FRAM71B
User’s Manual
FRAM71B 1.00
- 21 -
©Hans Brueggemann, 2016
11.2 Accessing the OD Signal with a Modified Module in Port #1
Unfortunately, the HP-71’s Card Reader bay doesn’t provide direct access to the OD signal,
which is required for switching between SYSROM and SYSRAM. Hence, it must be provided
by modifying a module (for example, HP82420A), which is plugged into Port #1. The modifi-
cation takes advantage of the fact that the OD signal is accessible at pin 11 of Port #1, and is
done by connecting a two-pin header with two thin isolated wires to PIN1 (VDD) and PIN11
(OD) of the module. Installing the jumper will then switch on the OD signal and thereby disa-
ble the SYSROM.
Only one of the SYS-areas (either SYSRAM or SYSROM) should be enabled at any one time.
As a protective measure against bus contention, FRAM71B’s bus drivers have been designed
with relatively high output impedances of 1 k
Ω
. In case of both SYS-areas accidentially being
enabled, SYSROM takes precedence over SYSRAM, and the calculator will function normally
without damage. However, current consumption will be increased.
Picture 2: Modified 4 KByte RAM module with connected two-pin header. Module case is
built from scratch using styrene sheets from
http://www.evergreenscalemodels.com/
, but original
module case can be used as well. Use very thin isolated wires or enameled wires.
Picture 3: Modified 4 KByte RAM module installed into PORT #1 with its jumper set. Mod-
ule’s functionality is not affected.
v
If you don't want to modify a module, have a look at Nate Martin's 3D-
printed module, especially designed by him for this purpose.
The module is available at:
http://www.shapeways.com/product/2AW7Y2T4U/fram71-front-port-housing?optionId=57838008
PORT #1
