X.) “X means connected to ground” and only one via was installed and I finished at code 1001 (.XX.) which means both middle vias installed. In other words, I needed to lie to HP that my 60nS SIMMs actually are the 70nS items.
The deception was successful, as verified by the test printout.
The overall result of the modification is looking like this: